Effects of Bias on the Responsivity of GaN Metal–Semiconductor–Metal Photodiodes

We report on the fabrication and characterization of AlGaN MSM photodetectors, which show a low dark current density and a sharp cutoff, with a visible rejection of four to five orders of magnitude, at 5 V bias. The devices behave linearly with optical power, for illumination over and below the bandgap. The study of the responsivity of AlGaN MSM photodiodes reveals a gain mechanism which is only active at bias over 2 V, and for excitation over the bandgap. This mechanism is responsible for the superlinear increase of the responsivity with bias, and also for the enhancement of the UV/visible contrast observed in these devices. A NEP* lower than 2 pW/Hz1/2 has been obtained in GaN MSM photodetectors, at 28 V bias.