A solution to the photoconductive response of a high-field channel, such as that typified by a FET, is presented for the case of low-level generation. The theory of transit-time photoconductivity is extended to include arbitrary channel field, thickness, and carrier-density profiles, arbitrary carrier generation and injection conditions, carrier heating, and Poisson self-consistency. Several limiting cases are examined and the results are then applied to a Si MOSFET and a GaAs MESFET as two examples.