Jan 30, 2014 Signals can flow in either direction across a MOSFET switch. In this application the drain and source of a MOSFET exchange places depending on the voltages of each electrode compared to that of the gate and the direction of current flow. Application Note 3 of 10 V 1.0 2018-07-20 Applications for depletion MOSFETs How to use a depletion MOSFET 2 ow to use a depletion OST Shorting gate and source, as shown in Figure 4, results in a load current independent of the applied drain-to.
MOSFET, showing gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
The metalâoxideâsemiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metalâoxideâsilicon transistor (MOS transistor, or MOS),[1] is a type of field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. It was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in November 1959. The MOSFET is the basic building block of modern electronics, and the most widely manufactured device in history, with an estimated total of 13sextillion (1.3 Ã 1022) MOSFETs manufactured between 1960 and 2018.
The MOSFET is by far the most widely used transistor in digital electronics as well as analog circuits, and is considered to be possibly the most important invention in electronics. It was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses, revolutionizing the electronics industry and in turn the world economy. It has been central to the microcomputer revolution, digital revolution, information revolution, silicon age, and information age. MOSFETs are the basis for high-densityintegrated circuits (ICs) such as memory chips and microprocessors, and in turn modern computers and mobile devices, each typically using millions to billions of MOSFETs. Discrete power MOSFETs are also the most common power devices. MOSFETs are used for numerous applications, such as computers, smart devices, telecommunications, power supplies, RF amplifiers, audio amplifiers, electronic musical instruments, display devices, television receivers, image sensors, automotive electronics, the Internet, wireless mobile networks, home entertainment, video games, industrial equipment, pocket calculators, X-ray detectors, spacecraft, and satellites, among many other uses. The US Patent and Trademark Office calls the MOSFET a 'groundbreaking invention that transformed life and culture around the world'.
A key advantage of a MOSFET is that it requires almost no input current to control the load current, when compared with bipolar transistors (bipolar junction transistors, or BJTs). In an enhancement mode MOSFET, voltage applied to the gate terminal increases the conductivity of the device. In depletion mode transistors, voltage applied at the gate reduces the conductivity.[2] MOSFETs are also capable of high scalability (Moore's law and Dennard scaling),[3] with increasing miniaturisation,[4] and can be easily scaled down to smaller dimensions.[5] They also consume much less power, and allow higher density, than bipolar transistors.[6] The MOSFET is also cheaper[7] and has relatively simple processing steps, resulting in a high manufacturing yield.[5] Since MOSFETs can be made with either p-type or n-type semiconductors (PMOS or NMOS logic, respectively), complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS (complementary MOS) logic.
The name 'metalâoxideâsemiconductor' (MOS) typically refers to a metal gate, oxide insulation, and semiconductor (typically silicon).[1] However, the 'metal' in the name MOSFET is sometimes a misnomer, because the gate material can also be a layer of polysilicon (polycrystalline silicon). Along with oxide, different dielectric materials can also be used with the aim of obtaining strong channels with smaller applied voltages.
A cross-section through an nMOSFET when the gate voltage VGS is below the threshold for making a conductive channel; there is little or no conduction between the terminals drain and source; the switch is off. When the gate is more positive, it attracts electrons, inducing an n-type conductive channel in the substrate below the oxide, which allows electrons to flow between the n-doped terminals; the switch is on.
Simulation of formation of inversion channel (electron density) and attainment of threshold volÂtage (IV) in a nanowire MOSFET. Note: threshold voltage for this device lies around 0.45 V
Origins[edit]Background[edit]
The basic principle of the field-effect transistor (FET) was first proposed by Austro-Hungarian physicist Julius Edgar Lilienfeld in 1925.[8] However, his early FET concept was not a practical design.[9] The FET concept was later also theorized by Oskar Heil in the 1930s and William Shockley in the 1940s,[10] but there was no working practical FET built at the time.[9] Shockley's research team initially attempted to build a working FET, by trying to modulate the conductivity of a semiconductor, but they were unsuccessful, mainly due to problems with the surface states, the dangling bond, and the germanium and copper compound materials. In the course of trying to understand the mysterious reasons behind their failure to build a working FET, this led them to instead invent the bipolar point-contact and junction transistors.[11][12] None of these early FET proposals involved thermally oxidizedsilicon, which later made the MOS transistor possible.[13] Semiconductor companies initially focused on junction transistors in the early years of the semiconductor industry. However, the junction transistor was a relatively bulky device that was difficult to manufacture on a mass-production basis, which limited it to a number of specialised applications. FETs were theorized as potential alternatives to junction transistors, but researchers could not get FETs to work properly, largely due to the troublesome surface state barrier that prevented the external electric field from penetrating into the material.[14]
Invention[edit]
Mohamed M. Atalla (left) first proposed the MOSFET. Atalla then invented the first MOSFET with Dawon Kahng (right) in November 1959.
A breakthrough came with the work of Egyptian engineer Mohamed M. Atalla in the late 1950s.[12] He investigated the surface properties of silicon semiconductors at Bell Labs, where he adopted a new method of semiconductor device fabrication, coating a silicon wafer with an insulating layer of silicon oxide, so that electricity could reliably penetrate to the conducting silicon below, overcoming the surface states that prevented electricity from reaching the semiconducting layer. This is known as surface passivation, a method that later became critical to the semiconductor industry as it made possible the mass-production of silicon semiconductor technology, such as integrated circuit (IC) chips.[9][15][16] For the surface passivation process, he developed the method of thermal oxidation, which was a breakthrough in silicon semiconductor technology.[17] The surface passivation method, which substantially reduced the influence of the dangling bond that had prevented Shockley's research team from building a working FET,[12] was presented by Atalla in 1957.[18] Building on the surface passivation method, Atalla developed the metalâoxideâsemiconductor (MOS) process,[15] with the use of thermally oxidized silicon.[13][19] He proposed that the MOS process could be used to build the first working silicon FET, which he began working on building with the help of Korean recruit Dawon Kahng.[15]
The MOSFET was invented by Mohamed Atalla and Dawon Kahng in 1959.[10][16] They fabricated the device in November 1959,[20] and presented it as the 'siliconâsilicon dioxide field induced surface device' in early 1960,[21] at the Solid-State Device Conference held at Carnegie Mellon University.[22] The device is covered by two patents, each filed separately by Atalla and Kahng in March 1960.[23][24] Operationally and structurally different from the bipolar junction transistor,[25] the MOSFET was made by putting an insulating layer on the surface of the semiconductor and then placing a metallic gate electrode on that. It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator. The silicon MOSFET did not generate localized electron traps at the interface between the silicon and its native oxide layer, and thus was inherently free from the trapping and scattering of carriers that had impeded the performance of earlier attempts at building a field-effect transistor.
Commercialization[edit]
The MOSFET was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses.[14] Despite the breakthrough, the MOSFET was initially overlooked and ignored by Bell Labs in favour of bipolar transistors, which led to Atalla resigning from Bell Labs and joining Hewlett Packard in 1961.[16] However, the MOSFET generated significant interest at RCA and Fairchild Semiconductor.[10] In December 1960, Karl Zaininger and Charles Meuller at RCA began fabricating a MOS device based on Atalla's device.[26] Similarly based on Atalla's work,[27]Chih-Tang Sah at Fairchild in late 1960 began building an MOS-controlled tetrode,[10] which he presented in 1961.[28] MOS devices were later commercialized by General Microelectronics and Fairchild in 1964, with p-channel devices for logic and switching applications.[10] The development of MOS technology, which was capable of increasing miniaturisation, eventually became the focus of RCA, Fairchild, Intel and other semiconductor companies in the 1960s, fuelling the technological and economic growth of the early semiconductor industry in California (centred around what later became known as Silicon Valley)[29] as well as Japan.[30]
Importance[edit]
The MOSFET forms the basis of modern electronics,[31] and is the basic element in most modern electronic equipment.[32] It is the most widely used semiconductor device in the world,[33] and the most common transistor in electronics[9] and communications technology.[34] It has been described as the 'workhorse of the electronics industry' due to being the building block of every microprocessor, memory chip and telecommunication circuit in use,[35] as well as siliconintegrated circuits (ICs),[36]visual displays,[37] digital wirelesstelecommunication networks,[38][39]graphics processing units (GPU),[40] the computer industry, digital telecommunication systems, video games, pocket calculators, and digital wristwatches, among many other uses.[41]
The MOSFET was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses.[14] This led to a revolution in the electronics industry, which has since impacted daily life in almost every way.[42] The MOSFET has been called the most important transistor,[43] the most important device in the electronics industry,[44] the most important device in the computing industry,[45] one of the most important developments in semiconductor technology,[46] the birth of modern electronics,[47] and possibly the most important invention in electronics.[48]
The MOSFET is the most widely manufactured device in history.[49][50] The MOSFET generates annual sales of $295 billion as of 2015.[51] Between 1960 and 2018, an estimated total of 13sextillion MOS transistors have been manufactured, accounting for at least 99.9% of all transistors.[49] Digital integrated circuits such as microprocessors and memory devices contain thousands to billions of integrated MOSFET transistors on each device, providing the basic switching functions required to implement logic gates and data storage. There are also memory devices which contain at least a trillion MOS transistors, such as a 256GBmicroSDmemory card, larger than the number of stars in the Milky Waygalaxy.[35]
Some of the earliest electronic fields to be revolutionized by the MOSFET include power electronics,[52] electronic signal processing, control systems, and computers.[53] The MOSFET was central to the electronics revolution,[54]microelectronics revolution,[55]silicon revolution,[56][57] and microcomputer revolution,[58] and has been the fundamental building block of modern digital electronics during the digital revolution, information revolution, information age,[59][60][61][62] and silicon age.[56][57] MOSFETs have been the driving force behind the computer revolution, and the technologies enabled by it, such as computers, the Internet, and smartphones.[34][63] The rapid progress of the electronics industry during the late 20th to early 21st centuries was achieved by rapid MOSFET scaling (Dennard scaling and Moore's law), down to the level of nanoelectronics in the early 21st century.[64]
Power MOSFETs and MOS integrated circuits are fundamental to the Internet, and the communications infrastructure that enables the Internet.[65][66] MOSFETs are fundamental to digital wireless telecommunication networks, such as mobile networks.[38][39] Most of the essential elements of mobile networks use MOSFETs, including the handheld mobile devices themselves, along with the mobile tranceivers, base station modules, routers, and RF power amplifiers.[39] Advances in MOS technology has been the most important contributing factor in the rapid rise of internet access bandwidth in telecommunications networks, with online bandwidth doubling every 18 months, from bits per second to terabits per second.[67] The thin-film transistor (TFT), a type of MOSFET,[37] is the basis for display technologies such as TFT LCDs (liquid-crystal displays), LCD televisions,[68]active-matrix LCD (AM LCD), and OLED (organic light-emitting diode), as well as X-ray detectors.[69]
The mass-production of silicon MOSFETs and MOS integrated circuit chips, along with continuous MOSFET scaling miniaturization at an exponential pace (as predicted by Moore's law), has led to revolutionary changes in technology, economy, culture and thinking.[56] The US Patent and Trademark Office calls the MOSFET a 'groundbreaking invention that transformed life and culture around the world'[34] and the Computer History Museum credits it with 'irrevocably changing the human experience' as 'the base technology' of the late 20th to early 21st centuries.[59] The MOSFET is included on the list of IEEE milestones in electronics,[70] and its inventors Mohamed Atalla and Dawon Kahng entered the National Inventors Hall of Fame in 2009.[9][15] The MOSFET was the basis for Nobel Prize winning breakthroughs such as the quantum Hall effect[71] and the charge-coupled device (CCD),[72] yet there was never any Nobel Prize given for the MOSFET itself.[73] In 2018, the Royal Swedish Academy of Sciences which awards the science Nobel Prizes acknowledged that the invention of the MOSFET by Atalla and Kahng was one of the most important inventions in microelectronics and in information and communications technology (ICT).[74]
Composition[edit]
Photomicrograph of two metal-gate MOSFETs in a test pattern. Probe pads for two gates and three source/drain nodes are labeled.
Usually the semiconductor of choice is silicon. Recently, some chip manufacturers, most notably IBM and Intel, have started using a chemical compound of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good semiconductor-to-insulator interfaces, and thus are not suitable for MOSFETs. Research continues[when?] on creating insulators with acceptable electrical characteristics on other semiconductor materials.
To overcome the increase in power consumption due to gate current leakage, a high-κ dielectric is used instead of silicon dioxide for the gate insulator, while polysilicon is replaced by metal gates (e.g. Intel, 2009[75]).
The gate is separated from the channel by a thin insulating layer, traditionally of silicon dioxide and later of silicon oxynitride. Some companies have started to introduce a high-κ dielectric and metal gate combination in the 45 nanometer node.
When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an inversion layer or channel at the semiconductor-insulator interface. The inversion layer provides a channel through which current can pass between source and drain terminals. Varying the voltage between the gate and body modulates the conductivity of this layer and thereby controls the current flow between drain and source. This is known as enhancement mode.
Operation[edit]
Metalâoxideâsemiconductor structure on p-type silicon
Metalâoxideâsemiconductor structure[edit]
The traditional metalâoxideâsemiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO
2) on top of a silicon substrate, commonly by thermal oxidation and depositing a layer of metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor.
When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a p-type semiconductor (with NA{displaystyle N_{text{A}}} the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB{displaystyle V_{text{GB}}}, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB{displaystyle V_{text{GB}}} is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator.
Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage. When the voltage between transistor gate and source (VGS) exceeds the threshold voltage (Vth), the difference is known as overdrive voltage.
This structure with p-type body is the basis of the n-type MOSFET, which requires the addition of n-type source and drain regions.
MOS capacitors and band diagrams[edit]Power Mosfet Application
The MOS capacitor structure is the heart of the MOSFET. Consider a MOS capacitor where the silicon base is of p-type. If a positive voltage is applied at the gate, holes which are at the surface of the p-type substrate will be repelled by the electric field generated by the voltage applied. At first, the holes will simply be repelled and what will remain on the surface will be immobile (negative) atoms of the acceptor type, which creates a depletion region on the surface. Remember that a hole is created by an acceptor atom, e.g. Boron, which has one less electron than Silicon. One might ask how can holes be repelled if they are actually non-entities? The answer is that what really happens is not that a hole is repelled, but electrons are attracted by the positive field, and fill these holes, creating a depletion region where no charge carriers exist because the electron is now fixed onto the atom and immobile.
As the voltage at the gate increases, there will be a point at which the surface above the depletion region will be converted from p-type into n-type, as electrons from the bulk area will start to get attracted by the larger electric field. This is known as inversion. The threshold voltage at which this conversion happens is one of the most important parameters in a MOSFET.
In the case of a p-type bulk, inversion happens when the intrinsic energy level at the surface becomes smaller than the Fermi level at the surface. One can see this from a band diagram. Remember that the Fermi level defines the type of semiconductor in discussion. If the Fermi level is equal to the Intrinsic level, the semiconductor is of intrinsic, or pure type. If the Fermi level lies closer to the conduction band (valence band) then the semiconductor type will be of n-type (p-type). Therefore, when the gate voltage is increased in a positive sense (for the given example), this will 'bend' the intrinsic energy level band so that it will curve downwards towards the valence band. If the Fermi level lies closer to the valence band (for p-type), there will be a point when the Intrinsic level will start to cross the Fermi level and when the voltage reaches the threshold voltage, the intrinsic level does cross the Fermi level, and that is what is known as inversion. At that point, the surface of the semiconductor is inverted from p-type into n-type. Remember that as said above, if the Fermi level lies above the Intrinsic level, the semiconductor is of n-type, therefore at Inversion, when the Intrinsic level reaches and crosses the Fermi level (which lies closer to the valence band), the semiconductor type changes at the surface as dictated by the relative positions of the Fermi and Intrinsic energy levels.
Structure and channel formation[edit]
Channel formation in nMOS MOSFET shown as band diagram: Top panels: An applied gate voltage bends bands, depleting holes from surface (left). The charge inducing the bending is balanced by a layer of negative acceptor-ion charge (right). Bottom panel: A larger applied voltage further depletes holes but conduction band lowers enough in energy to populate a conducting channel
CâV profile for a bulk MOSFET with different oxide thickness. The leftmost part of the curve corresponds to accumulation. The valley in the middle corresponds to depletion. The curve on the right corresponds to inversion
A MOSFET is based on the modulation of charge concentration by a MOS capacitance between a body electrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer. If dielectrics other than an oxide are employed, the device may be referred to as a metal-insulator-semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping.
If the MOSFET is an n-channel or nMOS FET, then the source and drain are n+ regions and the body is a p region. If the MOSFET is a p-channel or pMOS FET, then the source and drain are p+ regions and the body is a n region. The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.
The occupancy of the energy bands in a semiconductor is set by the position of the Fermi level relative to the semiconductor energy-band edges.
With sufficient gate voltage, the valence band edge is driven far from the Fermi level, and holes from the body are driven away from the gate.
At larger gate bias still, near the semiconductor surface the conduction band edge is brought close to the Fermi level, populating the surface with electrons in an inversion layer or n-channel at the interface between the p region and the oxide. This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between the two electrodes. Increasing the voltage on the gate leads to a higher electron density in the inversion layer and therefore increases the current flow between the source and drain. For gate voltages below the threshold value, the channel is lightly populated, and only a very small subthreshold leakage current can flow between the source and the drain.
When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages. When a voltage less negative than the threshold value (a negative voltage for the p-channel) is applied between gate and source, the channel disappears and only a very small subthreshold current can flow between the source and the drain. The device may comprise a silicon on insulator device in which a buried oxide is formed below a thin semiconductor layer. If the channel region between the gate dielectric and the buried oxide region is very thin, the channel is referred to as an ultrathin channel region with the source and drain regions formed on either side in or above the thin semiconductor layer. Other semiconductor materials may be employed. When the source and drain regions are formed above the channel in whole or in part, they are referred to as raised source/drain regions.
Modes of operation[edit]
Source tied to the body to ensure no body bias:
top left: Subthreshold, top right: Ohmic mode, bottom left: Active mode at onset of pinch-off, bottom right: Active mode well into pinch-off â channel length modulation evident
Example application of an n-channel MOSFET. When the switch is pushed, the LED lights up.[77]
The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used.[78] Modern MOSFET characteristics are more complex than the algebraic model presented here.[79]
For an enhancement-mode, n-channel MOSFET, the three operational modes are:
When VGS < Vth:
where VGS{displaystyle V_{text{GS}}} is gate-to-source bias and Vth{displaystyle V_{text{th}}} is the threshold voltage of the device.
According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. A more accurate model considers the effect of thermal energy on the FermiâDirac distribution of electron energies which allow some of the more energetic electrons at the source to enter the channel and flow to the drain. This results in a subthreshold current that is an exponential function of gate-source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.
In weak inversion where the source is tied to bulk, the current varies exponentially with VGS{displaystyle V_{text{GS}}} as given approximately by:[80][81]
where ID0{displaystyle I_{text{D0}}} = current at VGS=Vth{displaystyle V_{text{GS}}=V_{text{th}}}, the thermal voltage VT=kT/q{displaystyle V_{text{T}}=kT/q} and the slope factor n is given by:
with Cdep{displaystyle C_{text{dep}}} = capacitance of the depletion layer and Cox{displaystyle C_{text{ox}}} = capacitance of the oxide layer. This equation is generally used, but is only an adequate approximation for the source tied to the bulk. For the source not tied to the bulk, the subthreshold equation for drain current in saturation is[82][83]
where the κ{displaystyle kappa } is the channel divider that is given by:
with CD{displaystyle C_{text{D}}} = capacitance of the depletion layer and Cox{displaystyle C_{text{ox}}} = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once VDSâ«VT{displaystyle V_{text{DS}}gg V_{text{T}}}, but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on). Frequently, threshold voltage Vth for this mode is defined as the gate voltage at which a selected value of current ID0 occurs, for example, ID0 = 1μA, which may not be the same Vth-value used in the equations for the following modes.
Some micropower analog circuits are designed to take advantage of subthreshold conduction.[84][85][86] By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio, namely: gm/ID=1/(nVT){displaystyle g_{m}/I_{text{D}}=1/left(nV_{text{T}}right)}, almost that of a bipolar transistor.[87]
The subthreshold IâV curve depends exponentially upon threshold voltage, introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering. The resulting sensitivity to fabricational variations complicates optimization for leakage and performance.[88][89]
MOSFET drain current vs. drain-to-source voltage for several values of VGSâVth{displaystyle V_{text{GS}}-V_{text{th}}}; the boundary between linear (Ohmic) and saturation (active) modes is indicated by the upward curving parabola
Cross section of a MOSFET operating in the linear (Ohmic) region; strong inversion region present even near drain
Cross section of a MOSFET operating in the saturation (active) region; channel exhibits channel pinching near drain
When VGS > Vth and VDS < VGS â Vth:
The transistor is turned on, and a channel has been created which allows current between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:
where μn{displaystyle mu _{n}} is the charge-carrier effective mobility, W{displaystyle W} is the gate width, L{displaystyle L} is the gate length and Cox{displaystyle C_{text{ox}}} is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.
When VGS > Vth and VDS ⥠(VGS â Vth):
![]()
The switch is turned on, and a channel has been created, which allows current between the drain and source. Since the drain voltage is higher than the source voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. Although the channel does not extend the full length of the device, the electric field between the drain and the channel is very high, and conduction continues. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate-source voltage, and modeled approximately as:
The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is:
where the combination Vov = VGS â Vth is called the overdrive voltage,[94] and where VDSsat = VGS â Vth accounts for a small discontinuity in ID{displaystyle I_{text{D}}} which would otherwise appear at the transition between the triode and saturation regions.
Another key design parameter is the MOSFET output resistance rout given by:
rout is the inverse of gDS where gDS=âIDSâVDS{displaystyle g_{text{DS}}={frac {partial I_{text{DS}}}{partial V_{text{DS}}}}}. ID is the expression in saturation region.
If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.
As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in VGS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In the ballistic regime, the carriers travel at an injection velocity that may exceed the saturation velocity and approaches the Fermi velocity at high inversion charge density. In addition, drain-induced barrier lowering increases off-state (cutoff) current and requires an increase in threshold voltage to compensate, which in turn reduces the saturation current.
Body effect[edit]
Band diagram showing body effect. VSB splits Fermi levels Fn for electrons and Fp for holes, requiring larger VGB to populate the conduction band in an nMOS MOSFET
The occupancy of the energy bands in a semiconductor is set by the position of the Fermi level relative to the semiconductor energy-band edges. Application of a source-to-substrate reverse bias of the source-body pn-junction introduces a split between the Fermi levels for electrons and holes, moving the Fermi level for the channel further from the band edge, lowering the occupancy of the channel. The effect is to increase the gate voltage necessary to establish the channel, as seen in the figure. This change in channel strength by application of reverse bias is called the 'body effect'.
Simply put, using an nMOS example, the gate-to-body bias VGB positions the conduction-band energy levels, while the source-to-body bias VSB positions the electron Fermi level near the interface, deciding occupancy of these levels near the interface, and hence the strength of the inversion layer or channel.
The body effect upon the channel can be described using a modification of the threshold voltage, approximated by the following equation:
where VTB is the threshold voltage with substrate bias present, and VT0 is the zero-VSB value of threshold voltage, γ{displaystyle gamma } is the body effect parameter, and 2ÏB is the approximate potential drop between surface and bulk across the depletion layer when VSB = 0 and gate bias is sufficient to ensure that a channel is present.[95] As this equation shows, a reverse bias VSB > 0 causes an increase in threshold voltage VTB and therefore demands a larger gate voltage before the channel populates.
The body can be operated as a second gate, and is sometimes referred to as the 'back gate'; the body effect is sometimes called the 'back-gate effect'.[96]
Circuit symbols[edit]
A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel. Sometimes three line segments are used for enhancement mode and a solid line for depletion mode (see depletion and enhancement modes). Another line is drawn parallel to the channel for the gate.
The bulk or body connection, if shown, is shown connected to the back of the channel with an arrow indicating pMOS or nMOS. Arrows always point from P to N, so an NMOS (N-channel in P-well or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is connected to the source (as is generally the case with discrete devices) it is sometimes angled to meet up with the source leaving the transistor. If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the source may be used in the same way as for bipolar transistors (out for nMOS, in for pMOS).
Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols. The orientation of the symbols, (most significantly the position of source relative to drain) is such that more positive voltages appear higher on the page than less positive voltages, implying current flowing 'down' the page:[97][98][99]
In schematics where G, S, D are not labeled, the detailed features of the symbol indicate which terminal is source and which is drain. For enhancement-mode and depletion-mode MOSFET symbols (in columns two and five), the source terminal is the one connected to the triangle. Additionally, in this diagram, the gate is shown as an 'L' shape, whose input leg is closer to S than D, also indicating which is which. However, these symbols are often drawn with a 'T' shaped gate (as elsewhere on this page), so it is the triangle which must be relied upon to indicate the source terminal.
For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source (i.e., the black triangles in the diagrams in columns 2 and 5). This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors.
Types of MOSFET[edit]PMOS and NMOS logic[edit]
P-type MOS (PMOS) logic uses p-channel MOSFETs to implement logic gates and other digital circuits. N-type MOS (NMOS) logic uses n-channel MOSFETs to implement logic gates and other digital circuits.
For devices of equal current driving capability, n-channel MOSFETs can be made smaller than p-channel MOSFETs, due to p-channel charge carriers (holes) having lower mobility than do n-channel charge carriers (electrons), and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, neglecting leakage current, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place.
Mohamed Atalla and Dawon Kahng, after they invented the MOSFET, fabricated both pMOS and nMOS devices with a 20 µm process in 1960.[16] Their original MOSFET devices had a gate length of 20µm and a gate oxide thickness of 100 nm.[100] However, the nMOS devices were impractical, and only the pMOS type were practical working devices.[16] A more practical NMOS process was developed several years later. NMOS was initially faster than CMOS, thus NMOS was more widely used for computers in the 1970s.[101] With advances in technology, CMOS logic displaced NMOS logic in the mid-1980s to become the preferred process for digital chips.
Complementary MOS (CMOS)[edit]
The MOSFET is used in digital complementary metalâoxideâsemiconductor (CMOS) logic,[102] which uses p- and n-channel MOSFETs as building blocks. Overheating is a major concern in integrated circuits since ever more transistors are packed into ever smaller chips. CMOS logic reduces power consumption because no current flows (ideally), and thus no power is consumed, except when the inputs to logic gates are being switched. CMOS accomplishes this current reduction by complementing every nMOSFET with a pMOSFET and connecting both gates and both drains together. A high voltage on the gates will cause the nMOSFET to conduct and the pMOSFET not to conduct and a low voltage on the gates causes the reverse. During the switching time as the voltage goes from one state to another, both MOSFETs will conduct briefly. This arrangement greatly reduces power consumption and heat generation.
CMOS was developed by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963.[103] CMOS had lower power consumption, but was initially slower than NMOS, which was more widely used for computers in the 1970s. In 1978, Hitachi introduced the twin-well CMOS process, which allowed CMOS to match the performance of NMOS with less power consumption. The twin-well CMOS process eventually overtook NMOS as the most common semiconductor manufacturing process for computers in the 1980s.[101]
Floating-gate MOSFET (FGMOS)[edit]
The floating-gate MOSFET (FGMOS) is a type of MOSFET where the gate is electrically isolated, creating a floating node in DC, and a number of secondary gates or inputs are deposited above the floating gate (FG) and are electrically isolated from it. The first report of a floating-gate MOSFET (FGMOS) was made by Dawon Kahng (co-inventor of the original MOSFET) and Simon Sze in 1967.[104]
The FGMOS is commonly used as a floating-gate memory cell, the digital storage element in EPROM, EEPROM and flash memories. Other uses of the FGMOS include a neuronal computational element in neural networks, analog storage element, digital potentiometers and single-transistor DACs.
Depletion-mode[edit]
There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. To control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device. In essence, the depletion-mode device is equivalent to a normally closed (on) switch, while the enhancement-mode device is equivalent to a normally open (off) switch.[105]
Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in TV sets.
Depletion-mode MOSFET families include BF960 by Siemens and Telefunken, and the BF980 in the 1980s by Philips (later to become NXP Semiconductors), whose derivatives are still used in AGC and RF mixer front-ends.
Metal-insulator-semiconductor field-effect transistor (MISFET)[edit]
Metal-insulator-semiconductor field-effect-transistor,[106][107][108] or MISFET, is a more general term than MOSFET and a synonym to insulated-gate field-effect transistor (IGFET). All MOSFETs are MISFETs, but not all MISFETs are MOSFETs.
The gate dielectric insulator in a MISFET is silicon dioxide in a MOSFET, but other materials can also be employed. The gate dielectric lies directly below the gate electrode and above the channel of the MISFET. The term metal is historically used for the gate material, even though now it is usually highly dopedpolysilicon or some other non-metal.
Insulator types may be:
Thin-film transistor (TFT)[edit]
The thin-film transistor (TFT) is a type of MOSFET distinct from the standard bulk MOSFET.[37] The first TFT was invented by Paul K. Weimer at RCA in 1962, building on the earlier work of Atalla and Kahng on MOSFETs.[110]
The idea of a TFT-based liquid-crystal display (LCD) was conceived by Bernard Lechner of RCA Laboratories in 1968.[111] Lechner, F. J. Marlowe, E. O. Nester and J. Tults demonstrated the concept in 1968 with an 18x2 matrix dynamic scattering LCD that used standard discrete MOSFETs, as TFT performance was not adequate at the time.[112]
Power MOSFET[edit]
Two power MOSFETs in D2PAKsurface-mount packages. Operating as switches, each of these components can sustain a blocking voltage of 120V in the off state, and can conduct a conÂtiÂnuous current of 30 A in the on state, dissipating up to about 100 W and controlling a load of over 2000 W. A matchstick is pictured for scale.
Cross section of a power MOSFET, with square cells. A typical transistor is constituted of several thousand cells
Power MOSFETs have a different structure.[113] As with most power devices, the structure is vertical and not planar. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current. The voltage rating of the transistor is a function of the doping and thickness of the N-epitaxial layer (see cross section), while the current rating is a function of the channel width (the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the 'silicon estate'. With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.[114]
Power MOSFETs with lateral structure are mainly used in high-end audio amplifiers and high-power PA systems. Their advantage is a better behaviour in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications.[115]
The power MOSFET, which is commonly used in power electronics, was developed in the early 1970s.[116] The power MOSFET enables low gate drive power, fast switching speed, and advanced paralleling capability.[117]
Double-diffused metalâoxideâsemiconductor (DMOS)[edit]
There are VDMOS (vertical double-diffused metal oxide semiconductor) and LDMOS (lateral double-diffused metal oxide semiconductor). Most power MOSFETs are made using this technology.
Insulated-gate bipolar transistor (IGBT)[edit]
The insulated-gate bipolar transistor (IGBT) is a power transistor with characteristics of both a MOSFET and bipolar junction transistor (BJT).[118]
Multi-gate field-effect transistor (MuGFET)[edit]
A FinFET (fin field-effect transistor), a type of multi-gate MOSFET.
The dual-gate MOSFET (DGMOS) has a tetrode configuration, where both gates control the current in the device. It is commonly used for small-signal devices in radio frequency applications where biasing the drain-side gate at constant potential reduces the gain loss caused by Miller effect, replacing two separate transistors in cascode configuration. Other common uses in RF circuits include gain control and mixing (frequency conversion). The tetrode description, though accurate, does not replicate the vacuum-tube tetrode. Vacuum-tube tetrodes, using a screen grid, exhibit much lower grid-plate capacitance and much higher output impedance and voltage gains than triode vacuum tubes. These improvements are commonly an order of magnitude (10 times) or considerably more. Tetrode transistors (whether bipolar junction or field-effect) do not exhibit improvements of such a great degree.
The FinFET is a double-gate silicon-on-insulator device, one of a number of geometries being introduced to mitigate the effects of short channels and reduce drain-induced barrier lowering. The fin refers to the narrow channel between source and drain. A thin insulating oxide layer on either side of the fin separates it from the gate. SOI FinFETs with a thick oxide on top of the fin are called double-gate and those with a thin oxide on top as well as on the sides are called triple-gate FinFETs.[119][120]
A double-gate MOSFET transistor was first demonstrated in 1984 by Electrotechnical Laboratory researchers Toshihiro Sekigawa and Yutaka Hayashi.[121][122] A GAAFET (gate-all-around MOSFET), a type of multi-gate non-planar 3D transistor, was first demonstrated in 1988 by a Toshiba research team including Fujio Masuoka, H. Takato and K. Sunouchi.[123][124] The FinFET (fin field-effect transistor), a type of 3D non-planar double-gate MOSFET, originated from the research of Digh Hisamoto and his team at Hitachi Central Research Laboratory in 1989.[125][126] The development of nanowire multi-gate MOSFETs have since become fundamental to nanoelectronics.[127]
Radiation-hardened-by-design (RHBD)[edit]
Semiconductor sub-micrometer and nanometer electronic circuits are the primary concern for operating within the normal tolerance in harsh radiation environments like outer space. One of the design approaches for making a radiation-hardened-by-design (RHBD) device is enclosed-layout-transistor (ELT). Normally, the gate of the MOSFET surrounds the drain, which is placed in the center of the ELT. The source of the MOSFET surrounds the gate. Another RHBD MOSFET is called H-Gate. Both of these transistors have very low leakage current with respect to radiation. However, they are large in size and take more space on silicon than a standard MOSFET. In older STI (shallow trench isolation) designs, radiation strikes near the silicon oxide region cause the channel inversion at the corners of the standard MOSFET due to accumulation of radiation induced trapped charges. If the charges are large enough, the accumulated charges affect STI surface edges along the channel near the channel interface (gate) of the standard MOSFET. Thus the device channel inversion occurs along the channel edges and the device creates an off-state leakage path, causing the device to turn on. So the reliability of circuits degrades severely. The ELT offers many advantages. These advantages include improvement of reliability by reducing unwanted surface inversion at the gate edges that occurs in the standard MOSFET. Since the gate edges are enclosed in ELT, there is no gate oxide edge (STI at gate interface), and thus the transistor off-state leakage is reduced considerably. Low-power microelectronic circuits including computers, communication devices and monitoring systems in the space shuttle and satellites are very different to what is used on earth. They require radiation (high-speed atomic particles like proton and neutron, solar flare magnetic energy dissipation in Earth's space, energetic cosmic rays like X-ray, gamma ray etc.) tolerant circuits. These special electronics are designed by applying different techniques using RHBD MOSFETs to ensure safer journeys and space-walks for astronauts.
Applications[edit]
The MOSFET is by far the most widely used transistor in both digital circuits and analog circuits, and it is the backbone of modern electronics. A common use in analog circuits is the construction of differential amplifiers, used as input stages in op-amps, video amplifiers, high-speed comparators, and many other analog circuits.[128]
Discrete MOSFET devices are widely used in applications such as switch mode power supplies, variable-frequency drives and other power electronics applications where each device may be switching thousands of watts. Radio-frequency amplifiers up to the UHF spectrum use MOSFET transistors as analog signal and power amplifiers. Radio systems also use MOSFETs as oscillators, or mixers to convert frequencies. MOSFET devices are also applied in audio-frequency power amplifiers for public address systems, sound reinforcement and home and automobile sound systems.[citation needed]
MOS integrated circuits[edit]
Following the development of clean rooms to reduce contamination to levels never before thought necessary, and of photolithography[129] and the planar process to allow circuits to be made in very few steps, the SiâSiO2 system possessed the technical attractions of low cost of production (on a per circuit basis) and ease of integration. Largely because of these two factors, the MOSFET has become the most widely used type of transistor in integrated circuits (ICs), and it is the most critical device component in modern ICs.[68]
Mohamed Atalla first proposed the concept of the MOS integrated circuit (MOS IC) in 1960, followed by Dawon Kahng in 1961, both noting that the MOS transistor's ease of fabrication made it useful for integrated circuits.[14][20] The earliest experimental MOS IC to be demonstrated was a 16-transistor chip built by Fred Heiman and Steven Hofstein at RCA in 1962.[7]General Microelectronics later introduced the first commercial MOS integrated circuits in 1964, consisting of 120 p-channel transistors.[130] It was a 20-bit shift register, developed by Robert Norman[7] and Frank Wanlass.[131] In 1967, Bell Labs researchers Robert Kerwin, Donald Klein and John Sarace developed the self-aligned gate (silicon-gate) MOS transistor, which Fairchild Semiconductor researchers Federico Faggin and Tom Klein used to develop the first silicon-gate MOS IC.[132]
Large-scale integration (LSI)[edit]
With its high scalability,[3] and much lower power consumption and higher density than bipolar junction transistors,[6] the MOSFET made it possible to build high-density ICs.[1] MOS technology enabled the integration of more than 10,000 transistors in a single LSI (large-scale integration) chip by the early 1970s,[133] and later very large-scale integration (VLSI).[5][53]
One of the earliest influential consumer electronic products enabled by MOS LSI circuits was the electronic pocket calculator.[133] In 1965, the Victor 3900 desktop calculator was the first MOS LSI calculator, with 29 MOS LSI chips.[134] In 1967 the Texas Instruments Cal-Tech was the first prototype electronic handheld calculator, with three MOS LSI chips, and it was later released as the Canon Pocketronic in 1970.[135] The Sharp QT-8D desktop calculator was the first mass-produced LSI MOS calculator in 1969,[134] and the Sharp EL-8 which used four MOS LSI chips was the first commercial electronic handheld calculator in 1970.[135] The first true electronic pocket calculator was the Busicom LE-120A HANDY LE, which used a single MOS LSI calculator-on-a-chip from Mostek, and was released in 1971.[135]
By 1972, MOS LSI circuits were commercialized for numerous applications, including automobiles, trucks, home appliances, business machines, electronic musical instruments, computer peripherals, cash registers, calculators, data transmission and telecommunication equipment.[136]
Microprocessors[edit]
The MOSFET is the basis of every microprocessor,[35] and was responsible for the invention of the microprocessor.[137] The earliest microprocessors were all MOS chips, built with MOS LSI circuits. The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearchMP944 in 1970, were developed with multiple MOS LSI chips. The first commercial single-chip microprocessor, the Intel 4004, was developed by Federico Faggin, using his silicon-gate MOS IC technology, with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima.[138] With the arrival of CMOS microprocessors in 1975, the term 'MOS microprocessors' began to refer to chips fabricated entirely from PMOS logic or fabricated entirely from NMOS logic, contrasted with 'CMOS microprocessors' and 'bipolar bit-slice processors'.[139]
CMOS circuits[edit]Digital[edit]
The growth of digital technologies like the microprocessor has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor.[140] A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs. Bipolar transistor-based logic (such as TTL) does not have such a high fanout capacity. This isolation also makes it easier for the designers to ignore to some extent loading effects between logic stages independently. That extent is defined by the operating frequency: as frequencies increase, the input impedance of the MOSFETs decreases.
Analog[edit]
The MOSFET's advantages in digital circuits do not translate into supremacy in all analog circuits. The two types of circuit draw upon different features of transistor behavior. Digital circuits switch, spending most of their time either fully on or fully off. The transition from one to the other is only of concern with regards to speed and charge required. Analog circuits depend on operation in the transition region where small changes to Vgs can modulate the output (drain) current. The JFET and bipolar junction transistor (BJT) are preferred for accurate matching (of adjacent devices in integrated circuits), higher transconductance and certain temperature characteristics which simplify keeping performance predictable as circuit temperature varies.
Nevertheless, MOSFETs are widely used in many types of analog circuits because of their own advantages (zero gate current, high and adjustable output impedance and improved robustness vs. BJTs which can be permanently degraded by even lightly breaking down the emitter-base).[vague] The characteristics and performance of many analog circuits can be scaled up or down by changing the sizes (length and width) of the MOSFETs used. By comparison, in bipolar transistors the size of the device does not significantly affect its performance.[citation needed] MOSFETs' ideal characteristics regarding gate current (zero) and drain-source offset voltage (zero) also make them nearly ideal switch elements, and also make switched capacitor analog circuits practical. In their linear region, MOSFETs can be used as precision resistors, which can have a much higher controlled resistance than BJTs. In high power circuits, MOSFETs sometimes have the advantage of not suffering from thermal runaway as BJTs do.[dubious] Also, MOSFETs can be configured to perform as capacitors and gyrator circuits which allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices on a chip (except for diodes, which can be made smaller than a MOSFET anyway) to be built entirely out of MOSFETs. This means that complete analog circuits can be made on a silicon chip in a much smaller space and with simpler fabrication techniques. MOSFETS are ideally suited to switch inductive loads because of tolerance to inductive kickback.
Some ICs combine analog and digital MOSFET circuitry on a single mixed-signal integrated circuit, making the needed board space even smaller. This creates a need to isolate the analog circuits from the digital circuits on a chip level, leading to the use of isolation rings and silicon on insulator (SOI). Since MOSFETs require more space to handle a given amount of power than a BJT, fabrication processes can incorporate BJTs and MOSFETs into a single device. Mixed-transistor devices are called bi-FETs (bipolar FETs) if they contain just one BJT-FET and BiCMOS (bipolar-CMOS) if they contain complementary BJT-FETs. Such devices have the advantages of both insulated gates and higher current density.
In the late 1980s, Asad Abidi pioneered RF CMOS technology, which uses MOS VLSI circuits, while working at UCLA. This changed the way in which RF circuits were designed, away from discrete bipolar transistors and towards CMOS integrated circuits. As of 2008, the radio transceivers in all wireless networking devices and modern mobile phones are mass-produced as RF CMOS devices.[141]
Analog switches[edit]
MOSFET analog switches use the MOSFET to pass analog signals when on, and as a high impedance when off. Signals flow in both directions across a MOSFET switch. In this application, the drain and source of a MOSFET exchange places depending on the relative voltages of the source/drain electrodes. The source is the more negative side for an N-MOS or the more positive side for a P-MOS. All of these switches are limited on what signals they can pass or stop by their gate-source, gate-drain and sourceâdrain voltages; exceeding the voltage, current, or power limits will potentially damage the switch.
Single-type[edit]
This analog switch uses a four-terminal simple MOSFET of either P or N type.
In the case of an n-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS switch passes all voltages less than Vgate â Vtn. When the switch is conducting, it typically operates in the linear (or ohmic) mode of operation, since the source and drain voltages will typically be nearly equal.
In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than Vgate â Vtp (threshold voltage Vtp is negative in the case of enhancement-mode P-MOS).
Dual-type (CMOS)[edit]
This 'complementary' or CMOS type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the single-type switch. The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential (VDD) and the body of the N-MOS is connected to the low potential (gnd). To turn the switch on, the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential. For voltages between VDD â Vtn and gnd â Vtp, both FETs conduct the signal; for voltages less than gnd â Vtp, the N-MOS conducts alone; and for voltages greater than VDD â Vtn, the P-MOS conducts alone.
The voltage limits for this switch are the gate-source, gate-drain and source-drain voltage limits for both FETs. Also, the P-MOS is typically two to three times wider than the N-MOS, so the switch will be balanced for speed in the two directions.
Tri-state circuitry sometimes incorporates a CMOS MOSFET switch on its output to provide for a low-ohmic, full-range output when on, and a high-ohmic, mid-level signal when off.
Power electronics[edit]Power MOSFETs[edit]
The power MOSFET is the most widely used power device in the world.[117] Advantages over bipolar junction transistors in power electronics include MOSFETs not requiring a continuous flow of drive current to remain in the ON state, offering higher switching speeds, lower switching power losses, lower on-resistances, and reduced susceptibility to thermal runaway.[142] The power MOSFET had an impact on power supplies, enabling higher operating frequencies, size and weight reduction, and increased volume production.[143]
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MOSFETs are commonly used in switched-mode power supplies (SMPS)[142] and DC-to-DC converters.[144] Other common applications include portable information appliances, power integrated circuits, cell phones, notebook computers, Internetcommunications infrastructure,[65]computer power supplies, television receiver circuits, home entertainment equipment, industrial technology, instrumentation and test gear applications,[145]cellularwireless networks, consumer electronics, digital television, digital cameras, mobile computers,[146]3D printers, LED lighting, battery-powered applications (such as for notebook and desktop computers), audio amplifiers, power amplifiers, telecommunications, television systems, radar systems, and military communications, among other uses.[147]
Switching power supplies are the most common applications for power MOSFETs, which are also widely used for pulsedDC motor drives and Class Daudio amplifiers.[148] MOSFETs are also the most widely used RF power amplifiers.[38] MOSFET-based RF power amplifiers enabled the transition of mobile networks from analog to digital in the 1990s, leading to the wide proliferation of wireless mobile networks, which revolutionised telecommunication systems.[38] The LDMOS in particular is the most widely used power amplifier in mobile networks, such as 2G, 3G,[38] and 4G.[39]Over 50billion discrete power MOSFETs are shipped annually as of 2018. They are widely used for automotive, industrial and communications systems in particular.[149] Power MOSFETs are commonly used in automotive electronics, particularly as switching devices in electronic control units.[150] The insulated-gate bipolar transistor (IGBT), a hybrid MOS-bipolar transistor, is also used for applications such as household appliances, cars, solar panels, fluorescent lighting, medical equipment, and bullet trains.[151]
Space research[edit]
MOSFET devices were adopted by NASA for space research in 1964, for its Interplanetary Monitoring Platform (IMP) program.[152] The use of MOSFETs was a major step forward in spacecraft electronics design. Data gathered by IMP spacecraft and satellites were used to support the Apollo program, enabling the first manned Moon landing with the Apollo 11 mission in 1969.[153]
The CassiniâHuygens mission to Saturn in 1997 had power distribution accomplished 192 solid-statepower switches (SSPS), which also functioned as circuit breakers in the event of an overload condition. The switches were developed from a combination of two semiconductor devices with switching capabilities: the MOSFET and the ASIC (application-specific integrated circuit). This combination resulted in advanced power switches that had better performance characteristics than traditional mechanical switches.[154]
MOS memory[edit]
The first modern memory cells for computer memory was introduced in 1965, when John Schmidt at Fairchild Semiconductor designed the first MOS semiconductor memory, a 64-bit MOS SRAM (static random-access memory).[155] SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each bit of data.[156]
MOS technology is the basis for DRAM (dynamic random-access memory). In 1966, Dr. Robert H. Dennard at the IBMThomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of building capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell.[156] In 1967, Dennard filed a patent under IBM for a single-transistor DRAM (dynamic random-access memory) memory cell, based on MOS technology.[157] MOS memory enabled higher performance, was cheaper, and consumed less power, than magnetic-core memory, leading to MOS memory overtaking magnetic core memory as the dominant computer memory technology by the early 1970s.[158]
Frank Wanlass, while studying MOSFET structures in 1963, noted the movement of charge through oxide onto a gate. While he did not pursue it, this idea would later become the basis for EPROM (erasable programmable read-only memory) technology.[159] In 1967, Dawon Kahng and Simon Sze proposed that floating-gate memory cells, consisting of floating-gate MOSFETs (FGMOS), could be used to produce reprogrammable ROM (read-only memory).[160] Floating-gate memory cells later became the basis for non-volatile memory (NVM) technologies including EPROM, EEPROM (electrically erasable programmable ROM) and flash memory.[161]
Quantum physics[edit]
The MOSFET enables physicists to study electron behavior in a two-dimensional gas. In a MOSFET, conduction electrons travel in a thin surface layer, and a 'gate' voltage controls the number of charge carriers in this layer. This allows researchers to explore quantum effects by operating high-purity MOSFETs at liquid helium temperatures.[71]
In 1978, the Gakushuin University researchers Jun-ichi Wakabayashi and Shinji Kawaji observed the Hall effect in experiments carried out on the inversion layer of MOSFETs.[162] In 1980, Klaus von Klitzing, working at the high magnetic field laboratory in Grenoble with silicon-based MOSFET samples developed by Michael Pepper and Gerhard Dorda, made the unexpected discovery of the quantum Hall effect.[71][163]
Image sensors[edit]
MOS capacitors are the basis for image sensors, including the charge-coupled device (CCD) and the CMOS active-pixel sensor, used in digital imaging and digital cameras.[72]Willard Boyle and George E. Smith developed the CCD in 1969. While researching the MOS process, they realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tiny MOS capacitor. As it was fairly straighforward to fabricate a series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next.[72] The CCD is a semiconductor circuit that was later used in the first digital video cameras for television broadcasting.[164]
The MOS active pixel sensor (APS) was developed by Tsutomu Nakamura at Olympus in 1985.[165] The CMOS active pixel sensor (CMOS sensor) was later developed by Eric Fossum and his team in the early 1990s.[166]
Construction[edit]Gate material[edit]
The primary criterion for the gate material is that it is a good conductor. Highly doped polycrystalline silicon is an acceptable but certainly not ideal conductor, and also suffers from some more technical deficiencies in its role as the standard gate material. Nevertheless, there are several reasons favoring use of polysilicon:
While polysilicon gates have been the de facto standard for the last twenty years, they do have some disadvantages which have led to their likely future replacement by metal gates. These disadvantages include:
Present high performance CPUs use metal gate technology, together with high-κ dielectrics, a combination known as high-κ, metal gate (HKMG). The disadvantages of metal gates are overcome by a few techniques:[167]
Insulator[edit]
As devices are made smaller, insulating layers are made thinner, often through steps of thermal oxidation or localised oxidation of silicon (LOCOS). For nano-scaled devices, at some point tunneling of carriers through the insulator from the channel to the gate electrode takes place. To reduce the resulting leakage current, the insulator can be made thinner by choosing a material with a higher dielectric constant. To see how thickness and dielectric constant are related, note that Gauss's law connects field to charge as:
with Q = charge density, κ = dielectric constant, ε0 = permittivity of empty space and E = electric field. From this law it appears the same charge can be maintained in the channel at a lower field provided κ is increased. The voltage on the gate is given by:
with VG = gate voltage, Vch = voltage at channel side of insulator, and tins = insulator thickness. This equation shows the gate voltage will not increase when the insulator thickness increases, provided κ increases to keep tins / κ = constant (see the article on high-κ dielectrics for more detail, and the section in this article on gate-oxide leakage).
The insulator in a MOSFET is a dielectric which can in any event be silicon oxide, formed by LOCOS but many other dielectric materials are employed. The generic term for the dielectric is gate dielectric since the dielectric lies directly below the gate electrode and above the channel of the MOSFET.
Junction design[edit]
MOSFET showing shallow junction extensions, raised source and drain and halo implant. Raised source and drain separated from gate by oxide spacers
Download razor1911 crack for gta 4 eflc. The source-to-body and drain-to-body junctions are the object of much attention because of three major factors: their design affects the current-voltage (I-V) characteristics of the device, lowering output resistance, and also the speed of the device through the loading effect of the junction capacitances, and finally, the component of stand-by power dissipation due to junction leakage.
The drain induced barrier lowering of the threshold voltage and channel length modulation effects upon I-V curves are reduced by using shallow junction extensions. In addition, halo doping can be used, that is, the addition of very thin heavily doped regions of the same doping type as the body tight against the junction walls to limit the extent of depletion regions.[168]
The capacitive effects are limited by using raised source and drain geometries that make most of the contact area border thick dielectric instead of silicon.[169]
These various features of junction design are shown (with artistic license) in the figure.
Scaling[edit]
Trend of Intel CPU transistor gate length
MOSFET version of gain-boosted current mirror; M1 and M2 are in active mode, while M3 and M4 are in Ohmic mode, and act like resistors. The operational amplifier provides feedback that maintains a high output resistance.
Over the past decades, the MOSFET (as used for digital logic) has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but modern integrated circuits are incorporating MOSFETs with channel lengths of tens of nanometers. Robert Dennard's work on scaling theory was pivotal in recognising that this ongoing reduction was possible. The semiconductor industry maintains a 'roadmap', the ITRS,[170] which sets the pace for MOSFET development. Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process, the need to use very low voltages, and with poorer electrical performance necessitating circuit redesign and innovation (small MOSFETs exhibit higher leakage currents and lower output resistance). As of 2019, the smallest MOSFETs in production are 5 nmFinFETsemiconductor nodes, manufactured by Samsung Electronics and TSMC.[171][172]
Smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area. This results in a chip with the same functionality in a smaller area, or chips with more functionality in the same area. Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer. Hence, smaller ICs allow more chips per wafer, reducing the price per chip. In fact, over the past 30 years the number of transistors per chip has been doubled every 2â3 years once a new technology node is introduced. For example, the number of MOSFETs in a microprocessor fabricated in a 45 nm technology can well be twice as many as in a 65 nm chip. This doubling of transistor density was first observed by Gordon Moore in 1965 and is commonly referred to as Moore's law.[173] It is also expected that smaller transistors switch faster. For example, one approach to size reduction is a scaling of the MOSFET that requires all device dimensions to reduce proportionally. The main device dimensions are the channel length, channel width, and oxide thickness. When they are scaled down by equal factors, the transistor channel resistance does not change, while gate capacitance is cut by that factor. Hence, the RC delay of the transistor scales with a similar factor. While this has been traditionally the case for the older technologies, for the state-of-the-art MOSFETs reduction of the transistor dimensions does not necessarily translate to higher chip speed because the delay due to interconnections is more significant.
Producing MOSFETs with channel lengths much smaller than a micrometre is a challenge, and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology. Though processes such as ALD have improved fabrication for small components, the small size of the MOSFET (less than a few tens of nanometers) has created operational problems:
Timeline[edit]PMOS and NMOS[edit]
CMOS (single-gate)[edit]
Multi-gate MOSFET (MuGFET)[edit]
Other types of MOSFET[edit]
See also[edit]References[edit]
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=MOSFET&oldid=919444632'
MOSFET is basically a transistor which uses field effect. MOSFET stands for Metal Oxide Field Effect Transistor, which has a gate. The gate voltage determines the conductivity of the device. Depending on this gate voltage we can change the conductivity and thus we can use it as a switch or as an amplifier like we use Transistor as a switch or as an amplifier.
Bipolar Junction Transistor or BJT has base, emitter, and collector, whereas a MOSFET has gate, drain and source connection. Other than the pin configuration, BJT needs current for operation and MOSFET needs voltage.
MOSFET provides very high input impedance and it is very easy to bias. So, for a linear small amplifier, MOSFET is an excellent choice. The linear amplification occurs when we bias the MOSFET in the saturation region which is a centrally fixed Q point.
In the below image, a basic N-channel MOSFETs internal construction is shown. The MOSFET has three connections Drain, Gate, and Source. There is no direct connection present between the gate and the channel. The gate electrode is electrically insulated and due to this reason, it is sometimes referred to as IGFET or Insulated Gate Field Effect Transistor.
Here is the image of widely popular MOSFET IRF530N.
Types of MOSFETs
Based on the operating modes, there are two different types of MOSFETs available. These two types further have two subtypes
Depletion type MOSFET
Depletion type of MOSFET is normally ON at zero Gate to Source voltage. If the MOSFET is N-Channel Depletion-type MOSFET then there will be some thresholds voltage, which is needed to make the device turn off. For example, an N-Channel Depletion MOSFET with a threshold voltage of -3V or -5V, the gate of the MOSFET needs to be pulled negative -3V or -5V to turn off the device. This threshold voltage will be Negative for the N channel, and positive in case of P channel. This type of MOSFET is generally used in logic circuits.
Enhancement type MOSFET
In Enhancement type of MOSFETs, the device remains OFF at zero Gate voltage. To turn on the MOSFET , we must provide a minimum Gate to Source voltage (Vgs Threshold voltage). But, the drain current is highly dependable on this gate-to-source voltage, if the Vgs is increased, the drain current also increases in the same manner. Enhancement type MOSFETs are ideal for constructing an Amplifier circuit. Also, similarly like depletion MOSFET, it also has the NMOS and PMOS subtypes.
Characteristics and Curves of MOSFET
By providing the stable voltage across drain to source, we can understand the I-V curve of a MOSFET. As stated above, the drain current is highly dependable on the Vgs, gate to source voltage. If we vary the Vgs the Drain current will also vary.
Let's see the I-V curve of a MOSFET.
In the above image, we can see the I-V slope of an N-Channel MOSFET, the drain current is 0 when the Vgs voltage is below the threshold voltage, during this time the MOSFET is in the cut-off mode. After that when gate-to-source voltage start increasing, the drain current also increases.
Let's see a practical example of IRF530 MOSFETâs I-V Curve,
The curve showing that when the Vgs is 4.5V, the maximum drain current of IRF530 is 1A at 25 degree C. But when we increase the Vgs to 5V, the Drain current is almost 2A, and finally at 6V Vgs, it can provide 10A of Drain Current.
DC Biasing of MOSFET and Common-Source Amplification
Well, now it is the time to use a MOSFET as a linear Amplifier. It is not a tough job if we determine how to bias the MOSFET and use it in a perfect operation region.
MOSFET work in three operation modes: Ohmic, Saturation and Pinch off point. The saturation region also called as Linear Region. Here we operate the MOSFET in saturation region, it provides perfect Q-point.
If we provide a small signal (time-varying) and apply the DC bias at gate or input, then under the right situation the MOSFET provides linear amplification.
In the above image, a small sinusoidal signal ( Vgs) is applied to the MOSFET gate, resulting in a fluctuation of drain current synchronous to the applied sinusoidal input. For the small signal Vgs, We can draw a straight line from the Q point which has a slope of gm = dId / dVgs.
The slope can be seen in the above image. This is the transconductance slope. It is an important parameter for the amplification factor. At this point the drain current amplitude is
Now, if we look at the schematic given above, the drain resistor Rd can control the drain current as well as the drain voltage using the equation
AC output signal will be ß¡Vds = -ß¡Id x Rd = -gm x ß¡Vgs x Rd
Now by the equations, the gain will be
So, the overall gain of the MOSFET Amplifier is highly dependable on the transconductance and the Drain resistor.
Basic Common Source Amplifier Construction with single MOSFET
To make a simple common source Amplifier using N channel single MOSFET, the important thing is to achieve DC biasing condition. To serve the purpose, a generic voltage divider is constructed using two simple resistors: R1 and R2. Two more resistors are also required as Drain resistor and Source resistor.
To determine the value we need step by step calculation.
A MOSFET is provided with high input impedance, thus in operating condition, there is no current flow present in the gate terminal.
Now, if we look into the device, we will find that there are three resistors associated with VDD (Without the biasing resistors). The three resistors are Rd, MOSFETâs internal resistance and Rs. So, if we apply Kirchoffâs Voltage law then the voltages across those three resistors are equal to the VDD.
Now as per the Ohms law, if we multiply the current with resistor we will get voltage as V = I x R. So, here the current is Drain current or ID. Thus, the voltage across Rd is V = ID x Rd, same applies for the Rs as the current is the same ID, so the Voltage across Rs is Vs = ID x Rs. For the MOSFET, the Voltage is VDS or Drain-to-source voltage.
Now as per the KVL,
We can further evaluate it as
Other two resistors values can be determined by the formula VG = VDD (R2 / R1 +R2)
If you do not have the value, you can get it from the formula VG = VGS + VS
Fortunately, maximum values can be available from the MOSFET datasheet. Based on the specification we can build the circuit.
Two coupling capacitors are used to compensating the cut-off frequencies and to block the DC coming from the input or getting to the final output. We can simply get the values by finding out the equivalent resistance of the DC bias divider and then selecting the desired cutoff frequency. The formula will be
For High power Amplifier design, we previously build a Power amplifier of 50 Watt using Two MOSFET as Push-pull configuration, follow the link for practical application.
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