Radiative Measurement of Temperature in a MESFET Channel

NASA's Jet Propulsion Laboratory, Pasadena, California

A technique based on infrared-emission spectroscopy has been found to be useful for noncontact measurement of the temperature of a hot spot in the gate channel of a GaAs metal/semiconductor field-effect transistor (MESFET). Temperature measurements are important for the development of high-power GaAs MESFET and other advanced semiconductor devices because hot spots can affect operation and reduce operational lifetimes. An older passive infrared-sensing technique provides temperature measurements with a spatial resolution of 15 µm, which is much too coarse for determining local distributions of temperature in state-of-the-art devices with submicron-sized gate structures. The present technique affords a spatial resolution of about 0.5 µm.

The technique involves the use of parts of an apparatus called the "microelectronic advanced laser scanner" (MEALS), which was described in a number of articles in NASA Tech Briefs during the years 1992 and 1993. As shown in Figure 1, the beam from a helium/neon laser is aimed at the device under test. A micromanipulator stage with a resolution of 0.1 µm is used to position the device under test so that the laser beam impinges at the gate position of the device. Light emitted from the illuminated spot on the device is collected by a mirror, chopped, and focused onto a spectrophotometer. The light is also low-pass filtered to discriminate against the light at the illuminating laser wavelength. The photodetector in the spectrophotometer is a cooled photomultiplier tube, and its output is processed through a lock-in amplifier synchronized with the chopper. The output of the lock-in amplifier is digitized, then processed in a personal computer to obtain the spectrum of infrared emission stimulated by the laser beam.

Figure 1. A Laser Beam Impinges on a small spot on the device under test. Spectral measurement of the resulting infrared emission can be used to determine the temperature in the illuminated spot.

The technique exploits the temperature dependence of the wavelength of the peak of the stimulated-infrared-emission spectrum. In general, this wavelength increases approximately linearly with temperature — a consequence of the fact that the energy-band gap of a semiconductor material decreases with temperature. Thus, if the temperature dependence of this wavelength is known, it can be used to determine the local temperature in the device operating at various power levels.

Figure 2. These Infrared Emission Spectra were acquired at different device temperatures. Note the wavelength shift between the spectral peaks for the two temperatures.

The technique was demonstrated on a GaAs MESFET, with no power applied to it, at temperatures of 84.7 and 299 K. As shown in Figure 2, the peak wavelength for the higher temperature exceeded that for the lower temperature by about 10 nm. In another measurement run not indicated in the figure, the exterior of the device was maintained at 84.7 K and power was applied; the peak wavelength for this case was 4.5 nm greater than that for the zero-power case at 84.7 K. By linear interpolation, this wavelength shift indicates that under power, the temperature of the gate rose about 96 K above the exterior temperature of 84.7 K.

This work was done by Quiesup Kim and Sammy A. Kayali of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 149on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

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