paper

Cryogenic Subthreshold Swing Saturation in FD-SOI MOSFETs described with Band Broadening

arXiv:1903.05409 · doi:10.1109/LED.2019.2903111

Abstract

In the standard MOSFET description of the drain current $I_{D}$ as a function of applied gate voltage $V_{GS}$ , the subthreshold swing $S S (T) \equiv d V_{GS} / d lo g I_{D}$ has a fundamental lower limit as a function of temperature $T$ given by $S S (T) = ln 10 k_{B} T / e$ . However, recent low-temperature studies of different advanced CMOS technologies have reported $S S$ (4K or lower) values that are at least an order of magnitude larger. Here, we present and analyze the saturation of $S S (T)$ in 28nm fully-depleted silicon-on-insulator (FD-SOI) devices for both n- and p-type MOSFETs of different gate oxide thicknesses and gate lengths down to 4K. Until now, the increase of interface-trap density close to the band edge as temperature decreases has been put forward to understand the saturation. Here, an original explanation of the phenomenon is presented by considering a disorder-induced tail in the density of states at the conduction (valence) band edge for the calculation of the MOS channel transport by applying Fermi-Dirac statistics. This results in a subthreshold $I_{D} \sim e^{e V_{GS} / k_{B} T_{0}}$ for $T_{0} = 35$ K with saturation value $S S (T < T_{0}) = ln 10 k_{B} T_{0} / e$ . The proposed model adequately describes the experimental data of $S S (T)$ from 300 down to 4K using $k_{B} T_{0} ≃ 3$ meV for the width of the exponential tail and can also accurately describe $S S (I_{D})$ within the whole subthreshold region. Our analysis allows a direct determination of the technology-dependent band-tail extension forming a crucial element in future compact modeling and design of cryogenic circuits.