Field-effect transistor
The field-effect transistor (FET) is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. The concept of the field effect transistor predates the bipolar junction transistor (BJT), however FETs were implemented after BJTs due to the limitations of semiconductor materials and relative ease of manufacturing BJTs compared to 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.
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 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 (or lack of voltage) applied across the gate and source terminals. (For ease of discussion, this assumes body and source are connected). This conductive channel is the "stream" through which electrons flow from source to drain.
Consider 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. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.
Now consider an n-channel "enhancement-mode" device. A positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. However, enough electrons must first be attracted near the gate to counter the dopant ions added to the body of the FET, this process is called channel inversion and the phenomenom 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 free to create a conductive channel from source to drain.
At drain-to-source voltages somewhat less than gate-to-source voltages, changes to the gate voltage will alter the channel resistance. In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode. This mode is not employed when amplification is needed.
If a larger drain-to-source voltage is applied, 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 depletion region becomes "pinched-off" near the drain end of the channel. If the potential difference becomes even larger, the pinched-off point of the channel begins to move away from the drain towards the source. The FET is said to be in saturation mode.
Even though the conductive channel no longer connects source to drain, rather than entirely blocking the electrons from flowing from source to drain, electrons flow from source to drain in a differently-controlled manner. Any attempted increase of the drain-to-source voltage will also lengthen the depletion region, increasing the resistance proportionally with the applied drain-to-source voltage. This proportinal change causes the channel current to remain relatively fixed independent of the drain-to-source voltage and quite unlike the linear mode operation. In saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current in the channel.
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: | |||