Abstract
Our photoconductive detector is basically a p-i-n-FET, integrated into a single device. Its function is based on the light-induced increase of the conductance in the n-layer of a p-i-n structure, which is provided with two ohmic contacts to the n-layer and one to the p-layer. The goal of designing this device is to obtain the largest possible photoconductive response in the n-channel with a minimum number of photons incident on the detector. Apart from optimizing the quantum efficiency of the p-i-n diode, this goal can be achieved by minimizing the capacitance of the device. Therefore we use a sophisticated sample design with a small detection area, which surrounds a spatially separated large absorption area. Figure 1 shows a SEM picture of the device. The n-layer of the large absorption area is always depleted (etched down with respect to the detection area) and thus does not contribute to the device capacitance.1 Because of the ‘giant ambipolar diffusion constant of such structures,2 the diffusion of the photogenerated carriers from the inner absorption area to the surrounding detection area is fast enough (e.g., τ diff < 50 ps if the diameter of the absorption area is less than 20 μm). This design allows the combination of narrow contact spacings, necessary for a high photoconductive gain, with small RCRC and diffusion time constants. This results in high gain-bandwidth values (>20 GHz). The 3-dB frequency is adjustable by the integrated load resistor.
© 1995 Optical Society of America
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