<title>High-gain GaAs MSM photodetector</title>

Abstract

Photodetectors with metal-semiconductor -metal (MSM) structures having very high quan turn efficiency are investigated to identify the gain mechanism. From the temperature dependence studies impact ionisation of carriers in a region under the edge of the cathode is shown to be the predominant gain mechanism. Metal-Semiconductors-Metal (MSM) structure has recently emerged" 2 as an attractive device for photodetection due to its simple planar tech nology which can be easily adapted to optoelec tronic integration. The other features of MSM photodetector are their high sensitivity, low capacitance, low dark current and high speed. Substatial amount of work has been done on GaAs MSM photodiodes which are useful in near infrared wavelength regime. Devices with very low dark current (lOOpA at 1OV for 200x200pm geometry) and low rise and fall times (23 and 55 ps respecti vely) have been recently fabricated on semi-insu lating (SI) GaAs substrates1. The other interest ing feature of MSM structure is that it has in ternal gain. Despite intensive study of these devices there is still confusion about the physi cal origin of the gain in these devices. Ito et a13 attribute the gain to hole injection at the forward biased anode contact. Measurements of Zirngibl et al however, point to avalanche mechanism of gain in these devices. In this paper we report the results of experimental in vestigation of gain mechanism in GaAs MSM struc tures which have extremely high gain (100).