Dependence of conduction-band discontinuity on aluminum mole fraction in GaAs/AlGaAs heterojunctions

The dependence of the conduction band discontinuity (CBD) on the Al mole fraction in GaAs/AlGaAs heterojunctions was examined for the case of an Al mole fraction over 0.45. AlGaAs has an indirect bandgap when the Al mole fraction exceeds 0.45, in which case the X-valley has its lowest value. When the mole fraction surpasses 0.45, the bandgap increases slowly with the mole fraction, implying the energy bandgap difference increases slowly although the valence-band discontinuity increases quickly. However, since the sum of the valence- and conductance-band discontinuities has to equal the bandgap difference, the CBD has to decrease with an increasing Al mole fraction beyond 0.45. Calculations show that the maximum CBD is about 0.365 eV at a mole fraction of about 0.45, although the precise Al mole fraction of the direct-indirect transition is not yet known.