Cryogenic Subthreshold Swing Saturation in FDSOI MOSFETs Described With Band Broadening
Abstract
In the standard MOSFET description of the drain current $I_{D}$ as a function of applied gate voltage $V_{GS}$, the subthreshold swing $SS(T)\equiv dV_{GS}/d\log I_{D}$ has a fundamental lower limit as a function of temperature $T$ given by $SS(T) = \ln10~k_BT/e$. However, recent lowtemperature studies of different advanced CMOS technologies have reported $SS$(4K or lower) values that are at least an order of magnitude larger. Here, we present and analyze the saturation of $SS(T)$ in 28nm fullydepleted silicononinsulator (FDSOI) devices for both n and ptype MOSFETs of different gate oxide thicknesses and gate lengths down to 4K. Until now, the increase of interfacetrap 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 disorderinduced tail in the density of states at the conduction (valence) band edge for the calculation of the MOS channel transport by applying FermiDirac statistics. This results in a subthreshold $I_{D}\sim e^{eV_{GS}/k_BT_0}$ for $T_0=35$K with saturation value $SS(T<T_0) = \ln 10~k_BT_0/e$. The proposed model adequately describes the experimental data of $SS(T)$ from 300 down to 4K using $k_BT_0 \simeq 3$meV for the width of the exponential tail and can also accurately describe $SS(I_{D})$ within the whole subthreshold region. Our analysis allows a direct determination of the technologydependent bandtail extension forming a crucial element in future compact modeling and design of cryogenic circuits.
 Publication:

IEEE Electron Device Letters
 Pub Date:
 May 2019
 DOI:
 10.1109/LED.2019.2903111
 arXiv:
 arXiv:1903.05409
 Bibcode:
 2019IEDL...40..784B
 Keywords:

 Condensed Matter  Mesoscale and Nanoscale Physics
 EPrint:
 IEEE Electron Device Letters, 5 March 2019