Increase of critical current density and voltage for triggering avalanche injection through use of graded collector doping

In this paper it is shown that both critical current density and voltage of an epitaxial bipolar power transistor with an inductive load, which are taken as a measure of its susceptibility to avalanche injection, can be significantly increased by using double graded collector doping profiles. The first graded portion gives the required current protection level and the steeper second graded profile can be used to increase the voltage protection level. Calculations are carried out for optimum collector parameters to achieve minimum resistance for the required open base breakdown voltage. Results are compared with uniformly doped profile for the same collector resistance and open base voltage. It is shown that a device with graded collector is less prone to failure due to avalanche injection.