Recent work has demonstrated that light can be used to both phase-lock and switch IMPATT diodes. However, because the light that was used has energy E higher than band gap Ea, it cannot effectively reach the deeply-buried active region of the diode. The result hds been low optical coupling efficiencies (< 2%). We find that Burstein shift and internal photoemission can be used to enhance the efficiency by tuning E around Eg. These two mechanisms also can be used to define a better phase relationship with the locked microwaves with adjusting the distribution of photoinduced current injected into the active region. Our photoresponse study of Ti/W-GaAs Schottky IMPATT diodes has shown that efficiencies can be increased to 50% and 13% at E = 1.42 eV (using Burstein shift) and 1.38 eV (using internal photoemission), respectively. In the former case, the current is generated over the entire active region. In the latter case, the current is injected from a narrow region, i.e., the junction area.