AlGaN metal-semiconductor-metal photodiodes

We report on the fabrication and characterization of AlGaN metal-semiconduc tor-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 absen ce of photoconductive gain related to space-charge regions. No persistent p hotoconductivity effects have been detected. Time response is limited by th e RC product of the measurement system, the transit time of the device bein g far below 10 ns. The normalized noise equivalent power at 28 V bias is lo wer than 17 pW/Hz(1/2) in GaN detectors, and about 24 pW/Hz(1/2) in Al0.25G a0.75N photodiodes. (C) 1999 American Institute of Physics. [S0003-6951(99) 03522-6].