Silicon carbide microwave field-effect transistor: Effect of field dependent mobility

Abstract

Cutoff frequency f t and corresponding power P m ft of a SiC microwave field-effect transistor (FET) are calculated using an analytical model that allows for nonlinear dependence of the carriers drift velocity on electric field. A simple analytical expression for the cutoff frequency is derived as a function of the basic FET parameters such as channel depth a, doping level N, gate length L, and material parameters: saturation drift velocity v s and carriers mobility μ. Conditions for the short and long channel devices are discussed. Comparison with the linear piece-wise approximation shows that for α-SiC FET with a = 0.15 μm, N = 1 × 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 μm. This can partially explain the difference between previous theoretical calculations and experimental microwave performance of SiC FETs, and can lead to future optimization of SiC-based devices.