A focal-plane array (FPA) of GaAs-based quantum-well infrared photodetectors (QWIPs) that would detect images in two wavelength bands simultaneously is undergoing development. From ratios between image intensities at the two wavelengths, one can calculate the temperatures of imaged objects by use of Planck's radiation law. In several respects, this device is similar to the one described in "Two-Wavelength Focal-Plane Array of QWIPs" (NPO-19658) NASA Tech Briefs, Vol. 22, No. 1 (January 1998), page 8a. Both devices are intended to serve as prototypes of multispectral imaging devices for a variety of scientific, industrial, and military infrared instruments.

QWIPs that operate in medium-wave infrared (MWIR) and long-wavelength infrared (LWIR) bands have been undergoing development in recent years. The best previously available two-wavelength QWIP contains two stacked, voltage-tunable QWIP structures - one for MWIR and one for LWIR. Two difficulties arise in connection with attempts to utilize that device in an FPA: (1) the device must be supplied with two voltages, which cannot be obtained from any currently available readout multiplexers; and (2) a high bias (>8V) must be supplied to the LWIR segment to switch on LWIR detection.

MWIR and LWIR QWIPs Would Be Short-Circuited in alternate rows so that the array would detect interlaced LWIR and MWIR images. This is a schematic partial cross-sectional view looking along the rows.

The present device is designed to overcome these difficulties. It is based partly on the QWIP FPA in a portable camera developed recently by the same innovators. The array would contain 512 × 484 pixels. Each pixel would contain 25 periods of an MWIR QWIP structure stacked with 25 periods of an LWIR QWIP structure. The two stacks would be separated by a heavily doped intermediate GaAs contact layer. The responses of the MWIR and LWIR QWIP structures would peak at wavelengths of 4.5 and 9 μ m, respectively. This entire stack as described thus far would be sandwiched between doped GaAs contact layers and grown on a semi-insulated GaAs substrate by molecular-beam epitaxy. A GaAs cap layer would be added on top of a thin AlGaAs stop-etch layer on top of the device to fabricate a light-coupling optical cavity.

All pixels would contain both the MWIR and LWIR structures. However, during a metallization sequence that would be part of the fabrication process, the MWIR QWIPs on odd-numbered rows and the LWIR QWIPs on even-numbered rows of the FPA would be short-circuited (see figure). This interlaced short-circuiting of the MWIR and LWIR detectors would eliminate complicated voltage tuning and the necessity for very high bias voltage to operate the LWIR QWIPS. The QWIP FPA would be hybridized to a 512 × 484 complementary metal oxide/semiconductor (CMOS) multiplexer, which would read the alternating LWIR and MWIR rows to produce both an MWIR and an LWIR image of the same scene.

This work was done by Sarath D. Gunapala of Caltech and Kwong Kit Choi of the U.S. Army Research Laboratory for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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Refer to NPO-19837

This Brief includes a Technical Support Package (TSP).
Improved two-wavelength focal-plane array of QWIPs

(reference NPO19837) is currently available for download from the TSP library.

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This article first appeared in the July, 1998 issue of Electronics Tech Briefs Magazine.

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