Non-equilibrium modeling of mixed tunneling and avalanche breakdown effects in transit-time diodes.

Abstract

The objective of this thesis is to theoretically investigate the operation of GaAs and InP transit-time diodes at frequencies near 94 GHz and above, in which impact ionization, interband tunneling and Schottky leakage are the major carrier generation mechanisms. A non-equilibrium transport model, named the average-energy model, is employed to take into account the electron energy relaxation effects of GaAs and InP, which are not modeled by the traditional drift and diffusion formulation. Comparison of transport models shows that the average-energy transport model in homogeneous GaAs is close to the Boltzmann transport model for frequencies into the Terahertz range. DC and large-signal computer simulation programs based on the average-energy transport model have been developed. The dead space for impact ionization is inherently contained in the average-energy simulation programs. A classical method using the band diagram to determine the dead space for interband tunneling is presented. A method of incorporating Schottky leakage current in the boundary condition has been developed and implemented. Much effort has been devoted to determining the material parameters at high fields required by the simulation program. The interband tunneling rates were determined by simulating a TUNNETT structure actually measured in the laboratory. A three-valley Monte Carlo technique was employed to determine most of the electron parameters. Contact resistance and Schottky leakage currents were calculated by solving Schrodinger's equation in the potential barrier formed at the metal-semiconductor junction. Several GaAs and InP diodes of various structures have been studied using the simulation programs developed in this work. These investigations provide very useful information and insight into device operation and design at high frequencies and the potential of these devices.