Computer simulation of negative-differential-conductivity effects, including trapping

A macroscopic computer model to simulate negative differential conductivity effects is described, and simulation results for trap-free diodes are presented. The model is based on a simple approximation of GaAs; the relation between voltage, field, and diode length takes into account the internal voltage due to doping nonuniformities. The simulation is performed without restrictive assumptions on the field or carrier concentrations at the electrodes, and the results focus on parastable diodes. It is found that the field outside an anode parastable layer depends on the doping at the layer edge. Consequently, the steady-state current in a parastable diode is independent of voltage if the doping is uniform at the layer edge, but varies with the voltage if there is a doping gradient at the layer edge. An anode layer can remain stable below the threshold voltage, and a doping increase towards the anode can make a diode unstable just above threshold, and parastable at higher voltages.