Static Characterisation of Gallium Nitride HEMTs at Cryogenic Temperatures

Understanding the cryogenic performance of Gal-lium Nitride (GaN) devices is essential for determining their suitability for use in the electrification of large-scale transport such as aircraft, where the power electronics may need to withstand cryogenic temperatures. In this paper we investigate the temperature relationship of the channel on-resistance and gate threshold voltage of various GaN devices, using liquid nitrogen as an inexpensive and effective cryogenic medium. Two cascode-configuration high electron mobility transistors (Cascode HEMT) and two enhancement mode high electron mobility transistors (E-HEMT) devices, each from different manufacturers were selected to ensure a range of technologies were covered. The on-resistance of GaN devices was found to decrease approximately linearly with reducing temperature, until a knee-point is reached whereupon the on-resistance rises. One E-HEMT device did not reach a knee point, indicating the need for further testing at lower temperatures to determine whether, or where such a knee point exists. The device with the lowest normalised on resistance at -196.6°C exhibited 32.8% of its room temperature value, while the device with the highest normalised on resistance showed 89.3% of its room temperature value. It was also found that the gate thresh-old voltage shows temperature dependency. For both Cascode manufacturers, the threshold voltage increased linearly with decreasing temperature. For one of the E-HEMTs the threshold voltage increased non-linearly with decreasing temperature. For the other E-HEMTs the threshold voltage decreased linearly with decreasing temperature. Initial results indicate that GaN devices are compatible with operation in or near cryogenic environments. Further characteristics need investigation to fully confirm suitability. Output characteristics, as well switching rise and fall times may change in cryogenic conditions, potentially affecting suitability.