We present an experimental and theoretical analysis of the nonlinear response of cooled extrinsic Ge photoconductors of the type used to detect far-infrared radiation. State of the art p-type Ge devices with nearly ideal noise performance in the absence of drive show characteristic subharmonic and chaotic nonlinear phenomena when periodically driven, with a corresponding degredation in noise performance by a factor ≈ 10 4. A systematic series of experiments and corresponding simulations of a standard model of extrinsic photoconductors, including measurings and simulations of the I–V curves, transient and small signal response, as well as the response to periodic drive, identify the physical transport processes responsible: the nonlinear electric field dependence of the rates of free hole capture and impact ionization due to carrier heating at relatively modest applied electric fields E > 0.1 V/cm.