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⁺-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 ⁺-n GaAs diodes with Zn dopant concentrations ranging from 6.3×10¹⁷ to 1.3×10¹⁹ cm^-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_ie²D_n was determined, where n_ie is the effective intrinsic carrier concentration and D_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 10¹⁹ cm^-3. These results demonstrate the importance of treating bandgap narrowing effects when modeling and designing HBTs