We present the characteristics of a nitride-based UV metal-semiconductor-metal photodetector (MSM PD) with an in situ low-temperature (LT) grown AlN cap layer. From atomic-force microscopy scan images, we could clearly see that surface pits on the sample surface are almost invisible with an LT AlN cap layer but can be observed in a conventional cap layer. Compared with conventional MSM PDs, it was found that we achieved smaller dark current and larger UV-to-visible rejection ratio for the PDs with an LT AlN cap layer. With a 5 V applied bias, the UV-to-visible rejection ratio between 360 and 400 nm was 7.26x10(2) for the MSM PDs with an LT AlN cap layer. The calculated noise equivalent power and normalized detectivity biased at 5 V for the MSM PDs with an LT AlN cap layer was 1.21x10(-11) W and 4.16x10(10) cm Hz(0.5) W-1, respectively. (C) 2008 The Electrochemical Society.