III nitrides have become the most exciting challenge in optoelectronic materials in the last decade. Their intrinsic properties and an intense technological effort have made possible the fabrication of reliable and versatile detectors for short wavelengths. In this work, materials and devices issues are considered to provide a full picture of the advances in nitride UV photodetection. First, basic structures like photoconductors, Schottky, p-i-n and metal-semiconductor-metal photodiodes and phototransistors are compared, with emphasis on their specific properties and performance limitations. The efforts in the design and fabrication of more advanced detectors, in the search for higher quantum efficiency, contrast, signal-to-noise or speed operation, are reviewed afterwards. Metal-insulator-semiconductor diodes, avalanche photodetectors and GaN array detectors for UV imaging are also described. Further device optimization is linked with present materials issues, mainly due to the nitride quality, which is a direct result of the substrate used. The influence of substrates and dislocations on detector behaviour is discussed in detail. As an example of AlGaN photodetector applications, monitoring of the solar UV-B radiation to prevent erythema and skin cancer is presented.

A model to explain the behaviour of GaN photoconductive detectors is proposed, and it is based on the idea of a volume modulation rather than a carrier density modulation. Space charge regions inside the semiconductor produce a variation of the conductive volume when carriers are photogenerated. The strong non-exponential photocurrent decays result from carrier capture processes over the barriers associated with space charge regions. By means of computer simulation, this model explains quite well the behaviour of current GaN photoconductor devices and predicts their time response, temperature dependence and responsivity properties.

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We report on the fabrication and characterization of AlGaN metal–semiconductor–metal photodiodes with sharp cutoff wavelengths from 365 to 310 nm. The detectors are visible blind, with an ultraviolet/visible contrast of about 4 orders of magnitude. The photocurrent scales linearly with optical power for photon energies both over and below the band gap, supporting the absence of photoconductive gain related to space-charge regions. No persistent photoconductivity effects have been detected. Time response is limited by the RC product of the measurement system, the transit time of the device being far below 10 ns. The normalized noise equivalent power at 28 V bias is lower than 17 pW/Hz1/2 in GaN detectors, and about 24 pW/Hz1/2 in Al0.25Ga0.75N photodiodes.

Schottky barrier photovoltaic detectors have been fabricated on n-AlxGa1−xN(0⩽x⩽0.35) and p-GaN epitaxial layers grown on sapphire. Their characteristics have been analyzed and modeled, in order to determine the physical mechanisms that limit their performance. The influence of material properties on device parameters is discussed. Our analysis considers front and back illumination and distinguishes between devices fabricated on ideal high-quality material and state-of-the-art heteroepitaxial AlxGa1−xN. In the former case, low doping levels are advisable to achieve high responsivity and a sharp spectral cutoff. The epitaxial layer should be thin (<0.5 μm) to optimize the ultraviolet/visible contrast. In present devices fabricated on heteroepitaxial AlxGa1−xN, the responsivity is limited by the diffusion length. In this case, thick AlxGa1−xN layers are advisable, because the reduction in the dislocation density results in lower leakage currents, larger diffusion length, and higher responsivity. In order to improve bandwidth and responsivity, and to achieve good ohmic contacts, a moderate n-type doping level (∼1018 cm−3) is recommended.