The effects of heavy impurity doping on AlGaAs/GaAs bipolar transistors

The effects of heavy impurity doping on the electrical performance of AlGaAs/GaAs heterojunction bipolar transistors are examined. Electrical measurements of GaAs diodes and transistors demonstrate that the electron current injected into p⁺-GaAs is unexpectedly large. These results provide evidence for a large effective bandgap shrinkage in p⁺-GaAs. The results are presented in a form suitable for device modeling. For the heavy p-type doping commonly used in the base of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor, the effective bandgap shrinkage is comparable in magnitude to the bandgap variation designed into the device by its compositional variation. Two examples demonstrate that such effects must be considered when analyzing or designing such devices