A calculation is described of the transient pulse response of a planar metal‐semiconductor‐metal photodetector consisting of Schottky contacts made to an active layer of semi‐insulating InGaAs:Fe that is supported on an InP:Fe substrate. The simulation uses a two‐dimensional, drift/diffusion calculation and includes external circuit elements. Realistic conditions are considered where the semi‐insulating material is represented by a two‐level compensation model with Fe as a deep acceptor and hole trap that compensates shallown‐type impurities. The calculated results are compared directly with experimental ones for micron‐scale devices described in the literature. The calculation gives a microscopic picture of how trapping controls particularly the falling side of the transient response, and it also shows how the measured performance of the device can reflect the influence of typical external circuit elements.