Impact of bond angle distortions on the temperature dependent band structure and subthreshold swing behavior of UTB SOI MOSFETs

Abstract An unexpected deviation has been observed in the subthreshold swing (SS) behavior at lower device temperatures, between theoretical predictions (expected to follow Boltzmann temperature scaling) and experimental results, which has mostly been attributed to the presence of band tail states. However, this non-Boltzmann like behavior of MOSFETs, at low device temperatures, may also be caused by other reasons such as various kinds of stress/strain effects, that might result in bond angle distortions, which require greater investigation. Hence, in this work, we consider bond angle distortions ( δ ) through variations of direction cosines, resulting in the modification of the band structure, impacting the channel electrostatics and current behavior, leading to non-Boltzmann-like behavior in SS. In order to study the device manifestations of these distortions, we consider an ultra-thin-body (UTB) double-gate (DG) silicon-on-insulator MOSFET where we incorporate bond angle distortions ( δ ) into the s p 3 d 5 s ∗ tight binding Hamiltonian, thus causing these distortions to be reflected in the channel thickness and temperature-dependent band structure, while also impacting the I–V characteristics and SS behavior. In addition, through considering two extreme channel lengths in our analysis (viz, L g = 10 nm and L g = 186 nm, respectively), we show the differing impact of bond angle distortions on device performance (including SS behavior), for different device temperatures, where even a slight distortion is shown to significantly impact the I–V behavior of short channel UTB MOSFETs, while on the other hand for a long channel UTB MOSFET, only with a considerable bond angle distortion is there likely to be any discernible impact on the device behavior.