Integrated-circuit image sensors of a proposed type would be capable of operation in either the wavelength band of 0.5 to 2.5 *m, the wavelength band of 2.8 to 5.8 *m, or both bands simultaneously. Called "dual function, thermal optical image sensors" (DTOISs), these sensors could be useful in a variety of scientific, industrial, and medical applications in which there are needs for visible and infrared imaging -- typically, for noncontact measurements of temperature distributions. In medicine, for example, such measurements could provide early indications of developing tumors.

Each pixel of a DTOIS (see figure) would contain a"lifted," bridged InGaAs Schottky diode for sensitivity in the 2.8-to-5.8-*m band, integrated with an InP junction field-effect transistor (JFET). Also integrated with the bridged Schottky diode and JFET would be the already developed InGaAs positive/intrinsic/negative (PIN) photodiode (not shown in figure) for sensitivity in the 0.5-to-2.5-*m band. Each of these sensory devices would be connected to a dedicated active pixel readout circuit so that the image in either or both wavelength band(s) could be read out. The pixel-addressing scheme of the readout circuits would involve the use of JFETs instead of conventional multiplexers.

The Sensory Devices in each pixel of a DTOIS would be a bridged Schottky diode (sensitive in the longer-wavelength infrared band) and a PIN-diode (sensitive in the shorter-wavelength visible/infrared bands, not shown in the figure) with JFET(s).

The planned development of the DTOISs would build on previous accomplishments in the continuing development of InGaAs active-pixel sensor arrays, each pixel of which contains an InGaAs photogate-type sensory device integrated with a low-leakage InP JFET. The planned development would also incorporate techniques developed previously for the fabrication of GaAs Schottky diodes for operation at frequencies from 240 to 640 GHz. Current plans for proof of the DTOIS concept call for development of a prototype DTOIS for a microbiological application. The major technical challenge will be to design a lifted Schottky diode to increase thermal sensitivity so that spatial, temporal, and thermal resolutions could be superior to those of pre-existing detectors. Inasmuch as an array of lifted Schottky diodes would supplant the silicon-resistor arrays of state-of-the-art thermal sensors, the thermal resolution of the DTOIS is expected to be finer than the value of 0.04 K claimed previously for a monolithic silicon focal-plane array. Also, because of the high charge-carrier mobility of InGaAs, the thermal response of the DTOIS is expected to be faster than that of a silicon sensor by a factor of about 6. The ability to detect photons in the 2.8-to-5.8-*m wavelength band would constitute an additional advantage over a silicon sensor.

This work was done by Quiesup Kim of Caltech forNASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the category Electronic Components and Circuits.

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-20430 , volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Dual-Function Microelectronic Sensors

(reference NPO20430) is currently available for download from the TSP library.

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This article first appeared in the January, 1999 issue of NASA Tech Briefs Magazine.

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