EXPERIMENTAL-DETERMINATION OF THE EFFECTS OF DEGENERATE FERMI STATISTICS ON HEAVILY P-DOPED GAAS

The effects of degenerate Fermi statistics on electron injection currents for p+ -GaAs grown by molecular beam epitaxy are presented. To achieve Be dopant concentrations of greater than 8 X 10(19) cm-3, the substrate temperature during growth was reduced to approximately 450-degrees-C from the usual 600-degrees-C. In this heavily doped material, we measure unexpectedly large electron injection currents which are interpreted in terms of an effective narrowing of the band gap. At extremely heavy doping densities, the Fermi level pushes into the valence band and degenerate Fermi statistics must be taken into account. For doping concentrations greater than 1 X 10(20) cm-3, effects due to degenerate Fermi statistics oppose the band-gap shrinkage effects; consequently, a reduction in the electron injection currents is observed. The result is a substantial reduction in gain for AlGaAs/GaAs heterostructure bipolar transistors when the base is doped above 10(20) cm-3.