Abstract
The purpose of this thesis is to investigate the operation of GaAs field-effect transistors with particular attention to the existence of negative resistance regions in the current-voltage characteristics, velocity overshoot effects, the role of substrate, and the role of heterojunctions. The approach used is to solve the electron transport equation using the Monte Carlo method which accounts for non-local effects in electron transport. Arguments are presented to support the contention that the negative resistance regions in the current-voltage characteristics observed in some experimental devices and produced by other researchers' computer simulations are attributed, in part, to the negative differential mobility of GaAs. The main reason of the existence of this negative resistance is related to the active layer thickness and it will be explained in terms of the rotation of the velocity vector. Electron velocity overshoot, a consequence of non-local effects, is examined in terms of gate length. The velocity overshoot becomes significant for FET structures with gates less than a micron in length and has many significant effects on the device performance. It is found also that velocity overshoot accounts for the undesirable saturation characteristics of submicron gate length GaAs FET which are observed in practical devices. However, it was also found that the presence of a low-doped n-type GaAs substrate below the active layer removes the negative resistance regions in the current-voltage characteristics. This is attributed to the effect of carrier injection from the active layer into the substrate which leads to the decrease of the effective channel thickness. This then will decrease the transconductance of the device, increase the gate pinchoff voltage and lower the device frequency response. This degradation of device's performance depends entirely on the purity and properties of the substrate. The performance of substrated FETs can be improved by preventing electron penetration into the substrate. This situation can be reached by using AlGaAs substrate whose energy band gap is higher than that of GaAs which then leads to electron confinement in the active layer. The use of AlGaAs in FETs can be in different forms. These will also be demonstrated in this thesis.