On the emitter ledge length effect for InGaP/GaAs heterojunction bipolar transistors

The emitter ledge length effect on the performance of InGaP/GaAs heterojunction bipolar transistors is comprehensively studied. It is shown that the undesired potential saddle point at the edge of emitter ledge is substantially presented. At the saddle point, the electron density and recombination rate are decreased with increasing the emitter ledge length. However, the longer emitter ledge length increases the base-collector junction area which in turn deteriorates the high frequency performance. Therefore, the length of emitter ledge should be carefully considered. Based on the theoretical analysis and experimental results, the optimum emitter ledge length is near 0.8 μm.