GaN ultraviolet photodiodes with (i.e. sample 1) and without (i.e. sample 2) Mg-doped AlGaN/GaN strained-layer superlattice structure were both fabricated. It was found that we could achieve a near constant dark current of around 3 and 7 nA/cm(2) for sample 1 and sample 2, respectively. It was also found that we could achieve a larger photocurrent and a larger photocurrent to dark current contrast ratio from sample 1. The zero-bias peak responsivity was found to be around 0.05 A/W at 356 nm and 0.006 A/W at 363 nm for sample 1 and sample 2, respectively. However, the transient response full-width-half-maximum (FWHM) of sample 1 was found to be longer. For devices with a 500 pm diameter, it was found that the transient response FWHM was 0.6 mus for sample 1 and 0.2 mus for sample 2. The difference in response time between sample 1 and sample 2 should be attributed to the distribution of depletion layer in between the p-layer and the n-type i-layer. (C) 2002 Elsevier Science Ltd. All rights reserved.