A novel technique to reduce leakage in metal-semiconductormetal photodetectors

High efficiency and low power photodetectors operating at low absorption regions of silica fibers are attractive for both long distance and chip scale communications. Optical interconnects are promising to alleviate many limitations faced by their electrical counterparts (Miller, 2000). Easy integration of photodetectors with mainstream Si-ICs is a key aspect to realize on-chip optical clocking/signaling on dense integrated systems. Among various photodetector structures, metal-semiconductor-metal photodetectors (MSM-PDs) are attractive for their high sensitivity-bandwidth product, low capacitance and remarkable ease of integration. However, relatively large dark current (I_dark) associated with a lower bandgap and thus low Schottky barrier MSMs (vs. PIN diodes (Oh et al.,2002)) like Ge and Si, poses additional power dissipation, an increasingly serious problem in today´s dense integrated systems. In addition, the resultant increase in noise level demands higher optical powers for minimum detectable signal. The paper have investigated the possibility to suppress leakage current by utilizing asymmetric-area contacts on a basic MSM structure with two back-to-back Schottky diodes. Under an applied bias, an identical current flowing through the unequal area electrodes, results in an enhanced depletion at the smaller-area contact due to higher electric field at this electrode. This in turn decreases the reach through voltage, the applied bias at which the sum of the depletion widths extends through the electrode separation (i.e. total depletion operating condition). 2D MEDICItrade simulations of Si-based interdigitated MSM structures were used to verify dark current reduction with this area asymmetry scheme