Modeled temperature contours in HEMT device during (a) calibration and (b) operation. Due to the difference in temperature distributions, the stress state during operation will be different than that of calibration. The differing stress states will contribute to errors in the measurement of temperature when utilizing the Stokes peak position.
Schematic showing layout of (a) the entirety of the package, (b) TLM, and (c) HEMT including the location of the Raman spot.
Calibration of (a) Stokes peak position, (b) Stokes linewidth, and (c) the ratio of Stokes to anti-Stokes intensity as a function of temperature at the midpoint of a packaged TLM device. The inset of (c) highlights the shifting, broadening, and reduction in intensity which occurs to the Stokes signal of the Raman response with an increase in temperature. Using these curves, any change in a spectral component may be transformed to temperature if stress induced effects are negligible.
Effect of uniaxial stress on (a) Stokes peak position, (b) Stokes linewidth, and (c) Stokes/anti-Stokes ratio applied on the epilayer stack. Only the peak position shows a dependence on this mechanically induced stress along the nonpolar planar direction.
Effect of stress arising from the inverse piezoelectric effect on (a) the Stokes peak position, (b) linewidth, and (c) the Stokes to anti-Stokes intensity ratio applied through biasing of a HEMT under pinch off conditions. Unlike that seen for loading along the nonpolar direction, the induced stress along the polar (vertical) direction of the crystal affects the linewidth signal.
Operating temperature of TLM device as a function of dissipated power at a package temperature of . The temperature measured using both the linewidth and Stokes/anti-Stokes ratio correlating well with the prediction of the computational model. The peak position significantly underpredicts the temperature as it is affected by the presence of the evolving thermoelastic stress.
Operating temperature of a HEMT at a package temperature of acquired from Raman measurements derived using a standard unpowered reference condition. Only the measurements obtained from the Stokes to anti-Stokes intensity ratio correlate with the predicted operating temperatures due to this aspect’s independence to both thermally and piezoelectric induced stresses.
Operating temperature of a HEMT measured using Raman spectroscopy with a reference taken under pinch off conditions. Using this nonstandard reference condition allows for piezoelectric induced effects to be removed thus allowing for the accurate measurement of temperature through use of the Stokes linewidth.
Material properties utilized in model.
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