The Effective Emitter Area of Power Transistors

The theoretical derivation in this paper is concerned first with the determination of an effective emitter area for a transistor with cylindrical geometry. From this it follows that out of the total emitter area only the border region in a width of one diffusion length is effective. This result is applied to a geometry of concentric emitter and base rings, each being considerably broader than two diffusion lengths. For comparing theory and experiment in such a geometry, different emitter rim lengths are realized easily when the different base rings are connected successively with one another and the emitter rings are also treated in the same manner. Theory and experiments agree. As a second theoretical contribution, the Appendix contains a rigorous solution for a special three-dimensional transistor model.