We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaNp-i-nultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunctionp-i-nphotodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunctionp-i-ndetectors. We analyzed the effect ofp-layer thickness of the GaNp-i-ndiodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes. ©1997 American Institute of Physics.