UV SCHOTTKY SENSORS BASED ON WIDE BANDGAP SEMICONDUCTORS P. ALLEGRINI, P. CALVANI, M. GIROLAMI, G. CONTE, M.C. ROSSI University of Roma Tre, Dept. of Electronic Engineering, Via della Vasca Navale 84 - 00146 Rome, Italy Planar Al/GaN/Ni Schottky diodes were realized on GaN films deposited on sapphire substrates and characterized in the dark and under illumination. The optoelectronic characteristics of GaN photodetectors appear largely influenced by structural defects and impurities, which are clearly detected in photocurrent yield measurements. In particular, an exponential increase of the photocurrent is observed and explained in terms of a barrier lowering photoeffect, hence a light induced shrinking of the space charge region, related to carrier trapping at defects and impurities. Trapping events are also responsible for a dispersive behavior of the AC responsivity with the light chopping frequency. Such effects point out the importance of a proper selection of bias voltage and working frequency for GaN photodetector operations. 1. Introduction In recent years, the increasing interest in the ultraviolet imaging for scientific instrument and astronomy applications has triggered a research effort to develop radiation detectors based on wide band gap materials, showing solar blind spectral response. Among suitable devices, photodetectors based on AlGaN alloys, show high sensitivity, large UV/visible contrast, extremely low dark current and negligible damage caused by operation in harsh environments [1-3]. The photoresponse of these devices can be tuned over the whole UV spectral range from 200 to 360 nm by changing the Al content in a Al/GaN/Ni planar Schottky diodes. However, GaN optoelectronic applications requires a deeper comprehension of fundamental questions about defect related detrimental effetcs. The purpose of this article is to concentrate on this aspect affecting the optoelectronic performance of GaN photodetectors. 2. Experimental GaN samples were grown on sapphire substrate by Molecular Beam Epitaxy (MBE) technique according to a procedure reported elsewhere [4]. For device fabrication samples were cleaned in acid before contacts realization. Planar arrays of ohmic Al contacts and Schottky Ni contacts were thermally evaporated on the sample slab in order to test photoconductor, Schottky photodiode and MSM Schottky detector structures. In each case about 1mm 2 free surface was left for device illumination. Electrical characteristics were measured in the dark by using an HP4140B picoammeter. Current-voltage measurements were performed in a helium cryostat in