Reverse-Bias Second Breakdown of High-Power Darlington Transistors

The reverse-bias second breakdown (RBSB) characteristics of high power Darlington transistors are discussed. The Darlingtons investigated are rated at 400 V maximum voltage and 100 A maximum current. Devices with and without speed-up diodes (connected between the bases of the input and output transistors) were studied. A nondestructive system for characterizing the RBSB behavior of these devices is described. The RBSB behavior was found to vary unpredictably with varying reverse base current magnitude. It was also found that the RBSB behavior of the Darlingtons was a function of the forward base current magnitude. This is in marked contrast to what has been found for discrete devices. The presence of a speed-up diode also influenced the RBSB behavior of these devices.