Milli-Kelvin Analysis Revealing the Role of Band-edge States in Cryogenic MOSFETs

We experimentally performed temperature-dependent current‒voltage (I‒V) analysis of Si n-MOSFETs down to 15 mK, for the first time. We found that the saturated subthreshold swing (SS) at a few Kelvins decreased again in the milli-Kelvin range in proportion to temperature, exhibiting a SS of 0.071 mV/dec at 15 mK. We provided a physical model to explain the behavior of the SS and threshold voltage (Vth) in the milli‒Kelvin range via calculation, thus elucidating the role of band-edge states in cryogenic MOSFETs. We found that the exponentially distributed immobile band-edge states were responsible for the I‒V behavior, indicating that considering the occupancy of band-edge states is key to understanding the operation mechanism of cryogenic MOSFETs. This study provides deep insights into the operation of MOS-based devices at cryogenic temperatures for cryogenic CMOS and silicon qubits.