The metal-semiconductor-metal (MSM) photodetector has the desirable attributes of large bandwidth and ease of fabrication. The lateral structure of the MSM detector allows easy incorporation into optoelectronic integrated circuits. In this paper, a new, simplified, broad-band model of the MSM detector is presented. Practical MSM detectors often exhibit an undesirable low frequency gain that is bias-dependent. It is shown in this paper that the trapping process involved in producing this gain can be modeled, in part, by including a passive equivalent circuit within the circuit model of the detector. The components of the equivalent circuit are related to the trap lifetime and the probability of a hole becoming trapped. The nonlinear effect resulting from the saturation of the electron velocity is modeled as a nonlinear current source whose magnitude is a function of bias voltage. The distributed nature of the interdigitated structure is modeled by a single set of coupled transmission lines. The trapping model and associated current sources are incorporated at the ends of the transmission lines to produce the overall model for the compete detector. Tests have shown that the proposed model provides good agreement with previously published experimental results.