In this paper, the performance of GaN metal-semiconductor-metal (MSM) photodetectors is analyzed based on the active layer parameters. The calculated results show the dependence of dark current on carrier concentration for n- and p-type active layers in GaN MSM structures. It is found that the increase in the finger spacing and layer thickness in the MSM structures increase the steady-state photocurrent but do not show strong influence on the transient photocurrent response. (C) 2003 Elsevier Ltd. All rights reserved.