A photoconductive model for superior GaAs THz photomixers

Theoretical methods are used to evaluate the THz output from photomixer structures consisting of interdigitated electrodes and planar antennas on top of a low-temperature-grown GaAs layer. Consistent with experiment, the THz power from a standard photomixer is found to be limited by low external quantum efficiency (similar to 1%). This arises primarily from low photoconductive gain, which is attributed to a long transit time (between electrodes) for the majority of photocarriers generated in the structure. The modeling is then applied to an improved structure containing a thinner absorbing layer (approximate to 0.34 mu m for lambda=0.85 mu m pump) with a dielectric mirror below it to induce resonant-cavity absorption near the surface where the gain is higher. Through increased gain and absorptivity, the model predicts approximate to 7x greater THz output for the same optical pump power. (C) 1999 American Institute of Physics. [S0003-6951(99)04430-7].