ZnO nanowire (NW) UV photodetectors with high internal photoconductive gain have been fabricated and characterized. These devices exhibit a photoconductive gain as high as G ~ 10^8, attributed to oxygen-related hole-trap states at the NW surface, which prevent charge-carrier recombination and prolong the photocarrier lifetime. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects. ZnO is emerging as a potential alternative to GaN in optoelectronic applications due to its wide band gap (Eg = 3.4 eV), low cost, and ease of manufacturing. ZnO NW photodetectors and optical switches have been the subject of extensive investigations. The high photosensitivity of ZnO NWs is due to the large surface-to-volume ratio and the presence of deep level surface trap states, which prolong the photocarrier lifetime, and the reduced dimensionality of the active area, which shortens the carrier transit time. The combination of long lifetime and short transit time of charge carriers results in substantial photoconductive gain. The photoconduction mechanism involves fast carrier thermalization and trapping at the NW surface and electron-hole recombination at extended and localized states. The results demonstrate the uniqueness of NWs for photosensing applications and enable the design of novel photodetector architectures. The ZnO NWs used in this study were grown by chemical vapor deposition (CVD) using a simple tube furnace at 925°C. The as-grown single-crystal ZnO NWs had diameter of 150–300 nm and length ranging from 10 to 15 μm. After growth, the NWs were transferred onto a thermally oxidized Si substrate and patterned with interdigitated electrodes. The results of photocurrent measurements performed on single NW devices in standard ambient conditions are summarized in Figure 1. The photocurrent increases significantly under illumination, with the current increasing from 2 to 5 orders of magnitude when the light intensity varies from 6.3 μW/cm² to 40 mW/cm². The photoconductive gain is defined as the ratio between the number of electrons collected per unit time and the number of absorbed photons per unit time (G = Nel / Nph), which can be derived from the equation. The extremely long photocarrier lifetime combined with the short carrier transit times due to the reduced dimensionality of the NW devices results in photoconductive gain as high as G = 2 × 10^8. The high gain values result in large gain-bandwidth products, implying that a significant photoresponse is expected in NW photodetectors even at high modulation frequencies. The photoconductivity of ZnO NWs is strongly dependentZnO nanowire (NW) UV photodetectors with high internal photoconductive gain have been fabricated and characterized. These devices exhibit a photoconductive gain as high as G ~ 10^8, attributed to oxygen-related hole-trap states at the NW surface, which prevent charge-carrier recombination and prolong the photocarrier lifetime. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects. ZnO is emerging as a potential alternative to GaN in optoelectronic applications due to its wide band gap (Eg = 3.4 eV), low cost, and ease of manufacturing. ZnO NW photodetectors and optical switches have been the subject of extensive investigations. The high photosensitivity of ZnO NWs is due to the large surface-to-volume ratio and the presence of deep level surface trap states, which prolong the photocarrier lifetime, and the reduced dimensionality of the active area, which shortens the carrier transit time. The combination of long lifetime and short transit time of charge carriers results in substantial photoconductive gain. The photoconduction mechanism involves fast carrier thermalization and trapping at the NW surface and electron-hole recombination at extended and localized states. The results demonstrate the uniqueness of NWs for photosensing applications and enable the design of novel photodetector architectures. The ZnO NWs used in this study were grown by chemical vapor deposition (CVD) using a simple tube furnace at 925°C. The as-grown single-crystal ZnO NWs had diameter of 150–300 nm and length ranging from 10 to 15 μm. After growth, the NWs were transferred onto a thermally oxidized Si substrate and patterned with interdigitated electrodes. The results of photocurrent measurements performed on single NW devices in standard ambient conditions are summarized in Figure 1. The photocurrent increases significantly under illumination, with the current increasing from 2 to 5 orders of magnitude when the light intensity varies from 6.3 μW/cm² to 40 mW/cm². The photoconductive gain is defined as the ratio between the number of electrons collected per unit time and the number of absorbed photons per unit time (G = Nel / Nph), which can be derived from the equation. The extremely long photocarrier lifetime combined with the short carrier transit times due to the reduced dimensionality of the NW devices results in photoconductive gain as high as G = 2 × 10^8. The high gain values result in large gain-bandwidth products, implying that a significant photoresponse is expected in NW photodetectors even at high modulation frequencies. The photoconductivity of ZnO NWs is strongly dependent