In contrast to BJTs, ,field effect transistors (FETs) are monopolar, with only one carrier type (holes or electrons) providing current flow through the device: n-channel FETs employ electrons, while p-channel devices use holes. In addition, while a BJT is a current controlled device, an FET is a voltage-controlled device, having a source-to-drain characteristic that is similar to that of a voltage-dependent variable resistor. There are different types of FET available

1. MESFET (metal semiconductor FET),
2. MOSFET (metal oxide semiconductor FET),
3. HEMT (high electron mobility transistor),
4. the PHEMT (pseudomorphic HEMT).

Metal Semiconductor Field Effect Transistor:

Figure shows the cross section of a typical n-channel GaAs MESFET. The gate junction is formed as a Schottky (metal and semiconductor) barrier. The desirable gain and noise features of this transistor are a result of the higher electron mobility of GaAs compared to silicon, and the absence of shot noise. The device is biased with a drain-to-source voltage, Vds, and a gateto- source voltage, Vgs . In operation, electrons are drawn from the source to the drain by the positive Vds supply voltage. An applied signal voltage on the gate then modulates these majority electron carriers, producing voltage amplification. The maximum frequency of operation is limited by the gate length; present FETs have gate lengths on the order of 0.2–0.6 μm, with corresponding upper frequency limits of 100 to 50 GHz.