A photoconductive model for superior GaAs THz photomixers

Theoretical methods are used to evaluate the THz output from photomixer str uctures 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 qua ntum efficiency (similar to 1%). This arises primarily from low photoconduc tive 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 lay er (approximate to 0.34 mu m for lambda=0.85 mu m pump) with a dielectric m irror below it to induce resonant-cavity absorption near the surface where the gain is higher. Through increased gain and absorptivity, the model pred icts approximate to 7x greater THz output for the same optical pump power. (C) 1999 American Institute of Physics. [S0003-6951(99)04430-7].