Modeling tunnel field effect transistors—From interface chemistry to non-idealities to circuit level performance

Abstract

We present a quasi-analytical model for Tunnel Field Effect Transistors (TFETs) that includes the microscopic physics and chemistry of interfaces and non-idealities. The ballistic band-to-band tunneling current is calculated by modifying the well known Simmons equation for oxide tunneling, where we integrate the Wentzel-Kramers-Brillouin (WKB) tunneling current over the transverse modes. We extend the Simmons equation to finite temperature and non-rectangular barriers using a two-band model for the channel material and an analytical channel potential profile obtained from Poisson's equation. The two-band model is parametrized first principles by calibrating with hybrid Density Functional Theory calculations, and extended to random alloys with a band unfolding technique. Our quasi-analytical model shows quantitative agreement with ballistic quantum transport calculations. On top of the ballistic tunnel current we incorporate higher order processes arising at junctions coupling the bands, specifically interface trap assisted tunneling and Auger generation processes. Our results suggest that both processes significantly impact the off-state characteristics of the TFETs -Auger in particular being present even for perfect interfaces. We show that our microscopic model can be used to quantify the TFET performance on the atomistic interface quality. Finally, we use our simulations to quantify circuit level metrics such as energy consumption. arXiv:1806.06331v3 [cond-mat.mtrl-sci]

Minimum-Energy Digital Computing With Steep Subthreshold Swing Tunnel FETs

“…where f clk is the clock operating frequency which is the inverse of the cycle time. A background on the derivation of (4)-(6) is given in [18]. The maximum clock frequency for any V dd < V th can be modeled as…”

“…where t d = KC gate V dd /I on represents the gate delay of a single inverter using the traditional CV/I approximation and K is a scaling factor used for calibration. Operation at the maximum frequency allowable by the critical path is a necessary condition for achieving minimum-energy operation due to a minimized integration time of leakage power without affecting dynamic energy [18]. Note that this result has an exponential dependence on V dd , so small linear variations in C gate across V dd can be considered negligible to preserve modeling simplicity.…”

“…In order to study the effect of non-idealities on minimumenergy optimization, we created a quasi-analytical model for double gated III-V TFETs [18]. The model includes major non-idealities such as trap-assisted tunneling (TAT) and Auger generation.…”

“…We consider trapassisted tunneling as a Fowler-Nordheim type type tunneling process through a tilted barrier around the trap. The TAT current per unit width can be written in the following compact form [18], [22] where Γ is the electric field enhancement due to TAT and thermionic emission processes, d is the width of the trap active region, and n i represents the intrinsic carrier concentration. The recombination velocity v rcmb = σ v th N t , where N t is the surface trap density per unit area at the midgap energy.…”

“…In this work, we implement a fully-analytical digital circuit model based on foundational modeling techniques from both CMOS circuits and TFET devices ( [13], [18]) that includes operating frequency and voltage regulation, and use it to evaluate energy minimization based on device parameters with emphasis on the SS. We demonstrate the impact of voltage regulation losses on the minimum-energy point and model the optimum supply voltage for minimizing energy based on the SS.…”