Response of semi-insulating GaAs detectors to near-infrared picosecond light pulses

Semi-insulating gallium arsenide (SI-GaAs) detectors, in the form of Schottky diodes, have been irradiated from the front or from the back contact or from the side, with light pulses from picosecond laser source, tunable in the range of 780–1020nm and focused to a 50–100 um spot on the surface of the detector. Energy per pulse was in the range of 4–25 pJ, with a 10 or 100 Hz repetition rate. Each pulse mimics the interaction ofa single energetic particle with the substrate, but with a photon mean free path modulated in the range of 1 mm–1 cm, depending on the light wavelength. The SI-GaAs substrates have a thickness of 200 um (VGF grown), or 600 um (LEC) or 1000 um (VGF). The time response ofthe detectors, analysed with the Transient Current Technique (using a fast digital sampling oscilloscope) in terms of peak signal amplitude and charge rise time, has been evaluated as a function of: reverse voltage bias (50–1200 V), light wavelength (780–1020 nm), side position between the electrodes, position on the front contact. The results are: (1) the shape ofthe transient current signal may show the presence of two peaks, about 1 ns apart; (2) the charge rise time, considered to be dominated by the charge collection time, is between 2 and 13 ns; (3) the charge rise time, as due to electron drift or hole drift in the region between the contacts, has been measured as 2–5 ns, also dependent on the applied bias; (4) the transient current (exponential) decay time, for front or back irradiation, increases with the bias voltage up to 2–20 ns and these data are considered to be consistent with the electron detrapping time; (5) under reverse bias, for 1mm pad side and a large ohmic contact on the back, the electric field extends around the front contact up to about 200 um.