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InGaN-GaN multiquantum-well (MQW) metal-semiconductor-metal (MSM) photodetectors (PDs) with the unactivated Mg-doped GaN cap layer were successfully fabricated. It was found that we could achieve a dark current by as much as six orders of magnitude smaller by inserting the unactivated Mg-doped GaN cap layer. For MSM photodetectors with the unactivated Mg-doped GaN cap layer, the responsivity at 380 nm was found to be 0.372 A/W when the device was biased at 5 V. The UV-to-visible rejection ratio was also estimated to be around 1.96 times 103 for the photodetectors with the unactivated Mg-doped GaN cap layer. With a 5-V applied bias, we found that minimum noise equivalent power and normalized detectivity of our PDs were 4.09 times 10-14 W and 1.18 times 1013 cmmiddotHz0.5W-1, respectively. Briefly, incorporating the unactivated Mg-doped GaN layer into the PDs beneficially brings about the suppression of dark current and a corresponding improvement in the device characteristics.