Impact ionization dynamics in silicon by MV/cm THz fields

We investigate the dynamics of the impact ionization (IMI) process in silicon in extremely high fields in the MV/cm range and at low initial carrier concentrations; conditions that are not accessible with conventional transport measurements. We use ultrafast measurements with high-intensity terahertz pulses to show that IMI is significantly more efficient at lower than at higher initial carrier densities. Specifically, in the case of silicon with an intrinsic carrier concentration (similar to 10(10) cm(-3)), the carrier multiplication process can generate more than 10(8) electrons from just a single free electron. The photoexcited carrier density dependence of the IMI rate shows that with decreasing initial carrier density the rate increases and approaches the fundamental Okuto limit imposed by energy conservation.