tion, high-speed chip-to-chip connection, and high-speed sampling. Traditionally, PIN (p-doped/intrinsic/n-doped) photo diodes and APD's (avalanche photodiodes) have been the preferred optoelectronic detectors for use in fiber communication systems. In recently years, MSM photodetectors have emerged as a viable alternative because their ease in fabrication, fabrication co mpatibility with field-effect transistors (FET's), and their improving performance.

    The capacitance of a MSM photodetector with equal finger spacing and width is only one quarter that of a PIN diode having same photosensitive area.Hence, despite the 50 % lower efficieny due to blocking of the incoming light with its metal fingers, the MSM detector combined with an integrating receiver would be expected to double the reponsivity-bandwidth product which could be obtained using a PIN diode. A MSM detector can be integrated on a chip with high-speed circuits with relatively minor fabrication modifications, in most cases without adding additional lithography steps.

2. Device Design Rules

    There are many elements that are important to high-speed operation
of MSM photodetectors. For transit-time-limited detectors, the
intrinsic response time depends strongly on the finger spacing and
applied bias. Moreover, when the photon penetration depth is larger
than the finger spacing, large number of carriers are generated deep
inside the semiconductor bulk. It takes a long time for these carriers
to reach the electrodes, so the photon penetration depth is also an
important factor to detector's speed. However, for recombination
limited detectors, the intrinsic response time depends only on the
carrier lifetime and the photon pen etration depth is insignificant to
the speed. As in all kinds of high-speed devices, parasitic elements
often play an important role in device operation and design.

    Through the experimental data, the simulation of impulse response,