Numerical Simulations of Gate-Granularity- Induced Subthreshold Characteristics Deterioration of MOSFETs Magnified at Cryogenic Temperatures

While gate workfunction fluctuation causes the threshold voltage shift of transistors, it leads to off- and on-state current variations with a given supply voltage and circuit performance degradation at room temperatures. However, the impacts of gate workfunction fluctuation on device electrical characteristics at cryogenic temperatures are still unclear yet. Based on self-consistent numerical simulations, we report in this work that cryogenic operation of n-channel transistors magnifies the influence of gate granularity on the device performance variation in terms of threshold voltage, subthreshold swing and drain currents. Gate granularity roughens the channel potential landscape and forms the local minima on the conduction band for electron transport, and which effectively reduces the device channel width. Our results highlight current deterioration in the subthreshold and weak inversion regions at cryogenic temperatures due to gate granularity. It may result in subthreshold swing saturation at cryogenic temperatures and pose a challenge to supply voltage scaling for power management. Device physics is explained and potential solution is discussed in this paper.