The topography of current density in bipolar transistors

A new method is proposed for determing the current distribution over the entire chip area of a bipolar transistor. The method is applied to a comb type of emitter-base geometry and takes into account the sheet resistances of base and emitter finger metallizations and also the sheet resistance of the base diffusion. Emphasis is placed on determining the variation of current density over the long dimension of each emitter finger with the transistor operating in the quasi-saturation or base-widened regime. Calculated results are checked against an actual device by comparing predicted and measured voltages while probing along each emitter finger. Good agreement is obtained for a 24 finger pattern which is typical of present high-voltage switching regulator designs.