GaN UV metal-semiconductor-metal photodetectors (MSM PDs) with an unactivated Mg-doped cap layer and sputtered indium tin oxide (ITO) were fabricated. Compared with conventional MSM PDs without a cap layer, it was found that we could achieve a significantly much smaller dark current, larger UV to visible rejection ratio, and larger normalized detectivity by inserting an unactivated Mg-doped GaN cap layer. The dark leakage current for the MSM PDs with an unactivated Mg-doped GaN cap layer was shown to be about ten orders of magnitude smaller than that for the conventional MSM PDs. Under a 0.5 V bias, the measured responsivity and UV-to-visible rejection ratio were 0.017 A/W and 1.44 x 10(4), respectively, for the MSM PDs with an unactivated Mg-doped GaN cap layer. This result could be attributed to the thicker and higher potential barrier and effective surface passivation after inserting this in situ grown cap layer. With a 1 V applied bias, it was also found that we could achieve a lower noise level and a higher normalized detectivity of 2.67 x 10(10) cm Hz(0.5) W-1 by inserting an unactivated Mg-doped GaN cap layer into MSM PDs. (c) 2008 The Electrochemical Society.