Circuit Models of the MESFET

Abstract

An idealized MESFET (metal-semiconductor field effect transistor) structure is shown in Fig. 2.1.1. An active layer of n-type GaAs is grown on top of a semi-insulating GaAs substrate. The drain and source electrodes make contact with the active layer through an n+ region under each. A third electrode, which is the gate, is placed directly on the active layer. From consideration of the energy band diagram at the metal-semiconductor interface, we find there is a depletion region under the gate whose height is controlled by a transverse electric field created by an applied gate voltage V _G. The undepleted region is called the channel. When a positive voltage V _D is applied to the drain with respect to the source, a longitudinal electric field is created that accelerates electrons in the channel from the source towards the drain. The resulting current in the channel, called the drain current, will depend on the gate voltage and the drain to source voltage. A MESFET is therefore a three-terminal device whose drain to source current is controlled by the voltage on the third electrode.