On the Transient Behavior of Semiconductor Rectifiers

Abstract

If the forward current through a p-n junction suddenly ceases, an emf appears due to nonequilibrium carrier concentrations at the junction barrier. Measurements of the barrier emf have been made with a wide range of injection currents and yield current vs barrier voltage characteristic curves. The open circuit case of voltage decay across a p-n junction in the range V≫kT/q is discussed. The voltage decay is observed to be linear with time with a slope kT/qτ. It follows from an analysis based on a simple model that the excess minority carrier concentrations decrease exponentially with the time constant τ. The floating emitter and collector voltages, VE(t) and VC(t), of p-n-p transistors are observed to decay like the open circuit voltage across a p-n junction, and it is noted that VE(t) ≈ VC(t) in the range V≫kT/q. The transient response of a semiconductor rectifier connected in parallel with a large external capacitance is analyzed. The response calculated for large negative excursions of voltage gives a simple relation by which the saturation current may be determined. Measurements are presented and discussed.