The maximum voltage, current and power ratings of junction transistors

Abstract

The paper discusses the properties of a junction transistor which determine the limits of the three basic ratings, namely the maximum collector voltage, collector current and power dissipation, and describes methods of measurement. The effects of the limitations on the various circuit configurations are also discussed.The maximum value of both the direct and alternating voltages that may be applied to a transistor are dependent upon the variation of the characteristics with voltage. The most important of these factors are surface leakage, avalanche multiplication and collector/ emitter punch-through. The last is an absolute limitation whereas the first two result in a variation of the current gain and output impedance. The effects of these variations are very dependent upon the circuit arrangement and the possible parameter tolerances. The collector leakage current can also result in thermal instability. In general the important value of the leakage current, from the circuit-performance aspect, is that at high temperature. However, to simplify measuring and for life considerations, a low-temperature test may be more suitable.The maximum current rating is normally determined by the decrease in current gain at high emitter currents. Again this limitation is dependent upon the circuit arrangement and it is not possible to set an absolute limit. For linear-amplifier applications the variation of current gain with emitter current should be low, whereas, in switching applications, only the gain at high current is of importance.The power dissipation rating of a transistor is basically determined by the effect of a high internal temperature on its life. However, a convenient method of expressing the rating, as a function of ambient temperature, is by means of a maximum junction temperature and a thermal resistance. Since the total thermal resistance is dependent upon the mounting arrangements, the user must consider both the electrical and thermal properties of any transistor circuit. The transient power rating of a transistor is also of importance in many switching circuits.