The field-effect transistor is a transistor that uses an electric field to control the
shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor
material. FETs are unipolar transistors as they involve single-carrier-type operation.
The concept of the FET predates the bipolar junction transistor, though it was not physically
implemented until after BJTs due to the limitations of semiconductor materials and the relative
ease of manufacturing BJTs compared to FETs at the time.
History
The field-effect transistor was first patented by Julius Edgar Lilienfeld in 1926 and by
Oskar Heil in 1934, but practical semiconducting devices were developed only much later after
the transistor effect was observed and explained by the team of William Shockley at Bell Labs
in 1947, immediately after the 20-year patent period eventually expired. The MOSFET, which
largely superseded the JFET and had a more profound effect on electronic development,
was invented by Dawon Kahng and Martin Atalla in 1960.
Basic information
FETs can be majority-charge-carrier devices, in which the current is carried predominantly
by majority carriers, or minority-charge-carrier devices, in which the current is mainly due
to a flow of minority carriers. The device consists of an active channel through which
charge carriers, electrons or holes, flow from the source to the drain. Source and drain
terminal conductors are connected to the semiconductor through ohmic contacts. The conductivity of
the channel is a function of the potential applied across the gate and source terminals.
The FET's three terminals are: Source, through which the carriers enter the
channel. Conventionally, current entering the channel at S is designated by IS.
Drain, through which the carriers leave the channel. Conventionally, current entering
the channel at D is designated by ID. Drain-to-source voltage is VDS.
Gate, the terminal that modulates the channel conductivity. By applying voltage to G, one
can control ID. More about terminals
All FETs have source, drain, and gate terminals that correspond roughly to the emitter, collector,
and base of BJTs. Most FETs have a fourth terminal called the body, base, bulk, or substrate.
This fourth terminal serves to bias the transistor into operation; it is rare to make non-trivial
use of the body terminal in circuit designs, but its presence is important when setting
up the physical layout of an integrated circuit. The size of the gate, length L in the diagram,
is the distance between source and drain. The width is the extension of the transistor,
in the direction perpendicular to the cross section in the diagram. Typically the width
is much larger than the length of the gate. A gate length of 1 µm limits the upper frequency
to about 5 GHz, 0.2 µm to about 30 GHz. The names of the terminals refer to their
functions. The gate terminal may be thought of as controlling the opening and closing
of a physical gate. This gate permits electrons to flow through or blocks their passage by
creating or eliminating a channel between the source and drain. Electrons flow from
the source terminal towards the drain terminal is influenced by an applied voltage. The body
simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually
the body terminal is connected to the highest or lowest voltage within the circuit, depending
on the type of the FET. The body terminal and the source terminal are sometimes connected
together since the source is often connected to the highest or lowest voltage within the
circuit, although there are several uses of FETs which do not have such a configuration,
such as transmission gates and cascode circuits. FET operation
The FET controls the flow of electrons from the source to drain by affecting the size
and shape of a "conductive channel" created and influenced by voltage applied across the
gate and source terminals. This conductive channel is the "stream" through which electrons
flow from source to drain. n-channel
In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion
region to expand in width and encroach on the channel from the sides, narrowing the
channel. If the depletion region expands to completely close the channel, the resistance
of the channel from source to drain becomes large, and the FET is effectively turned off
like a switch. This is called pinch-off, and the voltage at which it occurs is called the
pinch-off voltage. Conversely, a positive gate-to-source voltage increases the channel
size and allows electrons to flow easily. In an n-channel enhancement-mode device, a
conductive channel does not exist naturally within the transistor, and a positive gate-to-source
voltage is necessary to create one. The positive voltage attracts free-floating electrons within
the body towards the gate, forming a conductive channel. But first, enough electrons must
be attracted near the gate to counter the dopant ions added to the body of the FET;
this forms a region with no mobile carriers called a depletion region, and the voltage
at which this occurs is referred to as the threshold voltage of the FET. Further gate-to-source
voltage increase will attract even more electrons towards the gate which are able to create
a conductive channel from source to drain; this process is called inversion.
p-channel In a p-channel depletion-mode device, a positive
voltage from gate to body 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.
Operation For either enhancement- or depletion-mode
devices, at drain-to-source voltages much less than gate-to-source voltages, changing
the gate voltage will alter the channel resistance, and drain current will be proportional to
drain voltage. In this mode the FET operates like a variable resistor and the FET is said
to be operating in a linear mode or ohmic mode.
If drain-to-source voltage is increased, this creates a significant asymmetrical change
in the shape of the channel due to a gradient of voltage potential from source to drain.
The shape of the inversion region becomes "pinched-off" near the drain end of the channel.
If drain-to-source voltage is increased further, the pinch-off point of the channel begins
to move away from the drain towards the source. The FET is said to be in saturation mode;
although some authors refer to it as active mode, for a better analogy with bipolar transistor
operating regions. The saturation mode, or the region between ohmic and saturation, is
used when amplification is needed. The in-between region is sometimes considered to be part
of the ohmic or linear region, even where drain current is not approximately linear
with drain voltage. Even though the conductive channel formed
by gate-to-source voltage no longer connects source to drain during saturation mode, carriers
are not blocked from flowing. Considering again an n-channel enhancement-mode device,
a depletion region exists in the p-type body, surrounding the conductive channel and drain
and source regions. The electrons which comprise the channel are free to move out of the channel
through the depletion region if attracted to the drain by drain-to-source voltage. The
depletion region is free of carriers and has a resistance similar to silicon. Any increase
of the drain-to-source voltage will increase the distance from drain to the pinch-off point,
increasing the resistance of the depletion region in proportion to the drain-to-source
voltage applied. This proportional change causes the drain-to-source current to remain
relatively fixed, independent of changes to the drain-to-source voltage, quite unlike
its ohmic behavior in the linear mode of operation. Thus, in saturation mode, the FET behaves
as a constant-current source rather than as a resistor, and can effectively be used as
a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current
through the channel. Composition
The FET can be constructed from a number of semiconductors, with silicon being by far
the most common. Most FETs are made with conventional bulk semiconductor processing techniques,
using a single crystal semiconductor wafer as the active region, or channel.
Among the more unusual body materials are amorphous silicon, polycrystalline silicon
or other amorphous semiconductors in thin-film transistors or organic field-effect transistors
that are based on organic semiconductors; often, OFET gate insulators and electrodes
are made of organic materials, as well. Such FETs are manufactured using a variety of materials
such as silicon carbide, gallium arsenide, gallium nitride, and indium gallium arsenide.
In June 2011, IBM announced that it had successfully used graphene-based FETs in an integrated
circuit. These transistors are capable of about 2.23 GHz cutoff frequency, much higher
than standard silicon FETs. Types of field-effect transistors
The channel of a FET is doped to produce either an n-type semiconductor or a p-type semiconductor.
The drain and source may be doped of opposite type to the channel, in the case of depletion
mode FETs, or doped of similar type to the channel as in enhancement mode FETs. Field-effect
transistors are also distinguished by the method of insulation between channel and gate.
Types of FETs include: The JFET uses a reverse biased p–n junction
to separate the gate from the body. The MOSFET utilizes an insulator between the
gate and the body. The DGMOSFET is a FET with two insulated gates.
The DEPFET is a FET formed in a fully depleted substrate and acts as a sensor, amplifier
and memory node at the same time. It can be used as an image sensor.
The FREDFET is a specialized FET designed to provide a very fast recovery of the body
diode. The HIGFET is now used mainly in research.
[1] The MODFET uses a quantum well structure formed
by graded doping of the active region. The TFET is based on band-to-band tunneling.
The IGBT is a device for power control. It has a structure akin to a MOSFET coupled with
a bipolar-like main conduction channel. These are commonly used for the 200–3000 V drain-to-source
voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source
voltages of 1 to 200 V. The HEMT, also called a HFET, can be made
using bandgap engineering in a ternary semiconductor such as AlGaAs. The fully depleted wide-band-gap
material forms the isolation between gate and body.
The ISFET can be used to measure ion concentrations in a solution; when the ion concentration
changes, the current through the transistor will change accordingly.
The BioFET is a class of sensors/biosensors based on ISFET technology which are utilized
to detect charged molecules; when a charged molecule is present, changes in the electrostatic
field at the BioFET surface result in a measurable change in current through the transistor.
These include EnFETs, ImmunoFETs, GenFETs, DNAFETs, CPFETs, BioFETs, and FETs
based on ion-channels/protein binding. The MESFET substitutes the p–n junction
of the JFET with a Schottky barrier; and is used in GaAs and other III-V semiconductor
materials. The NOMFET is a nanoparticle organic memory
field-effect transistor.[2] The GNRFET uses a graphene nanoribbon for
its channel. The VeSFET is a square-shaped junctionless
FET with a narrow slit connecting the source and drain at opposite corners. Two gates occupy
the other corners, and control the current through the slit. [3] [4]
The CNTFET. The OFET uses an organic semiconductor in
its channel. The DNAFET is a specialized FET that acts
as a biosensor, by using a gate made of single-strand DNA molecules to detect matching DNA strands.
The QFET takes advantage of quantum tunneling to greatly increase the speed of transistor
operation by eliminating the traditional transistor's area of electron conduction.
Advantages of FET The main advantage of the FET is its high
input resistance, on the order of 100 MΩ or more. Thus, it is a voltage-controlled
device, and shows a high degree of isolation between input and output. It is a unipolar
device, depending only on majority current flow.. Because base current noise will increase
with shaping time, a FET typically produces less noise than a bipolar junction transistor,
and is thus found in noise sensitive electronics such as tuners and low-noise amplifiers for
VHF and satellite receivers. It is relatively immune to radiation. It exhibits no offset
voltage at zero drain current and hence makes an excellent signal chopper. It typically
has better thermal stability than a BJT. Disadvantages of FET
It has a relatively low gain-bandwidth product compared to a BJT. The MOSFET has a drawback
of being very susceptible to overload voltages, thus requiring special handling during installation.
The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable
to electrostatic damage during handling. This is not usually a problem after the device
has been installed in a properly designed circuit.
FETs often have a very low 'on' resistance and have a high 'off' resistance. However
the intermediate resistances are significant, and so FETs can dissipate large amounts of
power while switching. Thus efficiency can put a premium on switching quickly, but this
can cause transients that can excite stray inductances and generate significant voltages
that can couple to the gate and cause unintentional switching. FET circuits can therefore require
very careful layout and can involve trades between switching speed and power dissipation.
Uses of FET The most commonly used FET is the MOSFET.
The CMOS process technology is the basis for modern digital integrated circuits. This process
technology uses an arrangement where the p-channel MOSFET and n-channel MOSFET are connected
in series such that when one is on, the other is off.
In FETs, electrons can flow in either direction through the channel when operated in the linear
mode. The naming convention of drain terminal and source terminal is somewhat arbitrary,
as the devices are typically built symmetrically from source to drain. This makes FETs suitable
for switching analog signals between paths. With this concept, one can construct a solid-state
mixing board, for example. A common use of the FET is as an amplifier.
For example, due to its large input resistance and low output resistance, it is effective
as a buffer in common-drain configuration. IGBTs see application in switching internal
combustion engine ignition coils, where fast switching and voltage blocking capabilities
are important. See also FET
Chemical field-effect transistor Quantum field effect transistor
ISFET MOSFET
FET amplifier References
External links Field Effect Transistor Applications
PBS The Field Effect Transistor Junction Field Effect Transistor
CMOS gate circuitry Winning the Battle Against Latchup in CMOS
Analog Switches Nanotube FETs at IBM Research
Field Effect Transistors in Theory and Practice The Field Effect Transistor as a Voltage Controlled
Resistor