Novel modelling methods for microwave GaAs MESFET device

Abstract

As one of the most widely used microwave devices, the gallium arsenide metal semiconductor field effect transistor (GaAs MESFET) dominates in modern MIC/MMIC applications such as switches, power amplifiers, low noise amplifiers, oscillator, etc. Reliable modelling methodology and accurate device models of GaAs MESFET are currently extremely important and in great demand. In this thesis, both small signal and nonlinear large signal models of GaAs MESFETs have been investigated. This study first involves investigation and comparison of different small-signal parameter extraction techniques. A reliable analytical small signal model extraction approach is subsequently presented. For the first time, a novel analytical approach for extracting all the 15 equivalent circuit elements of GaAs MESFET devices has been proposed with no subsidiary circuit such as Cold-FET or Hot-FET techniques. On the other hand, for the relatively high operating frequencies, a new GaAs MESFET distributed model based on accurate EM simulation and quasi-optimization method has also been proposed in this thesis. This distributed model can be adopted to describe complex parasitic effects in device layouts and to predict the electrical characteristics of unconventional device structures for better MMIC performance. For the large-signal modelling of GaAs MESFET, a new empirical model is developed. To further refine the drain current description, a set of power series function is introduced in the improved drain current expression for the correlations between modulation parameters a, ? and biasing condition Vds & Vgs. Moreover, a new gate terminal charge model for Cgs and Cgd description is also proposed under gate charge conservation law. The model expressions and their derivatives are continuous over the entire device bias range. This new large signal model can be easily implemented in CAD software and is very useful in the nonlinear microwave circuit simulation. For complete model evaluation, a Ku-band power amplifier has been designed and fabricated using 0.18 um TOSHIBA? GaAs MESFET technology. Simulated and measured amplifier performances have been investigated and good agreement has been demonstrated.