Measurement of bandgap narrowing effects in p-GaAs and implications for AlGaAs/GaAs HBT performance

The authors report the first electrical measurements of bandgap narrowing effects in p/sup +/-GaAs and explore their influence on the performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs). A sequential etch technique was used to measure electron injection currents in MOCVD (metalorganic vapor-phase epitaxy)-grown p/sup +/-n GaAs diodes with Zn dopant concentrations ranging from 6.3*10/sup 17/ to 1.3*10/sup 19/ cm/sup -3/. The measurement consisted essentially of characterizing the n=1 electron injection current as a function of thickness of the p-type layer. From the measured n=1 saturation current density, the product, n/sub ie//sup 2/D/sub n/ was determined, where n/sub ie/ is the effective intrinsic carrier concentration and D/sub n/ is the minority-carrier diffusion coefficient. This product was observed to increase greatly with doping and was still a factor of two to three greater than expected, even when bandgap shrinkage was accounted for. This affects AlGaAs/GaAs n-p-n HBTs by increasing the electron current injected into the base. Bandgap narrowing effects increase the collector current by more than one order of magnitude when the base doping exceeds 10/sup 19/ cm/sup -3/. These results demonstrate the importance of treating bandgap narrowing effects when modeling and designing HBTs.<
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