Infrared observation of transistor current distribution during operation in the quasi-saturation regime

The current gain of transistors with wide, lightly doped collectors is is known to fall off during high current, low voltage operation. This "two-level saturation" effect, sometimes known as "quasi-saturation", has been attributed by some authors to a current density-induced widening of the base into the collector, and by others to lateral current injection at high current densities. In this paper we describe the results of experiments designed to determine whether or not lateral current injection is significant during quasi-saturation. In our experiments, the band-to-band radiative recombination observed between the base and emitter metallization in the course of transistor operation was used to determine the current distribution. This infrared radiation was observed using a very sensitive S-1 image tube fibre-optically coupled to a SIT TV camera. On the basis of the current distributions observed in a variety of transistors, we concluded that lateral injection is not the dominant factor in establishing a quasi-saturation regime.