Ultrafast metal-semiconductor-metal ultraviolet photodetectors on GaN with 0.3-mum finger width and spacing were fabricated and packaged with a specially designed fast circuit. The assembly was simulated using a distributed circuit approach with optical illumination at lambda = 270 nm. The results from simulations showed a 30-ps response time at a low illumination level. At high illumination, the response time increases significantly due to the space-charge screening effect. This is the first theoretical simulation report of this effect in ultrafast ultraviolet photodetectors on GaN. Comparison of simulations and measurements was made in a wide range of optical energies, and a close agreement was achieved with a single energy-scaling factor. (C) 2004 Elsevier Ltd. All rights reserved.