Infrared microradiometer studies of operating power transistors

This paper presents infrared temperature measurements on the surface of planar power transistors under steady-state operating conditions. These results present a clear picture of the thermal characteristics of silicon planar epitaxial power transistors as a function of construction and operating conditions. Isothermal contour maps constructed from consecutive infrared profiles clearly demonstrate the thermal characteristics to be radically different at different operating conditions due to hot spot or thermal instability development. The results are discussed in terms of thermal-electrical interaction effects which lead to the nonuniform current concentration causing the hot spots. At moderate voltages, this temperature or current nonuniformity develops continuously as power level is increased. At high voltage, however, device producing very high temperatures. This instability is shown to be independent of case temperature. The relationship between this current instability and "second breakdown" is discussed in detail.