The performances of large focal-plane arrays of quantum-well infrared photodetectors (QWIPs) would be improved, according to a proposal, by incorporating microlenses. In comparison with a similar QWIP array that lacked microlenses, a QWIP array with microlenses could be made to exhibit less dark current [and thus a greater signal-to-noise ratio (SNR)] at a given temperature. Alternatively, the QWIP array with microlenses could be made to exhibit a given SNR at a higher temperature; this would be advantageous in that it would open up the possibility of operating QWIPs in infrared cameras at higher temperatures, reducing the cost of cooling the QWIPs to obtain adequate SNRs.
It would be advantageous to reduce dark current for two reasons:
- The noise current of a photodetector is proportional to the square root of its dark current.
- Along with signal current, dark current contributes to filling of the charge-storage wells of a readout multiplexer. Because of this and because the amount of charge that can be stored is finite, the available charge-integration time decreases with increasing dark current. As the charge-integration time decreases, the SNR decreases.
Assuming that the QWIPs in a given array were designed to operate with back-side illumination, the microlenses would be formed on the back side (the substrate side) of the array (see figure). There would be one microlens for each pixel. The basic function of the microlenses would be to concentrate incident infrared radiation (or preserve optical area) into a fraction of the area of each pixel. Concomitantly, the active device area in each pixel would be reduced to encompass only the reduced illuminated area plus (if desired) an appropriate margin. Inasmuch as the dark current of a QWIP is proportional to its area, the dark current would be reduced accordingly.
Suppose, for example, that a QWIP array had a 50-µm pixel pitch with active pixel areas of 45 by 45 µm. Microlenses could be used to concentrate light into 15-by-15-µm active pixel areas. If the active pixel areas were reduced to 15 by 15 µm, then the dark current at a given temperature would be reduced to 1/9 of its previous value, and therefore, the noise current at that temperature would be reduced to 1/3 of its previous value.
This work was done by Sarath Gunapala, Sumith Bandara, and Fred Pool of Caltech for NASA's Jet Propulsion Laboratory.
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