Date of Award
Doctor of Engineering Science in Electrical Engineering
Raj Pratap Misra
Joseph J. Padalino
Paul O. Hoffmann
The purpose of this dissertation is to examine the behavior of electrons and holes in a semiconductor or diode under conditions of high current density as a function of temperature, and to relate this behavior to the phenomenon of Second Breakdown. The approach used is that of magnetohydrodynamics, the electrons and holes being treated as a plasma "gas" embedded in the dielectric of the semiconductor.
This approach is unique in the following respects:
- This is the first attempt to explain second breakdown in terms of magnetohydrodynamics.
- This is the first time an explanation of pinching in a solid at room temperature has been presented which does not rely on some type of crystal imperfection to initiate the pinching.
- This is the first time variations in the forbidden gap width have been considered as causing voltage drops, and therefore, electric fields in a semiconductor.
The author is convinced that there are really two types of second breakdown, depending upon the emitter bias. The first type of second breakdown occurs when the emitter-base junction is forward-biased, in which case current constrictions are due to pinching of electrons and holes in the base region of the device. This is examined in Part I of the dissertation, where the theory is developed for low electric fields.
Computer calculations of electron-hole concentration and temperature versus distance from the hot-spot center are presented along with infrared data obtained for temperature versus distance for several measured hot spots. Agreement between theory and data is very good.
The theory predicts that second breakdown is due to thermal effects at or near room temperature, and due to magnetic effects at or near liquid nitrogen temperature. This leads to the definition of the transition temperature as an indication of the temperature at which the transition occurs between second breakdown due to Joule heating, and second breakdown due to magnetic pinching.
The most striking conclusion to be drawn from the computer results is that the theory predicts that units are much more susceptible to failure at lower temperatures than at higher temperatures, contrary to popular opinion.
The second type of second breakdown occurs when the emitter-base junction is reverse-biased, in which case current constrictions are due to avalanche effects in the collector-base junction. This is examined in Part II of the dissertation, where the theory is developed for high electric fields. A critical current for the onset of second breakdown is determined as a function of electric field in the collector depletion region. Comparison of published data on the temperature dependence of second breakdown with theory is given first. Then data taken on a reverse-biased test set are presented. The temperatures investigated are 77°K, 195°K, 273°K, and 300°K.
Theory predicts that devices are much more easily driven into second breakdown at low temperatures than at high temperatures, and this is verified experimentally. Experimental agreement with the theory is excellent over the entire temperature range investigated, in complete agreement with the theoretical results obtained in Part I.
Actual devices will have flaws and defects in them, and will fail at power levels below their theoretical capabilities. This leads to the definition of a quality factor, which is a measure of the actual performance of a given device as compared to its theoretical capability.
Fabricius, Eugene David, "The mechanism of second breakdown in transistors" (1968). Dissertations. 1334.