Increase in response time of low-temperature-grown GaAs photoconductive switches at high voltage bias

The response time of photoconductive submillimeter-wave emitters based on low-temperature-grown (LTG) GaAs is known to increase at high applied bias, which limits the output power of these devices at frequencies near 1 THz. We performed measurements of an LTG GaAs photoconductor embedded in a coplanar waveguide with both static and dynamic illumination to investigate the increase in response time and an increase in direct-current photoconductance that occurs at the same bias voltages. We attribute both phenomena to a reduction of the electron capture cross section of donor states due to electron heating and Coulomb-barrier lowering. We discuss why the phenomena cannot be explained by space-charge-limited current or other injection-limited currents, or by impact ionization.