The Thermal Runaway Phenomenon of Unipolar Power Semiconductors in Cryogenic Power Electronics

Galliumnitride (GaN) high electron mobility transistor (HEMT) and silicon (Si) Metal-Oxide semiconductor field effect transistor (MOSFET) power devices exhibit greatly reduced conduction losses at cryogenic temperatures. Consequently, cryogenic cooling is currently largely investigated in aviation research. However these temperatures lead to a hitherto largely unnoticed, safety-critical phenomenon of “thermal runaway”, which becomes increasingly severe as the coolant temperature drops. This effect arises because the losses generated in the chip increase more rapidly with rising temperature than the heat that can be conducted away from the chip. Contrary to the well-known behavior at room temperature, a higher current density cannot be achieved by arbitrarily increasing the temperature swing. Instead, a tipping point is reached beyond which the system enters thermal self-destruction. Here, this effect is described theoretically and validated experimentally at liquid nitrogen temperatures using a GaN HEMT.