One of the more common FET transistor typologies is the MESFET (Metal Semiconductor field effect transistor). This active device is the oldest FET device concept. The MESFET is similar in structure to a JFET (Junction Field effect transistor) but includes a Schottky junction instead of a P-N junction.
The MESFET’s channel depends on three parameters: the velocity of the charge carriers, the density of these charge carriers, and the geometric cross section the carriers flow through. The gate electrode is connected directly to the semiconductor material, creating a Schottky diode. The MESFET is generally constructed from the compound semiconductor GaAs (Gallium Arsenide) to provide higher electron mobility. As shown, the substrate is semi-insulating to decrease parasitic capacitance.
The device works by limiting the electron flow from source to drain, similar to a JFET. The Schottky diode controls the resistance of the channel (size of depletion region). Varying the voltage across the Schottky gate changes the channel size. Similar to other FETs, there is a certain pinch off voltage that causes the current to be very small, making the MESFET a switch or variable resistor. MESFETs can be depletion mode or enhancement mode. The MESFET is often used in high frequency wireless communication devices such as cell phones or military radars.
(All information and photos obtained from “High Speed Electronics and Optoelectronics Devices and Circuits” by Sheila Prasad)