Abstract
The DC characteristics of InP-InGaAs and InAlAs-InGaAs HBT's with abrupt emitter-base junctions are studied using a thermionic-field emission boundary-condition model. The model incorporates tunneling and thermionic emission into a one-dimensional drift-diffusion numerical scheme and accounts for breakdown and bulk recombination mechanisms. The effects of abrupt heterojunction transport and electrical junction displacement on the current gain h/sub FE/ and on the turn-on voltage are investigated. The simulations indicate that the spacer layer design has a profound effect on the DC behavior of these devices. A detailed performance comparison of different emitter structures indicates that InP-emitter HBT's show a more uniform h/sub FE/ than InAlAs-emitter HBT's especially at low current densities. Experimental data from a fabricated InAlAs-InGaAs abrupt emitter single HBT was compared to the theoretical predictions of the model. The analysis reveals that several injection and recombination mechanisms are responsible for the emitter-base forward characteristics. In the collector, the exact velocity-field profile and an anomalous multiplication factor are responsible for kinks in the output common-emitter characteristics and for soft breakdown of the collector-base junction. >
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