Impulse response of metal-semiconductor-metal photodetector (MSM-PD) to a short light pulse is computed and discussed in terms of the distribution of photogenerated carriers and electric fields. A two-dimensional self-consistent time-dependent technique has been applied to investigate carrier drift processes in the active volume of high-speed MSM-PD structures with small contact separation. We analyze the limitations imposed by reduced dimensions of the interdigitated electrode system of the MSM-PD. We show that the heterobarrier structure in the light-absorbing region greatly affects the movement of optically generated carriers and enhances the response speed of InP/GaInAs MSM-diode. (C) 2002 Elsevier Science Ltd. All rights reserved.