Buried channel GaAs MESFET's—Scattering parameter and linearity dependence on the channel doping profile

The influence of the epitaxial structure (doping profile) in the channel region of a GaAs MESFET on its small signal scattering parameters is investigated both theoretically and experimentally. The large signal frequency behavior of FET's with a buried channel is compared with that of uniformally doped FET's. In the case of buried channel devices a much smaller variation of the s parameters with frequency is observed. This phenomenon can be understood by considering the inner transistor to consist of a two-dimensional transmission line. The principle of this new model, which is mainly based on technological data, is presented. Furthermore, the improvement of the nonlinear distortion is investigated. The FET's with a graded doping profile show very small intermodulation products, -42 dBm for an input power level of 4 dBm (P
out
= 9 dBm: 1-dB gain compression) at a relatively small drain-source voltage of only 4 V.