Tech Briefs

Semiconductor Bolometers Give Background-Limited Performance

These devices can be fabricated inexpensively by use of established silicon-processing techniques.

Semiconductor bolometers that are capable of detecting electromagnetic radiation over most or all of the infrared spectrum and that give background-limited performance at operating temperatures from 20 to 300 K have been invented. The term “background-limited performance” as applied to a bolometer, thermopile, or other infrared detector signifies that the ability to detect infrared signals that originate outside the detector is limited primarily by thermal noise attributable to the background radiation generated external to the bolometer. The signal- to-noise ratios and detectivities of the bolometers and thermopiles available prior to this invention have been lower than those needed for background-limited performance by factors of about 100 and 10, respectively.

Like other electrically resistive bolometers, a device according to the invention exhibits an increase in electrical resistance when heated by infrared radiation. Depending on whether the device is operated under the customary constant-current or constant-voltage bias, the increase in electrical resistance can be measured in terms of an increase in voltage across the device or a decrease in current through the device, respectively. In the case of a semiconductor bolometer, it is necessary to filter out visible and shorter-wavelength light that could induce photoconductivity and thereby counteract all or part of the desired infrared-induced increase in resistance.

The basic semiconductor material of a bolometer according to the invention is preferably silicon doped with one or more of a number of elements, each of which confers a different variable temperature coefficient of resistance. Suitable dopants include In, Ga, S, Se, Te, B, Al, As, P, and Sb. The concentration of dopant preferably lies in the range between 0.1 and 1,000 parts per billion. The dopant and its concentration are chosen to optimize the performance of the bolometer, taking account of the bolometer operating temperature, the temperature of the source of infrared radiation to be detected, and other relevant environmental factors.

An important practical advantage of the use of silicon, in contradistinction to other semiconductors, is that the art of fabrication of electronic devices from silicon is mature, enabling mass production at low cost per device. An additional advantage accrues when indium is used as the dopant: Indium can be incorporated into silicon over a wide range of concentrations with little consequent change in the basic structure of the silicon matrix. Hence, with impunity, the concentration of indium dopant can be set at almost any desired value in an effort to obtain the desired electrical impedance.

This work was done by John Goebel and Robert McMurray of Ames Research Center. For further information, access the Technical Support Package (TSP) free online at under the Semiconductors & ICs category.

This invention has been patented by NASA (U.S. Patent No. 6,838,669). Inquiries concerning rights for the commercial use of this invention should be addressed to the Ames Technology Partnerships Division at (650) 604-2954. Refer to ARC-14577.