Abstract

The development of the power MOSFET allows an electronic circuit designer greater flexibility in high power applications. Over the last several years, it has been demonstrated that there are marketable advantages of operating the power MOSFET devices at cryogenic temperature. This paper demonstrates by experiment and theoretical derivation that the on-resistance decreases significantly when the operating temperature decreases. Such a decrease leads to a considerable reduction in heat generation inside the MOSFET device. In other words, it becomes possible to push higher current through the MOSFET device with the same heat generation. It makes higher integration of MOSFET devices possible. This paper shows an experimental measurement of on-resistances at 77 K, 173 K, 243 K and 295 K by applying cryogenic cooling techniques. The results have been confirmed by comparing them with the threshold voltage calculations. Another important advantage of operating MOSFET devices under cryogenic temperature is that the thermal resistance of the device decreases. The thermal conductivity of silicon material increases about nine times when the temperature decreases from 295 K to 77 K (148 W/mK to 1340 W/mK). This will significantly decrease the internal thermal resistance of a MOSFET device. The junction temperature is the parameter under which all the characteristics of MOSFET should be evaluated. When the MOSFET is operating under an extremely high power condition, although there are several effective cooling techniques to maintain the heat sink temperature within a certain range, the junction temperature can be kept at a reasonable temperature only if the internal thermal resistance is small. By knowing the internal thermal resistance, heat generation rate and the sink temperature, the junction temperature can be calculated. This paper proposes a simple method to determine the internal thermal resistance under any power and any operating temperature. The experiment found that at liquid nitrogen temperature (77 K), the internal thermal resistance drops to 1/5 of that at room temperature (295 K). These results indicate that at cryogenic operating temperature, it is possible to obtain high power application while still maintain relatively low temperature at the junction.

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