Simulation of electron diffusion effect on plasma formation in silicon TRAPATT diodes

Abstract

Plasma formation and extraction processes in silicon n+np+ TRAPATT (TRApped Plasma Avalanche Triggered Transit) diodes were simulated. The drift-diffusion model was chosen for the simulation of the processes. This model is adequate for diodes under consideration with a total thickness of 6.5 µm. Two approximations of carrier diffusion coefficient dependence on the electric field above 20 kV/cm were used. A strong dependence of plasma density and oscillation period on n+n junction steepness was found in the case of a constant electron diffusion coefficient in the electric field range above 20 kV/cm. This behaviour depends on the impact ionization model in silicon. Two models were used. In one of them we included drift and diffusion current in the impact ionisation process. In the other model we included only the drift current in the impact ionisation process. In the second case the influence of the n+n junction steepness on the plasma formation process is much stronger. In the diodes with a highly abrupt n+n junction the TRAPATT mode is impossible. These results explain our experiments on TRAPATT diodes with an abrupt n+n junction.