Abstract:
The emitter-mesa surface (perimeter) and emitter-base bulk recombination currents, in heterojunction bipolar transistors (HBTs), are known to degrade device performance at low applied voltages, where these currents dominate. To study the effects of the etchant (used in defining the diode mesas) on the perimeter recombination current, GaAs homojunction diodes were given a short time etch in a selection of wet chemical etchants and the effect on the edge leakage current investigated. It was observed that the edge leakage current could be modified by the process used to fabricate the mesa. Following encouraging results, the work was extended to include HBTs, and an investigation was undertaken of the effects on recombination at the perimeter of the emitter-mesa and on bulk recombination current, with surface treatments. A simple model was used to separate the bulk and perimeter contributions from the recombination current. The surface recombination current was strongly dependent on the process used to define the mesa, and can therefore, by appropriate processing, be reduced.