6.1 Introduction

The GaAs MESFET has gained a well deserved position amongst microwave solid state devices as a low noise amplifying device up to 30 GHz or so. The performance of Schottky barrier FETs as mixers, particularly utilizing low IF frequencies has been somewhat disappointing in respect of noise figure performance although there are undoubted advantages in terms of conversion gain and high compression point over Schottky diode mixers. Extensive studies have been carried out by laboratories worldwide into the applications of both single and dual-gate MESFETs in up and down-conversion circuits. Recently, with the advent of gallium arsenide integrated circuits, new device and circuit techniques have been adopted which are realizing low-noise figures with high conversion gains at frequencies up to X-band (Pengelly, 1980). The reason for this latter success has been the realization of biasing and circuit designs giving optimum performance in respect of signal and local oscillator injection as well as a fuller understanding of FET requirements.

GaAs MESFETs utilize their nonlinearities to produce mixing (Pucel et al, 1975; Sitch et al, 1973)-namely, the I _GS ?V _GS nonlinearity and the I _DS ?V _GS nonlinearity. The first of these effects results from the Schottky barrier between the gate and source and it shows characteristics analogous to Schottky barrier diodes. The second nonlinearity is caused by the pinch-off effect described in Chapter 2.

6.2 The GaAs FET As A Mixing Element

Figure 6.1 shows the simplified equivalent circuit of a chip FET where...