what is Field effect transistor?

Added Date:2007-06-25

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.

The channel of a FET (explained below) 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 enhancement mode FETs, or doped of similar type to the channel as in depletion mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel 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.
* The IGBT (insulated-gate bipolar transistor) 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 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.

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 FETs electrons can flow in either direction through the channel when operated in the linear mode, and the naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example.

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