Emitter Softening in Diffused Silicon Transistors

IN SILICON, n-p-n, double-diffused mesa transistors, emitter softening is a term which has been coined to describe a unique form of device life failure. It is characterized by an increase in the emitter leakage current which occurs during power aging. This specific type of failure does not usually occur during storage aging at high temperatures. The rate of failure of a device in this mode has been found to be proportional to the current density at which the device is operating. Thus, the rate of failure is increased by localized areas of high current density. This paper summarizes the results of an investigation to determine the mechanism of emitter softening and possible solutions. Detailed examination of the voltage-current characteristics of the emitter diode before and after failure show that the reverse characteristic of the diode changes from a predominantly avalanche mode of breakdown to one which exhibits the characteristics of field, emission or tunneling. The presence of field emission suggests that the depletion layer is being modified to the extent that localized areas have very high electric field intensities. An investigation of the cause of the field emission indicated that emitter softening is caused by the diffusion of residual metal impurities into the emitter. Such diffusion occurs most rapidly in areas of localized high temperatures. This explains why softening is found to occur at regions of high current concentration, since more heat is generated in the portion of the collector junction adjacent.