Non Linear Modelling Techniques Of High Frequency MESFET Transistors

Abstract

This paper presents comparisons between the different methodologies of non-linear modelling for MESFET transistors. Also, different kinds of measurements for modelling purposes, measuring several devices; including power MESFET transistors, have.s been studied. In this sense, we have developed a continuous and pulsed automated set up along with a fitting program that permit us to obtain the access resistors $, R,, Rd, the Schottky current parameters I,, and a, and the parameters of the nonlinear Id, source. A software program to extract the linear equivalent circuit measuring the scattering parameters at the interest frequency band at several operation points, permits us to obtain the linear elements and the parameters of the nonlinear Schottky capacitor of the model. At last, a large signal MESFET model suitable for applications in commercial nonlinear microwave CAD have been developed. The originality of this woirk lies in the fact that multibias starting points (hot and cold device) for pulsed measurements are used to derive a unique expression for Ids that describes the DC as well as the small and large signal behaviour of a device biased at any point. This Ids current is modelled biy two nonlinear sources, one of them is a bias point dependent nonlinear equation and the other one represents the differences between DC and Pulsed characteristics at every bias point. Experimental pulsed characteristics and simulations, for different package transistors, using our Id, nonlinear equation at several quiescent bias points have been canid out, showing excellent agreement. Furthermore, successful comparisons between MDS nonlinear simulations using the extracted model and experimental power measurements of the transistor loaded by 50 Ohms at the input and output ports have been made. I INTRODUCTION. The actual civil and military applications, have allowed the HEMT and MESFET to be incorporated in an increasing numlber of nonlinear hybrid and MMIC circuits. To design these circuits, nonlinear software programs using harmonic balance or timedomain algorithms are used. These simulatioii tools are very powerful but they need accurate large signal device models to improve the performance of these circuits and to minimize the number of design and fabrication cycles required. Therefore, foundries need to have more advanced non-linear models than the traditional quasi-static approximation. An important problem in dealing with the nonlinear modelling of these devices is their anomalous low frequency behaviour: Ihe quasi-static approach is not fulfilled. This behaviour leads to the frequency dependence of transconductance and output resistance and its origin is due to trapping, surface state, etc, [l] [2] and implies that the DC characteristics are an