SIMULATION AND INTERPRETATION OF FAST RISE TIME LIGHT-ACTIVATED P-I-N-DIODE SWITCHES

Rise time and output power of light-activated silicon p-i-n diode switches with long transit time, under both high and low optical pulse excitations as well as various circuit rise times, are studied using a finite difference computer program. A new physical interpretation of the simulated results is presented. One-dimensional carrier transport equations and the circuit equation are solved self-consistently. For diodes with a long transit time, the simulated output rise time can be explained by a physical model including the cancellation electric field due to separated photocarriers, the induced electric field with various RC time delays, and the optical pulse energy rise time. Under high optical pulse excitations and without the instantaneous presence of the induced electric field associated with long circuit rise times, the collapse of the total electric field can be expected. Thus the output rise time is determined only by the electric field cancellation time and decreases with increasing optical pulse excitation. The rise time under low optical pulse excitations is determined by both the circuit rise time and the optical energy rise time, similar to the rise time of a conventional p-i-n photodiode with long transit time. Under high optical-pulse excitations and short circuit rise times, the cancellation electric field is instantaneously opposed by the induced electric field and the output rise time is similar to that of the low optical-pulse excitation case. In order to maximize output power for a given bias voltage, it is necessary to decrease the matched load impedance to maximize the short-circuit power and at the same time to increase the laser pulse energy to minimize electrical dissipation in the diode switch.