A metal–semiconductor–metal photodetector (MSM detector) is a photodetector device containing two Schottky contacts, i.e., two metallic electrodes on a semiconductor material, in contrast to a p–n junction as in a photodiode. It is thus a kind of Schottky barrier detector, but with two Schottky junctions. During operation, some electric voltage is applied to the electrodes. When light impinges on the semiconductor between the electrodes, it generates electric carriers (electrons and holes), which are collected by the electric field and thus can form a photocurrent.
One normally uses some kind of interdigited electrode structure, where the finger spacing can be as small as 1 μm. The electrode structure can also be ring-shaped, covering an approximately circular area. Light hitting the device from the side of the electrodes is partially blocked by the electrodes, which of course reduces the quantum efficiency – unless the light is fully directed to the area between electrodes. For higher quantum efficiencies, there are back-illuminated devices, where the light impinges from the other side, so that it is not hindered by the electrodes. Another possibility is to use top illumination in combination with extremely thin gold contacts, which are partially transparent. An advantage of top illumination is that the achieved detection bandwidth is usually higher, since the carriers are generated closer to the contact.
For highest speeds, traveling-wave configurations are used, where the input light is sent through an optical waveguide containing the absorbing layer. The electrodes are deposited on top of the waveguide, forming a coplanar waveguide line for the generated microwave signal.
MSM detectors can be made faster than photodiodes. Their detection bandwidths can reach hundreds of gigahertz (with an impulse response function narrower than 1 ps), making them suitable for very high-speed optical fiber communications.
A practically important aspect is that MSM photodetectors, having a relatively simple planar structure, are particularly suitable for monolithic integration with other components on photonic integrated circuits.
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