Technique of controlling avalanche injection in a transistor

Abstract

Bipolar power transistors operated under reverse- biased condition are prone to failure due to the occurrence of current mode second breakdown (CSB). There are two parameters, critical current density and critical voltage, which measure the susceptibility of a transistor to this type of failure. Previous studies of CSB have demonstrated that the critical current density can be increased by increasing the collector doping density. On the other hand, the collector density decreases with increase of breakdown voltage. Therefore, this conflicts with the requirement to support a high breakdown voltage of a power transistor with uniformly doped collector. A transistor with graded collector will offer more resistance to this failure. The transistor is usually designed to meet some requirements like specified breakdown voltage, maximum current gain, etc. In this thesis, the optimal collector parameters of single and double graded collectors are theoretically evaluated under certain given constraints. The results show that a double graded collector provides a substantial increase in doping density at the collector- substrate interface. For a specified breakdown voltage, this increase in doping density at the interface provides a large increase in critical current density at the onset of avalanche injection. However, this results in high current protection with a small penalty in terms of collector width compared to that offered by an optimized uniform collector.