THE STUDY OF EMITTER THICKNESS EFFECT ON THE HETEROSTRUCTURE EMITTER BIPOLAR-TRANSISTORS

AlGaAs/GaAs heterostructure emitter bipolar transistors were grown with emitter thicknesses varied from 300 to 900 angstrom and the emitter thickness effects on the current gain and offset voltage were studied. It was found that both the current gain and offset voltage are strongly dependent on the emitter thickness. The current gain decreases with increasing emitter thickness. For an emitter thickness smaller than 500 angstrom, the offset voltage decreases with increasing emitter thickness, but for an emitter thickness larger than 500 angstrom, the offset voltage stays at a nearly constant value. Offset voltage as low as 55 mV was obtained for an emitter thickness of 700 angstrom. This low offset voltage, mostly contributed by the geometric effect, indicates that the base-emitter junction is a homojunction. From the current gain and offset voltage considerations, the optimal emitter thickness was found to be about 300-500 angstrom.