Optimized design of GaAs FET's for low-noise microwave amplifiers

Abstract

In order to optimize the performance of GaAs Schottky-barrier gate FET's for small-signal amplification at a given microwave frequency, a design procedure has been established on the basis of simple analytical theories of the junction-gate FET. The design goal is to simultaneously satisfy the specifications of quantities for which there are trade-off's. These are the power gain, noise figure, and input reflection coefficient in 50-ohm lines, which represent major figure-of-merits for amplifying devices. An equivalent circuit model is constructed to include intrinsic small-signal elements calculated from the theory of Lehovec and Zuleeg, and noise current sources from van der Ziel's theory. Parasitic elements are also taken into account to reflect the state-of-the-art processing and packaging techniques. A simple, systematic method for analysing a complicated equivalent network including noise sources is devised and applied in computations. Three major design parameters of the Schottky-barrier FET, i.e. gate length, channel thickness and channel doping density are taken as independent variables in order to obtain the optimized design. An experiment is carried out to prove the validity of this theory. It is found that the theory predicts the optimized device performance with sufficient accuracy, and gives a set of optimum design parameters to a good approximation. The optimized unit, with a gate length of 1·5 μm, shows a unilateral gain as high as 21·5 dB, an F min as low as 2·6 dB, and an input reflection coefficient around 0·85 at 4 GHz.