Symmetric Double-gate MOSFETs Research Articles

An implicit analytical surface potential based model for long channel symmetric double-gate MOSFETs accounting for oxide and interface trapped charges

We present analytical models for calculating surface potential for both undoped (intrinsic) and doped (extrinsic) long channel symmetric double-gate MOSFETs. Starting with Poisson’s equation, we derive the implicit surface potential equation(s) for the symmetric double-gate MOSFET in the accumulation and depletion/inversion regions of operation. This results in a continuous surface potential curve throughout all operating regions of the device. By introducing a defect potential to surface potential equations for the symmetric double-gate MOSFET, the models add the effects of oxide-trapped charge and interface traps (both uniform and non-uniform distributions) that can build up in the gate dielectric of the device. A drain current model is presented for both the undoped and doped cases of the SDG MOSFET. The accuracy of the surface potential analytical models and drain current models are validated by comparing to numerical results from two dimensional TCAD simulations.

Physics based compact modeling of symmetric double gate MOS transistors with high mobility III-V channel material

In this work, we report a physics-based core model of surface potential, inversion charge, and drain current of a symmetric double-gate MOSFET with high mobility III-V channel material. The model efficiently captures all essential device physics of III-V MOSFETs. The carrier quantization effect inside the quantum well formed between oxide and channel region, is modeled by solving time-independent Schrödinger wave equation. The surface potential and inversion charge are obtained from the explicit solution of the Poisson and Schrödinger equation. A first-order correction has been applied to incorporate the modifications in sub-band energy due to applied gate bias voltages. The conduction band non-parabolicity effect is also included in our proposed model. The drain current has been derived using drift-diffusion transport including the velocity overshoot effect. The core model is free from any empirical parameters. The model predicted results have been validated with self-consistent Schrödinger-Poisson solver and commercial device simulator (TCAD) for different channel thicknesses, effective masses, a wide range of gate and drain bias voltages, and different non-parabolicity factor. The model predicted results are found to be in reasonable agreement with numerical simulation data.

Modeling of Short P-Channel Symmetric Double-Gate MOSFET for Low Power Circuit Simulation

In the present era, down scaling of complementary metal-oxide-semiconductor (CMOS) technology has lead the metal-oxide-semiconductor field-effect-transistor's (MOSFET) sizes to nanometer regime which in turn experiencing difficulties due to the effect of physical and technological perspective. Double-gate (DG) MOSFET is considered as a promising device to reduce the shortcoming and shrink down towards nanometer domain. This paper proposes electrostatic potential distribution and drain current models for the lightly doped symmetrical p-channel DG MOSFET. The analytic solution of potential distribution is derived by solving the 2D Poisson's equation incorporated with hole density through the superposition method. The drain current model has been explored by incorporating physical effects like threshold-voltage roll-off, channel length modulation and surface roughness scattering. Functionality of the models has been calculated in MATLAB and the obtained results are verified and compared with state of the art literature.

A 2D channel potential modelling of symmetric double-gate MOSFET at onset of threshold condition

ABSTRACT A two-dimensional (2D) channel potential distribution is analytically derived for lightly doped double-gate MOSFET at turn-on condition of the device by solving 2D Poisson’s equation with the application of suitable boundary condition. Threshold voltage roll-off is also modelled to predict the short channel behaviour of the device at sub-threshold condition. Our derived model closely follows the 2D numerical simulation without the inclusion of any fitting parameters. It also serves as a basic step towards computing the compact modelling of DG MOSFET at threshold condition.

2D Surface potential and mobility modelling of doped/undoped symmetric double gate MOSFET

The 2D surface potential and mobility models are proposed for symmetric doped/undoped channel double gate FET (DGFET) device. This is then used in drain current equation obtained by combining Pao-Sah's double integral formula with Pierret–Shields' type current model. The surface potential model and mobility model both are analytic and differently solved for both undoped and doped device. Being an explicit and continuous expression, the drain current model is used to describe the behaviour of the device at below and above threshold condition. Simulations are carried out using TCAD Sentaurus bundle of Synopsis tool. The accuracy of the proposed model is analysed and compared with the simulation result for different channel length and channel thickness value of the device.

A quasi-ballistic drain current, charge and capacitance model with positional carrier scattering dependency valid for symmetric DG MOSFETs in nanoscale regime

This paper presents a physically valid quasi-ballistic drain current model applicable for nanoscale symmetric Double Gate (SDG) MOSFETs. The proposed drain current model includes both diffusive and ballistic transport phenomena. The model considers the important positional carrier scattering dependency effect near the source region described in terms of transmission and reflection co-efficients related to the scattering theory. The significance of carrier transport near the bottleneck source region is illustrated where the carriers diffuse into the channel at a relatively lower velocity before accelerating ballistically. The results obtained demonstrate carrier scattering dependency at the critical layer defined near the low field source region on the drain current characteristics. The proposed model partly evolves from Natori’s ballistic bulk MOSFET model that is modified accordingly to be valid for a symmetric Double Gate MOSFET in the nanoscale regime. Carrier degeneracy and Fermi–Dirac statistics are included in the work so as to justify the complete physicality of the model. The model is further extended and is shown to be continuous in terms of terminal charges and capacitances in all regions of operation. A comparative analysis is also done between the proposed quasi-ballistic model and a hypothetical complete ballistic device.

Modeling of electrical behavior of undoped symmetric Double-Gate (DG) MOSFET using carrier-based approach

PurposeThis study aims to develop a compact analytical models for undoped symmetric double-gate MOSFET based on carrier approach. Double-Gate (DG) MOSFET is a newly emerging device that can potentially further scale down CMOS technology owing to its excellent control of short channel effects, ideal subthreshold slope and free dopant-associated fluctuation effects. DG MOSFET is of two types: the symmetric DG MOSFET with two gates of identical work functions and asymmetric DG MOSFET with two gates of different work functions. To fully exploit the benefits of DG MOSFETs, the body of DG MOSFETs is usually undoped because the undoped body greatly reduces source and drain junction capacitances, which enhances the switching speed. Highly accurate and compact models, which are at the same time computationally efficient, are required for proper modeling of DG MOSFETs.Design/methodology/approachThis paper presents a carrier-based approach to develop a compact analytical model for the channel potential, threshold voltage and drain current of a long channel undoped symmetric DG MOSFETs. The formulation starts from a solution of the 2-D Poisson’s equation in which mobile charge term has been included. The 2-D Poisson’s equation in rectangular coordinate system has been solved by splitting the total potential into long-channel (1-D Poisson’s equation) and short-channel components (remnant 2-D differential equation) in accordance to the device physics. The analytical model of the channel potential has been derived using Boltzmann’s statistics and carrier-based approach.FindingsIt is shown that the metal gate suppresses the center potential more than the poly gate. The threshold voltage increases with increasing metal work function. The results of the proposed models have been validated against the Technology Computer Aided Design simulation results with close agreement.Originality/valueCompact Analytical models for undoped symmetric double gate MOSFETs.

Unified Quantum and Reliability Model for Ultra-Thin Double-Gate MOSFETs

This paper presents a unified two-dimensional (2D) threshold voltage model for lightly doped symmetrical double-gate p-channel MOSFETs including quantum confinement effects and negative bias temperature instability (NBTI). The proposed model has been derived by solving the two-dimensional Poisson equation to obtain the NBTI potential model and the one-dimensional Schrodinger equation together with the 2D Poisson equation to obtain the quantum confinement model. The quantum expression was subsequently embedded in the NBTI solution to reach a unified model for both quantum confinement and NBTI. The model is simple and continuous, thereby ensuring compatibility for insertion in Verilog-A based device simulators. The effect of stress time on the degradation of the threshold voltage has been measured over a 10 year period. The accuracy of the model has been validated through comparisons with both 2D numerical simulations and experimental data. The results show matching within ±3% for channel lengths down to 7 nm and silicon thicknesses of 5 nm at 1 GHz operation after 10 years.

Analytical Modeling of Gate-Stack DG-MOSFET in Subthreshold Regime by Green’s Function Approach

An accurate analytical model of the gate-stack symmetric double-gate (DG) MOSFET using Green's function approach is developed in this brief. An exact analytical solution to 2-D Poisson's equation is derived in the subthreshold regime of operation, considering 2-D mixed boundary conditions and multizone techniques. It is observed that the Green's function approach of solving 2-D Poisson's equation in both oxides (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ox1</sub> and t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ox2</sub> ) and silicon region can accurately predict channel potential and subthreshold current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sub</sub> ) of gate-stack DG-MOSFET. We further extend our model to mitigate the fringe-induced barrier lowering (FIBL) effect in the gate-stack device, by optimizing the interfacial layer thickness. It is observed that selection of interfacial SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer as ~0.75 times of equivalent oxide thickness is necessary to completely mitigate the FIB' effect of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based gate-stack DG-MOSFET.

An analytical drain current model for symmetric double-gate MOSFETs

An analytical surface-potential-based drain current model of symmetric double-gate (sDG) MOSFETs is described as a SPICE compatible model in this paper. The continuous surface and central potentials from the accumulation to the strong inversion regions are solved from the 1-D Poisson’s equation in sDG MOSFETs. Furthermore, the drain current is derived from the charge sheet model as a function of the surface potential. Over a wide range of terminal voltages, doping concentrations, and device geometries, the surface potential calculation scheme and drain current model are verified by solving the 1-D Poisson’s equation based on the least square method and using the Silvaco Atlas simulation results and experimental data, respectively. Such a model can be adopted as a useful platform to develop the circuit simulator and provide the clear understanding of sDG MOSFET device physics.

Charge-Based Modeling of Radiation Damage in Symmetric Double-Gate MOSFETs

In this paper, a comprehensive charge-based predictive model of interface and oxide trapped charges in undoped symmetric long-channel double-gate MOSFETs is developed. The model involves essentially no fitting parameters, but first-principle calculations of both oxide and Si/oxide interface trapping. This charge-based approach represents an essential step toward compact modeling of ionizing dose and aging effects in advanced field effect devices. The soundness of this approach is confirmed by extensive comparisons with numerical TCAD simulations, while the analytical formulation helps understanding the most relevant parameters of the phenomena with respect to a specific technology. The model confirms its validity for all regions of operation, i.e., from deep depletion to strong inversion and from linear to saturation.

Parasitic Gate Resistance Impact on Triple-Gate FinFET CMOS Inverter

In this paper, based on a full intrinsic–extrinsic model for symmetric doped double-gate MOSFET, we analyze the impact of FinFET gate resistance over the inverter and ring oscillator performance. It is shown that, when the total number of fins remains constant, the propagation delay can be improved thanks to the multifinger configuration that translates into the gate resistance reduction. Furthermore, the fin spacing in addition to source/drain fin extension reduction are of primary importance to improve the digital circuit performance.

Analytical Current Model for Long-Channel Junctionless Double-Gate MOSFETs

In this paper, a SPICE compatible analytical surface-potential-based model for junctionless symmetric double-gate (JLDG) MOSFETs is described. By using the gradual-channel-approximation, the 1-D Poisson’s equation is solved to obtain the surface and central potential in the JLDG MOSFET for long channel case. A continuous drain current model with smooth transitions from fully depleted region to partially depleted and accumulation regions is then derived from the Pao-Sah’s dual integral as a function of the surface and central potential at the source and drain terminals. The model is verified and validated by numerical simulations over a wide range of doping concentrations and device geometries. The model has been implemented in a circuit simulator and used to simulate some circuit building blocks without any convergent problem.

3D simulation of single-event-transient effects in symmetrical dual-material double-gate MOSFETs

Dual-material double-gate (DMDG) structure is a promising structure for future ultra-scaled devices thanks to its capability to reduce short channel effects (SCEs) and hot-carrier induced effects (HCEs). This is due to a step in the surface-potential profile which screens the source side of the channel from drain-potential variations and reduces the drain electric field. In this work, we investigate the DMDG sensitivity to single-event transients. The impact of dual gate material workfunctions on the bipolar gain is particularly addressed. We show that DMDG is naturally less radiation immune than the usual single-material DG (SMDG) devices.

Modeling the drain current and its equation parameters for lightly doped symmetrical double-gate MOSFETs

A 2D model for the potential distribution in silicon film is derived for a symmetrical double gate MOSFET in weak inversion. This 2D potential distribution model is used to analytically derive an expression for the subthreshold slope and threshold voltage. A drain current model for lightly doped symmetrical DG MOSFETs is then presented by considering weak and strong inversion regions including short channel effects, series source to drain resistance and channel length modulation parameters. These derived models are compared with the simulation results of the SILVACO (Atlas) tool for different channel lengths and silicon film thicknesses. Lastly, the effect of the fixed oxide charge on the drain current model has been studied through simulation. It is observed that the obtained analytical models of symmetrical double gate MOSFETs are in good agreement with the simulated results for a channel length to silicon film thickness ratio greater than or equal to 2.

Novel attributes and design considerations of effective oxide thickness in nano DG MOSFETs

Impacts of effective oxide thickness on a symmetric double-gate MOSFET with 9-nm gate length are studied, using full quantum simulation. The simulations are based on a self-consistent solution of the two-dimensional (2D) Poisson equation and the Schrödinger equation within the non-equilibrium Green’s function formalism. Oxide thickness and gate dielectric are investigated in terms of drain current, on-off current ratio, off current, sub-threshold swing, drain induced barrier lowering, transconductance, drain conductance, and voltage. Simulation results illustrate that we can improve the device performance by proper selection of the effective oxide thickness.

10 nm이하 비대칭 이중게이트 MOSFET의 하단 게이트 전압에 따른 터널링 전류 분석

This paper analyzed the deviation of tunneling current for bottom gate voltage of sub-10 nm asymmetric double gate MOSFET. The asymmetric double gate MOSFET among multi gate MOSFET developed to reduce the short channel effects has the advantage to increase the facts to be able to control the channel current, compared with symmetric double gate MOSFET. The increase of off current is, however, inescapable if aymmetric double gate MOSFET has the channel length of sub-10 nm. The influence of tunneling current was investigated in this study as the portion of tunneling current for off current was calculated. The tunneling current was obtained by the WKB(Wentzel-Kramers-Brillouin) approximation and analytical potential distribution derived from Poisson equation. As a results, the tunneling current was greatly influenced by bottom gate voltage in sub-10 nm asymmetric double gate MOSFET. Especially it showed the great deviation for channel length, top and bottom gate oxide thickness, and channel thickness.

Analytical temperature dependent model for nanoscale double-gate MOSFETs reproducing advanced transport models

In this paper we extend our compact model for nanoscale double-gate (DG) MOSFETs which considers a hydrodynamic transport model to include the effect of the temperature dependence. Temperature dependence equations are incorporated to the expressions of the mobility and saturation velocity. For model validation we have considered a symmetric 22nm double-gate MOSFET template device optimized for low-stand-by-power applications. Comparison between the numerical 2D Monte Carlo (MC) simulations and the compact model shows a good degree of agreement.

Analytical Modeling of Dielectric Pocket Double-Gate MOSFET Incorporating Hot-Carrier-Induced Interface Charges

In this paper, the impact of interface charges on the performance of a short-channel symmetric dielectric pocket double-gate (DP-DG) MOSFET has been investigated. An analytical drain current model for DP-DG MOSFET has been developed, which is also useful for investigating the impact of dual-material gate on DP-DG architecture. The proposed model is verified using ATLAS 3-D simulator. In addition, exhaustive simulation has been carried out to address the impact of interface charges on the reliability issues of various devices, i.e., DP-DG, double-gate, and dielectric pocket architectures in terms of gate leakage current, electron temperature, and impact of interface charges on the threshold voltage lowering. Analog and digital performances have also been investigated and compared with the other devices.

The Impact of Body Doping Concentration on the Performance of Nano Dg-Mosfets: A Quantum Simulation

This paper presents the effects of p-type body doping concentration on a symmetric double-gate MOSFET with 9 nm gate length, using full quantum simulation. The simulations are based on a self-consistent solution of the 2D Poisson equation and Schrodinger equation with open boundary conditions, within the non-equilibrium Green's function formalism for a wide range of channel doping concentrations. The effects of varying the p-type body doping concentration parameter is investigated in terms of the drain current, potential energy profile, 2D electron density, on-off current ratio, subthreshold swing, drain induced barrier lowering, transconductance, drain conductance, voltage gain, and resistance. The simulation results show that a higher body doping improves the short channel effects.