Metal—Semiconductor Field-Effect Transistors

Abstract

The concept of a metal—semiconductor junction field-effect transistor (MESFET) is very similar to that of a junction field-effect transistor (JFET) discussed in Chapter 4. The geometry of the conducting channel in a MESFET is confined by the top and bottom gate depletion regions, which are controlled by the gate and drain voltages. Thus, before the two depletion regions touch each other (channel pinch-off), the MESFET, in essence, acts like a variable resistance. Beyond the channel pinch-off, however, the drain current increases only slowly with increasing drain voltage, and the device is operated in the saturation region. Unlike the JFET, which is a bulk device, the MESFET is a surface device; i. e., one of the boundaries of the primary region (channel region) is the interface between the metal and semiconductor. Because many trapping states can be present at the metal—semiconductor interface, current transport in the MESFET is fluctuated by the capture and release of free carriers at these surface states. As a result, the MESFET has a higher noise level than the JFET. The major difference between the MESFET and the metal—oxide semiconductor field-effect transistor (MOSFET), which is also a surface device, is that a MOSFET is normally off until a voltage greater than the threshold is applied to the gate, whereas the MESFET is normally on unless a large reverse voltage is applied to the gate to cut off the conducting channel. If the channel layer (or epilayer) thickness is made very thin, however, the MESFET will be off unless a forward voltage is applied to the gate. Here, the normally on MESFET will be emphasized.