Physical mechanisms responsible for nonlinear phenomena and anomalous transient response of cooled extrinsic far‐infrared photoconductors are discussed. A simple model describing carrier generation, trapping, and impact ionization is presented, which describes the transient response on fast time scales 10−3to 10−4sec, neglecting changes in space charge. Carrier heating by a dc electric field produces relatively fast, damped oscillatory response to external excitation. A small‐signal analysis of these equations is a test of stability. An analysis of the role of ideal electrical contacts and space charge is also presented. The very slow (∼1 sec) overshoot and transient response commonly observed in cooled extrinsic photoconductors is explained by the dynamics of trapped space charge near the injecting electrical contact. A small‐signal analysis determines the characteristic time constants for these processes, which are typically ∼1 sec. Calculated examples of the recombination and ionization coefficients, dcI‐Vcurves, differential equation flow diagrams, and transient response are presented for parameters typical of  p‐type Ge photoconductors doped with shallow acceptor levels, and suggestions for the design of more stable photoconductors are presented.