AlGaN metal–semiconductor–metal photodiodes

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/Hz^1/2 in GaN detectors, and about 24 pW/Hz^1/2 in ${\mathrm{Al}}_{\mathrm{0.25}}{\mathrm{Ga}}_{\mathrm{0.75}}\mathrm{N}$ photodiodes.