SILICON-CARBIDE MICROWAVE FIELD-EFFECT TRANSISTOR - EFFECT OF FIELD-DEPENDENT MOBILITY

Cutoff frequency f(t) and corresponding power P-m(ft) of a SIC microwa ve field-effect transistor (FET) are calculated using an analytical mo del that allows for nonlinear dependence of the carriers drift velocit y on electric field. A simple analytical expression for the cutoff fre quency is derived as a function of the basic FET parameters such as ch annel depth a, doping level N, gate length L, and material parameters: saturation drift velocity upsilon(s) and carriers mobility mu. Condit ions for the short and long channel devices are discussed. Comparison with the linear piece-wise approximation shows that for alpha-SiC FET with a = 0. 15 mu m, N = 1 x 10(18) cm(-3) at the drain voltage V-D = V-p = 20 V, where V-p is a channel pinch-off voltage, field dependent mobility reduces the value of f(t) more than 1.5 times for the device with L in the range of 0.9-1.5 mu m. This can partially explain the di fference between previous theoretical calculations and experimental mi crowave performance of SiC FETs, and can lead to future optimization o f SiC-based devices.