Abstract
The 2-kT current at the emitter-base heterojunction of AlGaAs/GaAs heterojunction bipolar transistor grown by liquid phase epitaxy is found to be much lower than that of a GaAs homojunction diode. By measuring spectral response and current-voltage characteristics of (n)AlxGa1−xAs-(p)Al0.05Ga0.95As diodes having a similar structure to the emitter-base heterojunction, it is found that the 2-kT surface recombination current is determined by the lower doped side of the heterojunction. Therefore, a higher 1- to 2-kT current ratio and thus higher current gain can be obtained when a high-band-gap emitter with lower doping concentration is used. It is also found that by inserting an undoped AlGaAs spacer layer in between the emitter-base heterojunction and applying an edge-thinning design, a high-gain heterojunction bipolar transistor can be routinely obtained.
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