Low-temperature avalanche multiplication in the collector-base junction of advanced n-p-n transistors

The collector-base junction avalanche in advanced n-p-n transistors in the temperature range of 293 to 83 K is described. The multiplication factor is shown to increase exponentially with decreasing temperature. The dependence decreases with increased collector doping concentration and, for the same device, with increased reverse bias. At a fixed collector bias, it is roughly constant at low current density, but varies with I_c at high-level injection due to space-charge modulation. Measurements at low temperatures excluded self-heating in the devices, and it was possible to study high-level injection effects at collector current densities higher than 10 mA/μm ². Extensive computer simulations were performed to study the effects of the field and carrier distributions. It was observed that, at very high current densities, when the injected carrier density in the collector junction exceeds 1×10¹⁷ cm^-3, there is an anomalous drop in the avalanche multiplication rate that conventional device simulators fail to predict. The latter is attributed to electron-electron scattering that retards impact ionization by quickly redistributing the excess energy through interparticle collisions