Simulation of impact ionization breakdown in MESFETS using Monte Carlo methods

We have investigated impact ionization in a small 1.2 mu m MESFET by means of Monte Carlo simulation. Whilst ionization in devices operating near pinch-off was found to occur in accordance with the nominal gate-drain electric field, it was found that at lower gate reverse bias the device was vulnerable to oscillating electric fields associated with the formation of accumulation layers. These fields caused significant impact ionization to occur at relatively low nominal gate-drain potentials. Indeed the newly formed holes could have a positive feedback effect on the potential causing the oscillations to increase and the device to eventually break down. Under certain circumstances, the accumulation layer oscillations were stable and the device oscillated with a frequency of approximate to 125 GHz, though as the displacement current was out of phase with the carrier current, the total current was found to have little more than a ripple on it.