A novel emitter edgehyphen;thinning structure was adopted forNpnAl0.5Ga0.5As/GaAs single heterojunction bipolar transistors grown by liquid phase epitaxy. In this structure, the emitter edge was etched down to approximately 0.1 mgr;m thick such that the surface and emitterhyphen;base junction depletion regions could touch each other. As a result, the current is blocked from the emitter periphery and the surface leakage current is reduced which improves the current gain especially at low operating current. The best device thus obtained shows a common emitter current gain of 12thinsp;500 at a collector current of 50 mA which is the highest gain reported to date for the heterojunction bipolar transistors. The current gain characteristics were indeed improved especially at a collector current below 10 mgr;A.
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