Electron and hole impact ionization coefficients in GaAs-AlxGa1−xAs superlattices

Electron and hole multiplication and impact ionization coefficients have been measured with pure carrier injection in p+-n−-n+ and p+-n diodes grown by molecular beam epitaxy. Values of the electron and hole ionization coefficient ratio α/β=2–5 are measured for superlattices with well width Lz≥100 Å and α/β>10 is measured in a graded band-gap superlattice with a total well and barrier width LB+LZ=120 Å. The ratio decreases and becomes less than unity for smaller well sizes. This is caused by an increase in β(E) while α(E) remains fairly constant. The results have been interpreted by considering varying hole confinement and scattering in the coupled quantum wells.