Current Characteristics Of Gallium Arsenide Pad

The characterization of circular pad GaAs detectors with guard rings fabricated at the University of Glasgow was performed in the Department of Physics of the Politecnico di Milano, with the use of the Politecnico's equipment and expertise. Current transient measurements were made for reverse and forward bias steps at room temperature. Current voltage curves were also obtained for the detectors. For one pad the temperature dependence of the reverse bias transients and I-V characteristics were obtained. An activation energy was calculated from these results to be 0.90 0.01eV. Current noise spectra were obtained for the detectors for two pad sizes, with reverse bias applied. Three measurements were also made on one of the detectors under forward bias. The noise spectra showed an excess noise component, with a low frequency corner at less than 1kHz, and a at region at higher frequencies. The magnitude of the white noise was approximately half that expected from shot noise theory for the given leakage currents. A fall in the magnitude of the noise was observed at 20kHz which was attributed to generation noise in the bulk. 1 The Detector Design The material used for the detectors was 200m thick semi-insulating gallium arsenide. The wafer had been thinned and polished at Glasgow University. The detectors were designed with a circular pad contact with a guard ring on top of the substrate. The bottom contact was a uniform contact that spread to the edge of the substrate. Two geometries were used one with a pad diameter of 2mm (GDP6) the other of 3mm (GDP3), the width of the guard was 200m and the pad guard separation was 10m for both diameters. The pad and guard metallization layers were identical and rectifying in nature. The reverse contact had a diierent recipe which would be Ohmic if annealed but no annealing had taken place and it was thus rectifying, however its saturation current was larger than the pad's due to its larger surface area. In the following, the direction of the detector bias is quoted with respect to that of the pad. That is reverse bias means that the front contact is at a negative potential with respect to the back contact. Measurements were carried out of the currents through the pad and the guard ring while they were both held at ground with respect to the bias applied to the back contact. The current that owed through …