Abstract: This invention discloses a dual base heterojunction bipolar transistor for use in a number of different application. Current is introduced into one of the base contacts such that current is forced through the base region of the transistor to the other base contact. Because of the different resistances in the base, there will be a voltage potential between one side of the emitter mesa adjacent one of the base contacts and the other side of the emitter mesa adjacent the other base contact. This lateral voltage potential creates current crowding which forces the current density to travel to the perimeter of the transistor. Because the current travels mostly through the perimeter regions of the transistor, this concept can be used for testing for defects in the bulk of the base region by comparing the current gain without current crowding and with current crowding. Also, this concept can be used strictly as a gain control for a heterojunction bipolar transistor.