Transit Time of Junction Transistors at High Bias Currents†

Abstract

A study is made of the behaviour of the small-signal base transit time of a transistor at high bias current levels, where increase of transit time occurs due to collector depletion layer contraction in combination with high level injection effects in the base. Computation of theoretical current dependence of transit time is carried out for devices of single-diffused base grading with alloyed emitter and collector junctions. A physical model of the devices, involving exponential base impurity density grading, is used as a basis of analysis. The physical parameters of this model are determined specifically for the transistor samples under study by interpretation of measured terminal properties under low-level injection conditions. Very close agreement between measured and computed dependence of base transit time on d. c. bias current is obtained, subject to appropriate allowance, in analysis, for variation of the operating temperature of the device with d. c. bias condition.