Infrared Observation of Current Distributions in Large Area Power Transistors

In this paper it is demonstrated that by imaging and studying the recombination radiation emitted by an operating power transistor, much useful information about the current distribution can be obtained in a routine manner. This information includes a semi-quantitative measure of the degree current crowding to one region of the pellet as a result of voltage losses in the metallization and thermal gradients, the effectiveness of the ballasting in preventing both thermally stable current crowding and hot spot formation, and the degree of crowding toward the edge of the emitter as a result of resistive losses in the base under the emitter. In general this detailed information cannot be obtained by any other method, and is very useful to device engineers in improving power transistor design.