The optimum geometry of an interdigitated Schottky barrier metal-semiconductor-metal photodetector (MSM-PD) is discussed. From the calculated MSM-PD capacitance and transit time of optically generated carriers, the response time and quantum efficiency are evaluated and analysed. We propose a simple scaling rule to achieve the best highspeed response of the MSM detector. The optimum interelectrode spacing for interdigitated MSM-PD has been established The potential of different semiconductor materials for high-speed MSM detectors is examined.