Field-effect transistor
The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. FETs are sometimes used as voltage-controlled resistors. The concepts related to the field effect transistor predated those of the bipolar junction transistor (BJT). Nevertheless, FETs were implemented only after BJTs due to the simplicity of manufacturing BJTs over FETs at the time.
Terminals
All FETs except J-FETs have four terminals, which are known as the gate, drain, source and body/base/bulk. Compare these to the terms used for BJTs: base, collector and emitter. BJTs and J-FETs have no body terminal.
The names of the terminals refer to their function. 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. Electrons flow from the source terminal towards the drain terminal if 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 type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit, however there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits.
Composition
Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel.officially
Types of field-effect transistors
The FET can be constructed from a number of semiconductors, silicon being by far the most common. The body of a FET is either doped to produce an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the body, in the case of enhancement mode FETs, or doped of similar type to the body as in depletion mode FETs. Field-effect transistors are also distinguished by the method of insulation between body and gate. Types of FETs are:
- The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body .
- The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.
- The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
- Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation between gate and body.
- The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.
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 and often apply organic gate insulators and electrodes.
FET Operation
The shape of the conducting channel in a FET is altered when a potential difference is applied to the gate terminal (potential relative to either source or drain.) In an n-channel "depletion-mode" device, a negative gate voltage causes a depletion region to expand in size and encroach on the channel from the side, narrowing the channel. If the depletion region completely closes the channel, the resistance of the channel becomes very large, and the FET is effectively turned off. Positive gate voltage attracts electrons from the surrounding semiconductor next to the gate, forming a conductive channel. At low source-to-drain voltages, small changes to the gate voltage will alter the channel resistance. In this mode the FET operates like a variable resistor. This mode is not employed when amplification is needed.
If a larger potential difference is applied between the source and drain terminals, this creates a significant current in the channel and produces a gradient of potential from source to drain. This also causes the shape of the depletion region to become asymmetrical–one end of the channel becomes narrow. If the potential difference is large enough, the depletion region begins to close the channel. The FET is said to be in saturation. Rather than entirely blocking the electrons from flowing from source to drain, electrons flow through the depletion region in a controlled manner. Any attempted increase of the drain-to-source voltage will lengthen the depletion region, increasing the channel resistance proportionally with the applied drain-to-source voltage which causes the value of drain current to remain relatively fixed. This mode of operation is called pinch-off. In this mode, the FET behaves as a constant-current source rather than as a resistor and can be used as a voltage amplifier. The value of gate voltage determines the value of the constant current in the channel.
An "enhancement-mode" device is of slightly more complex construction. Rather than only being one material, it is made of a three-piece sandwich: similar to either an npn or a pnp transistor with no base connection, but still with a gate. In this case, the device is "normally-off" since one of the two junctions will always be reverse biased. The npn device is called an n-channel device and the pnp device is called a p-channel device.
Uses
The most commonly used FET is the MOSFET. The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off. In CMOS logic devices, the p-channel device pulls up the output and the n-channel device pulls down the output. The great advantage of CMOS circuits is that they allow no current to flow (ideally), except during the transition from one state to the other, which is very short. The gates are capacitive, and the charging and discharging of the gates each time a transistor switches states is the primary source of power usage in fast CMOS logic circuits. However as integrated circuits become smaller, parasitic resistances are becoming more power consumptive than switching capacitance.
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.
FETs can switch signals of either polarity on the source or drain terminals, if their amplitude is significantly less than the gate swing, as the devices are typically symmetrical. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example.
The power MOSFET has a reverse-biased 'parasitic diode' shunting the conduction channel that has half the current capacity of the conduction channel. Sometimes this diode is used when driving inductive circuits, but in other cases it causes problems.
A more recent device for power control is the insulated-gate bipolar transistor, or IGBT. This has a control structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These have become quite popular in the 200-3000 V range of operation, as they overcome limitations of Power MOSFET in high voltage. Power MOSFETs are still the device of choice (and practically the only choice available) for low voltage (from less than 1 V to 200 V) applications.
See also
 | Bipolar junction transistor: |  | |
|---|---|---|---|
| Field-effect transistor: | |||
| Multiple transistors: | |||