Book Description

InP-based heterostructure field effect transistors (HFETs) have, over the past several years, demonstrated microwave performance capabilities superior to those of GaAs-based and Si-based transistors. In particular, InGaAs/InAlAs modulation-doped field effect transistors (MODFETs) have exhibited world-record unity current gain frequencies ($fsb{t}$s) as well as extremely high power cutoff frequencies ($fsb{rm max}$s) and have, therefore, become the optimum devices for small-signal applications at high frequencies, particularly in low-noise applications. Despite these strengths, InP-based HFETs have inherent weaknesses which limit their capabilities for large-signal, high output power applications. Due to a combination of the poor Schottky characteristics of InAlAs, which is often the material in contact with the metal gate, and the small bandgap of InGaAs, which is the material often used for the channel, the devices typically have lower breakdown voltages than their GaAs counterparts. However, because of the phenomenally high values of $fsb{t}$ and $fsb{rm max}$ obtainable for these devices, there has been a growing desire to overcome these weaknesses in order that the devices can be used for high-power applications at microwave frequencies. The subject of this work is the investigation of the possibility of designing InP-based HFETs for use as high-power devices. The emphasis is not on obtaining a world-record high frequency power device; instead, the focus is on the critical issues involved when designing the devices for high power applications. Hence, the goal is to obtain an in-depth understanding of the internal physics of the FETs when they are operating as power devices, and in so doing, attempt to arrive at designs and techniques which will overcome some of the limitations of InP-based HFETs.