Failure Mechanisms in High Power Four-Layer Diodes

High power four-layer diodes which failed during life tests were investigated in order to determine the reasons for their failure. The studies of the silicon chips have shown that the reliability of the diode can be limited by four failure mechanisms: nonuniform turn-on, hot spot development, thermal fatigue, and surface breakdown. Diode burn-out which occurs after the first few pulses at high current gives evidence of nonuniform turn-on of the diode resulting in local heating during a single pulse. The local heat can reach temparatures at which silicon and contact metal alloy further, finally short-circuiting the device. Nonuniform turn-on, usually caused by crystal defects, can be minimized by an appropriate design of the shorted emitter and proper triggering of the device. When the damage of the diode by local melting of the silicon metal alloy occurs after many pulses (of the order of 103 or more) at appropriate repetition rates it indicates the slow build-up of a hot spot and a subsequent burn-out. These hot spots are formed by lateral thermal instability. Two diode modifications were investigated with respect to hot spot build-up. The development of hot spots can be minimized by proper mounting of the silicon chip. Another failure mode shows a gradual deterioration of the device characteristics until the center junction is finally short-circuited without melting the silicon metal alloy. It can be speculated that the reason for this type of failure is either thermal fatigue or surface damage.