Transit Time and RC Time Constant Trade-offs in MSM Photodetectors

Abstract

The speed of response of Metal-Semiconductor-Metal (MSM) devices is determined by both the RC time constant and transit time of the optically generated carriers. A trade-off exists between these two parameters; reduction of transit time will increase the RC time constant, hence miniaturization of dimensions will not always increase speed of response. We have derived expressions which show that given the total available area, and minimum geometry allowed by the technology, what is the optimal design, both in terms of rise time and gain-bandwidth product, of the detector. This allows substantial improvement in device response with no processing penalty. IntroductionMetal-Semiconductor-Metal (MSM) photodetectors are simple planar structure consisting of two Schottky metal contacts on a semi-insulating semiconductor as schematically shown in Figure 1 in the usual interdigital configuration. These devices have attracted great deal of attention due to their ease of fabrication and process integrability with compound semiconductor electronic devices, specifically Metal Semiconductor Field-Effect Transistors (MESFETs) and Modulation Doped Field-Effect Transistors (MODFETs) both of which have found increasing application in MMIC circuits. In addition to monolithic integrability, MSM photodetectors have h g h speed of response, easily in tens of Gigahertz with highest cut-off frequency of 510 Gigahertz recently reported [ 11, low dark current, low noise, and improved sensitivity compared to PIN diodes [2],[3]. These devices are hence ideal as input ports to circuits merging photonic and MMIC circuitsThe speed of response of MSM devices is determined by the transit time of the optically generated carriers between the two contacts, the RC time constant of the device, recombination lifetime of the carriers, and traps in the substrate. The last factor can be reduced by epitaxial growth of the photosensitive area. Recombination lifetime contributes to the falling edge of time response and, due to reduced number of recombination centers in an epitaxially grown sample, does not influence FWHM calculations. The speed of device response is then dominated by the first two factors and can be approximated by where tr is the rise time, tt is transit time, and t RC is the RC time constant. The cut-off frequency, or 3dB bandwidth, of the device is then calculated from 0.35 f3db = 7 r L A trade-off exists between the two components of rise time; reduction of the distance between the interdigitated fingers, parameter g in Figure 1, will reduce the transit time of the optically generated carriers between electrodes but will increase the capacitance of the interdigitated structure. Hence, miniaturization of dimensions will not necessarily increase speed of response. A cross-over point exists beyond which reduction of distance, and hence geometry, will degrade performance. This optimal point is determined from (1) by analyzing its components as follows. metal film W semiconducmr substrate Fig. 1 Structure of the MSM Photodetector; g = finger ...

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