Commercial all-electric aircraft are projected to be flying as early as 2035, where hydrogen is selected to supply fuel and coolant. The cryogenic aircraft's powertrain includes the turbines, generators, power electronics and fan motors. As most of the powertrain is located at cryogenic temperature, it is preferable to co-locate the power electronics in the same region to reduce the engineering system's complexity. Silicon super-junction MOSFETs are known for their high efficiency, thus lower cooling requirements at cryogenic temperature. However, in phase-leg circuits, the behaviour of the MOSFET's intrinsic diode, and its output capacitance are challenging. These issues can be addressed by using the MOSFET with ancillary power devices to realise diode deactivation. This paper compares different permutations of intrinsic diode deactivation devices at room temperature and cryogenic temperature. The paper presents a demonstrator phase-leg built with different ancillary power devices. It is operated from a DC voltage of 270 V, and supplies a phase current of up to 10 A.