Thermal effects in JFET and MOSFET devices at cryogenic temperatures

Various JFET and MOSFET devices have been studied at LN and LHe temperatures. Transient and steady-state heating of the devices at 4.2 K is investigated and it is found that the active part of the device typically heats to a steady-state temperature of 40-60 K. A transient and steady-state heating model of the device is constructed and the results are found to be in good agreement with the measured temperatures and heating transients. Studies of the noise at the various ambient temperatures show that different physical phenomena are responsible for the noise. Low-frequency noise in JFET\´s seems to be of generation-recombination type. Thermal noise is prevalent in the frequency region between 100 kHz and 1 MHz. The noise in some of the MOSFET devices increases with decreased temperature and seems to be surface state or "flicker" noise, The noise in MOSFET devices is by factor 5-100 times larger than the noise in investigated JFET devices.