The Use of Current Gain as an Indicator for the Formation of Hot Spots Due to Current Crowding in Power Transistors

Under some power transistor operating conditions, current crowding leads to a region of localized power dissipation. Such hot spot operation should be avoided because it produces excessive localized junction temperatures that degrade electrical performance of devices and cause thermal fatigue. Its presence may indicate an abnormally high susceptibility to second breakdown. A non-destructive screening technique has been developed to cull out power transistors with hot spots due to current crowding. The technique is based on the use of d-c current gain, hFE, measured as a function of collector voltage with the collector current, IC, held constant. In this procedure, the base current of the transistor under test is measured as a function of collector voltage while the collector current and case temperature are held constant. Under these conditions, base current is proportional to the reciprocal of hFE. Hot spot formation is indicated by an increase in the base current, which normally decreases with increasing collector voltage. Hot spots were often found to occur within the published maximum ratings of the transistors, under operating conditions well below those leading to second breakdown. Typical examples are presented of base current (proportional to l/hFE) as a function of collectoremitter voltage for a variety of operating conditions under which hot spot formation occurs. Data are presented comparing the use of thermal resistance, emitter-base voltage, and hFE to indicate the formation of hot spots.