Heterojunction Transistors.

Abstract

The objective of this investigation is to study the properties of GaAs-based heterojunction transistors in order to better underst and their high-frequency performance. The high-frequency limitations of high-mobility materials are studied by comparing the switching performance of diodes and transistors based on two transport models: a conventional drift-diffusion transport model and a more accurate energy-momentum balance transport model. The drift-diffusion model predicts high-frequency performance limitations because of velocity saturation and hot electron effects. The energy-momentum model predicts poorer high-frequency performance based on velocity overshoot and cool electron effects. It also predicts an underdamped electron velocity due to the inductive nature of the electron velocity. The properties of heterojunction interfaces are studied by comparing three interface models: the Boltzmann model, the Fermi-Dirac model and a model based on quantum statistics. The choice of model is a tradeoff between accuracy and cost. Over the range of temperatures and interface conditions studied, the Fermi-Dirac model predicts interface conditions close to the more accurate quantum-based model using approximately the same amount of computer time as the simple Boltzmann model. The Fermi-Dirac interface model is used as a basis for a two-dimensional model for a high electron mobility transistor structure. The structure is studied over a range of bias, doping and Al fraction conditions in order to better underst and the device operation. The device operation is found to be closer to bipolar transistor operation than conventional field-effect transistor operation. The two-dimensional model is also used to study the properties of ballistic injection structures and ballistic transistors. A simple Monte Carlo model is used to study ballistic injection. The results show that very high velocities can be obtained by ballistic injection, but that the effects occur over relatively short distances. This limits the performance of ballistic transistors.