Determination of the Basic Device Parameters of a GaAs MESFET

In a gallium arsenide (GaAs) metal-semiconductor field effect transistor (MESFET), the properties of the active channel are fundamental in describing its operation. The channel properties can be characterized by the four basic parameters: gate length, gate width, channel thickness, and channel doping. In a recent paper,1 the maximally obtainable value of channel current was defined as the maximum channel current, Im. It was pointed out that Im differs from either (fully open channel) saturation current, Is, or zero-gate-bias drain current, which is often referred to as 7rfss. Currents Is and Id«« have conventionally been used to show upper limits of the drain current capability. However, neither I,, nor Ids« can represent the maximally obtainable value of channel current. It was emphasized that Im plays an important role in determining the maximum capability of large-signal operation of the device. Simple expres771 sions for Im were then obtained in terms of the four basic channel parameters, as a result of an extended study of Shockley's gradual channel approximation2 on Grebene-Ghandhi's two-section FET model3 with Fukui's concept on the current limiting mechanism.1 Among the four basic channel parameters, the total gate width, Z, is usually a given factor or merely a scaling factor. Therefore, the other three parameters are noted to be the most crucial variables in the design work. For these three parameters, their effective values were adopted in Ref. 1. This was essential, especially for gate length.