Cryogenic Subthreshold Swing Saturation in FD-SOI 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 $SS(T)\equiv d{V}_{GS}/d\log I_D$ has a fundamental lower limit as a function of temperature $T$ given by $SS(T)=\ln 10k_{B}T/e$. However, recent low-temperature 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 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 $SS(T<T_0)=\ln 10k_B{T}_0/e$. The proposed model adequately describes the experimental data of $SS(T)$ from 300 down to 4K using $k_B{T}_0\simeq 3$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 technology-dependent band-tail extension forming a crucial element in future compact modeling and design of cryogenic circuits.

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• Characterization and Modeling of Native MOSFETs Down to 4.2 K
• Generalized Boltzmann relations in semiconductors including band tails
• Physics-Based and Closed-Form Model for Cryo-CMOS Subthreshold Swing
• Revised theoretical limit of subthreshold swing in field-effect transistors
• Scalable on-chip multiplexing of silicon single and double quantum dots
• Ultra-steep slope cryogenic FETs based on bilayer graphene
• A Review on Quantum Computing: Qubits, Cryogenic Electronics and Cryogenic MOSFET Physics
• Subthreshold Swing Behavior in Amorphous Indium-Gallium-Zinc-Oxide Transistors from Room to Cryogenic Temperatures
• Cryogenic Behavior of High-Permittivity Gate Dielectrics: The Impact of the Atomic Layer Deposition Temperature and the Lithographic Patterning Method