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see transistortransistor,
three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all common applications.
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(Field Effect Transistor) One of two major categories of transistor; the other is bipolar. FETs use a gate element that, when charged, creates an electromagnetic field that changes the conductivity of a silicon channel and turns the transistor on or off. FETs are fabricated as individually packaged discrete components as well as by the hundreds of millions on a single chip.

FETs vs. Bipolar
FET-based silcon chips are easier to construct than their bipolar counterparts. FETs switch a little slower than bipolar transistors, but use less power. Once the gate terminal on an FET has been charged, no more current is needed to keep that transistor on (closed) for the duration of time required. By comparison, a bipolar transistor requires a small amount of current flowing to keep the transistor on. While the current for one transistor may be negligible, it adds up when millions are switching simultaneously. The heat dissipated on bipolar limits the total number of transistors that can be built on the chip, which is why CMOS logic (based on FETs) is used to build chips with millions of transistors.

The most widely used and widely known FETs are MOSFETs (metal oxide semiconductor FETs), which come in NMOS (n-channel) and PMOS (p-channel) varieties. On a chip, NMOS and PMOS transistors are wired together in a complementary fashion to create CMOS logic, which is the most predominant and used in almost every electronic device today. See MOSFET and n-type silicon.

There Are Many Kinds of FETs
Similar to MOSFETs are JFETs (junction FETs), which use a PN junction gate rather than a poly-crystalline gate. Used for microwave communications, MESFETs (metal semiconductor FETs) are similar to JFETs, but use a Schottky metal gate and are made from gallium arsenide or indium phosphide, not silicon. Evolving from MESFETs for higher-frequency applications are HEMTs and PHEMTs (high electron mobility transistors and pseudomorphic high electron mobility transistors). HEMTs are also called MODFETs, TEGFETs and SDHTs (modulation doped FETs, two-dimensional electron gas FETs and selectively doped heterojunction transistors).

Another high-frequency FET is the gallium arsenide-based CHFET (complementary heterostructure FET), which uses a complementary architecture similar to CMOS.

FETs vs. Bipolar
After the gate is charged in an FET, no more current flows, but the transistor remains closed (turned on) during the required time period. Bipolar transistors (BJTs) require current the entire time the transistor must be closed.
References in periodicals archive ?
The largest source of controllable loss in the vector modulator is the choice of the PHEMT devices.
The Smith chart in Figure 6 illustrates the ideal paired complementary control signals (I/IN) for a balanced attenuator assuming four ideal equal split (that is, 3 dB) Lange couplers and four matched PHEMT devices.
While PHEMT devices can be biased for excellent linearity under class AB bias conditions, it is also their high gain that contributes to the overall amplifier efficiency.
Figure 5 illustrates the linearity advantage of the PHEMT technology from a different viewpoint.
The load-pull characteristics of the E-mode pHEMTs were examined under continuous-wave, CDMA and EDGE-GSM modulation formats, all at 1.
Each reflected bit consists of a PHEMT switch, a capacitor and a tuned inductor.
The 2 mm E- and D-mode InGaP/AlGaAs PHEMT devices are characterized at 900 MHz using an automated on-wafer loadpull measurement system, which allows independent tuning of the fundamental and harmonic impedances presented to the device output.
A comparison of the modeled intermodulation characteristics of the PHEMT and two amplifier circuits incorporating the same PHEMT under identical bias-conditions underscores the importance of the matching networks and their influence on the gain-intermodulation relationship.