Control of avalanche injection in bipolar transistors through the use of graded collector impurity profiles

An analysis is presented to show that bipolar transistors can be made resistant to failure from avalanche injection (leading to current mode second breakdown) by incorporating an appropriately graded impurity profile within the collector. The improvement is achieved directly from an increase in the current density required to trigger the mechanism for any given collector-emitter voltage. This is in addition to benefits obtained through the reduction of current gain with an increase in current density, which in practice also contribute to the ability of a transistor to survive momentary overloads. Calculations have been performed for both single and double graded collector layers and a comparison has been made with those of uniform doping. The results are presented in a form that allows the optimum collector parameters for minimum resistance to be deduced, subject to a given breakdown voltage and current density protection level.