15 research outputs found
Inflection Phenomenon in Cryogenic MOSFET Behavior
This brief reports the analytical modeling and measurements of the inflection
in the MOSFET transfer characteristics at cryogenic temperatures. Inflection is
the inward bending of the drain current versus gate voltage, which reduces the
current in weak and moderate inversion at a given gate voltage compared to the
drift-diffusion current. This phenomenon is explained by introducing a Gaussian
distribution of localized states centered around the band edge. The localized
states are attributed to disorder and interface traps. The proposed model
allows to extract the density of localized states at the interface from the dc
current measurements
Cryogenic MOS Transistor Model
This paper presents a physics-based analytical model for the MOS transistor
operating continuously from room temperature down to liquid-helium temperature
(4.2 K) from depletion to strong inversion and in the linear and saturation
regimes. The model is developed relying on the 1D Poisson equation and the
drift-diffusion transport mechanism. The validity of the Maxwell-Boltzmann
approximation is demonstrated in the limit to zero Kelvin as a result of dopant
freeze-out in cryogenic equilibrium. Explicit MOS transistor expressions are
then derived including incomplete dopant-ionization, bandgap widening, mobility
reduction, and interface charge traps. The temperature dependency of the
interface-trapping process explains the discrepancy between the measured value
of the subthreshold swing and the thermal limit at deep-cryogenic temperatures.
The accuracy of the developed model is validated by experimental results on a
commercially available 28-nm bulk CMOS process. The proposed model provides the
core expressions for the development of physically-accurate compact models
dedicated to low-temperature CMOS circuit simulation.Comment: Submitted to IEEE Transactions on Electron Device
Generalized Boltzmann relations in semiconductors including band tails
Boltzmann relations are widely used in semiconductor physics to express the
charge-carrier densities as a function of the Fermi level and temperature.
However, these simple exponential relations only apply to sharp band edges of
the conduction and valence bands. In this article, we present a generalization
of the Boltzmann relations accounting for exponential band tails. To this end,
the required Fermi-Dirac integral is first recast as a Gauss hypergeometric
function, followed by a suitable transformation of that special function, and a
zeroth-order series expansion using the hypergeometric series. This results in
simple relations for the electron and hole densities that each involve two
exponentials. One exponential depends on the temperature and the other one on
the band-tail parameter. The proposed relations tend to the Boltzmann relations
if the band-tail parameters tend to zero. This work comes timely for the
modeling of classical semiconductor devices at cryogenic temperatures for
large-scale quantum computing
Physics-Based and Closed-Form Model for Cryo-CMOS Subthreshold Swing
Cryogenic semiconductor device models are essential in designing control
systems for quantum devices and in benchmarking the benefits of cryogenic
cooling for high-performance computing. In particular, the saturation of
subthreshold swing due to band tails is an important phenomenon to include in
low-temperature analytical MOSFET models as it predicts theoretical lower
bounds on the leakage power and supply voltage in tailored cryogenic CMOS
technologies with tuned threshold voltages. Previous physics-based modeling
required to evaluate functions with no closed-form solutions, defeating the
purpose of fast and efficient model evaluation. Thus far, only the empirically
proposed expressions are in closed form. This article bridges this gap by
deriving a physics-based and closed-form model for the full saturating trend of
the subthreshold swing from room down to low temperature. The proposed model is
compared against experimental data taken on some long and short devices from a
commercial 28-nm bulk CMOS technology down to 4.2 K.Comment: Accepted for publication in IEEE Transactions on Nanotechnolog
Energy filtering in silicon nanowires and nanosheets using a geometric superlattice and its use for steep-slope transistors
This paper investigates energy filtering in silicon nanowires and nanosheets by resonant electron tunneling through a geometric superlattice. A geometric superlattice is any kind of periodic geometric feature along the transport direction of the nanowire or nanosheet. Multivalley quantum-transport simulations are used to demonstrate the manifestation of minibands and minibandgaps in the transmission spectra of such a superlattice. We find that the presence of different valleys in the conduction band of silicon favors a nanowire with a rectangular cross section for effective energy filtering. The obtained energy filter can consequently be used in the source extension of a field-effect transistor to prevent high-energy electrons from contributing to the leakage current. Self-consistent Schrodinger-Poisson simulations in the ballistic limit show minimum subthreshold swings of 6 mV/decade for geometric superlattices with indentations. The obtained theoretical performance metrics for the simulated devices are compared with conventional III-V superlatticeFETs and TunnelFETs. The adaptation of the quantum transmitting boundary method to the finite-element simulation of 3-D structures with anisotropic effective mass is presented in Appendixes A and B. Our results bare relevance in the search for steep-slope transistor alternatives which are compatible with the silicon industry and can overcome the power-consumption bottleneck inherent to standard CMOS technologies. Published by AIP Publishing
Theoretical Limit of Low Temperature Subthreshold Swing in Field-Effect Transistors
This letter reports a temperature-dependent limit for the subthreshold swing in MOSFETs that deviates from the Boltzmann limit at deep-cryogenic temperatures. Below a critical temperature, the derived limit saturates to a value that is independent of temperature and proportional to the characteristic decay of a band tail. The proposed expression tends to the Boltzmann limit when the decay of the band tail tends to zero. Since the saturation is universally observed in different types of MOSFETs (regardless of dimension or semiconductor material), this suggests that an intrinsic mechanism is responsible for the band tail
Inflection Phenomenon in Cryogenic MOSFET Behavior
This brief reports the analytical modeling and measurements of the inflection in the MOSFET transfer characteristics at cryogenic temperatures. Inflection is the inward bending of the drain current versus gate voltage, which reduces the current in weak and moderate inversion at a given gate voltage compared to the drift-diffusion current. This phenomenon is explained by introducing a Gaussian distribution of localized states centered around the band edge. The localized states are attributed to disorder and interface traps. The proposed model allows to extract the density of localized states at the interface from the dc current measurements