Double-gate Metal Oxide Semiconductor Field Research Articles

A higher-order accurate operator splitting spectral method for the Wigner–Poisson system

An accurate description of 2-D quantum transport in a double-gate metal oxide semiconductor field effect transistor (dgMOSFET) requires a high-resolution solver for a coupled system of the 4-D Wigner equation and 2-D Poisson equation. In this paper, we propose an operator-splitting spectral method to evolve such Wigner–Poisson (WP) system in 4-D phase space with high accuracy. After the operator splitting of the Wigner equation, the resulting two sub-equations can be solved analytically with spectral approximation in phase space. Meanwhile, we adopt a Chebyshev spectral method to solve the Poisson equation. Spectral convergence in phase space and a fourth-order accuracy in time are both numerically verified. Finally, we apply the proposed solver to the simulation of a dgMOSFET, develop the steady states via long-time simulations and obtain numerically converged current–voltage (I–V) curves.

Group preserving scheme and reproducing kernel method for the Poisson–Boltzmann equation for semiconductor devices

This paper introduces that the nonlinear Poisson–Boltzmann equation for semiconductor devices describing potential distribution in a double-gate metal oxide semiconductor field effect transistor (DG-MOSFET) is exactly solvable. The DG-MOSFET shows one of the most advanced device structures in semiconductor technology and is a primary focus of modeling efforts in the semiconductor industry. Lie symmetry properties of this model is investigated in order to extract some exact solutions. The reproducing kernel Hilbert space method and group preserving scheme also have been applied to the nonlinear equation. Numerical results show that the present methods are very effective.

The effect of high-k gate dielectrics on device and circuit performances of a junctionless transistor

The impacts of high-k gate dielectric permittivity on the device and circuit performances of a double-gate junctionless transistor (DGJLT) are studied with the help of extensive device simulations. The results are compared with a conventional inversion mode double-gate metal oxide semiconductor field effect transistor (DG MOSFET) of same dimension. Drain induced barrier lowering, intrinsic gain $$(G_{m}R_{O})$$(GmRO), and unity gain cut-off frequency $$(f_{T})$$(fT) are degraded with an increase in gate dielectric permittivity $$(k)$$(k). The transconductance $$(G_{m})$$(Gm) and gate capacitance $$(C_{GG})$$(CGG) are slightly affected with increase in $$k$$k. The gain of CMOS single stage amplifier and delay of inverter are found to be decreasing and increasing, respectively, with increase in $$k$$k. In order to mitigate these short channel effects due to the high-k gate dielectrics, a hetero-gate-dielectric structure with symmetric double-gate junctionless transistor (HG-DGJLT) is studied. HG-DGJLT offers superior $$G_{m}, \, C_{GG}$$Gm,CGG and $$f_{T}$$fT compared to $$\hbox {SiO}_{2}$$SiO2-only and $$\hbox {HfO}_{2}$$HfO2-only DGJLT. However, intrinsic gain of HG-DGJLT is inferior to $$\hbox {SiO}_{2}$$SiO2-only and $$\hbox {HfO}_{2}$$HfO2-only DGJLT.

Exact analytical solution to the Poisson-Boltzmann equation for semiconductor devices

This paper shows that the nonlinear Poisson-Boltzmann equation for semiconductor devices describing potential distribution in a Double Gate-Metal Oxide Semiconductor Field Effect Transistor (DG-MOSFET) is exactly solvable. The DG-MOSFET represents one of the most advanced device structures in semiconductor technology and is a primary focus of modeling efforts in the semiconductor industry. Furthermore, exact analytical solution is obtained in the implicit form for further physical interpretation and a full discussion is given.

A two-dimensional model for the subthreshold swing of short-channel double-gate metal–oxide–semiconductor field effect transistors with a vertical Gaussian-like doping profile

An analytical two-dimensional (2D) model for the subthreshold swing of the short-channel double-gate (DG) MOSFET with a vertical Gaussian-like doping profile is presented in this paper. The 2D potential function obtained by solving the 2D Poisson’s equation by using the evanescent mode analysis has been used to formulate the subthreshold current. The concept of the effective conduction path effect of uniformly doped DG MOSFETs has been extended to the nonuniformly doped DG MOSFETs to obtain the subthreshold swing variations against doping profile parameters as well as device parameters of the present device. The results of the model show excellent matching with the numerical simulation data obtained by using the commercially available ATLASTM, a two-dimensional device simulator from SILVACO.

Application of the homotopy analysis method to the Poisson–Boltzmann equation for semiconductor devices

This paper describes the application of a recently developed analytic approach known as the homotopy analysis method to derive an approximate solution to the nonlinear Poisson–Boltzmann equation for semiconductor devices. Specifically, this paper presents an analytic solution to potential distribution in a DG-MOSFET (Double Gate-Metal Oxide Semiconductor Field Effect Transistor). The DG-MOSFET represents one of the most advanced device structures in semiconductor technology and is a primary focus of modeling efforts in the semiconductor industry.

Compact Current Modeling of Fully Depleted Symmetric Double-Gate Metal–Oxide–Semiconductor Field Effect Transistors with Doped Short-Channel

The current behaviors of fully depleted (FD) symmetric double-gate (DG) metal–oxide–semiconductor field effect transistors (MOSFETs) with doped short-channel were parametrically modeled with the simple closed-form in all operational regions. In the diffusion current model, a physical parameter DG as a function of gate bias (VGS), drain bias (VDS), silicon body width (WB), channel length (L), and channel doping concentration (Nb) was introduced to consider the VGS dependence. Also, the subthreshold slope (SS) of DG MOSFETs with doped channel was modeled accurately with DG. DD which is dependent on VDS was introduced to consider the drain-induced barrier lowering (DIBL) in the diffusion current model. After the strong inversion, the drift current of doped DG MOSFETs was modeled by considering inversion-layer capacitance based on charge-sheet approximation. The channel length modulation by VDS was considered for accuracy in the current modeling of DG MOSFETs with doped short-channel. Our simple compact models predicted accurately DC characteristics of the devices with the channel length to 20 nm and shown good agreement with two-dimensional (2D) simulation.

Fullband Simulation of Nano-Scale MOSFETs Based on a Non-equilibrium Green's Function Method

The analysis of multiband quantum transport simulation in double-gate metal oxide semiconductor field effects transistors (DGMOSFETs) is performed based on a non-equilibrium Green's function (NEGF) formalism coupled self-consistently with the Poisson equation. The empirical sp3s* tight binding approximation (TBA) with nearest neighbor coupling is employed to obtain a realistic multiband structure. The effects of non-parabolic bandstructure as well as anisotropic features of Si are studied and analyzed. As a result, it is found that the multiband simulation results on potential and current profiles show significant differences, especially in higher applied bias, from those of conventional effective mass model.

Quantum electron transport modeling in double-gate MOSFETs based on multiband non-equilibrium Green's function method

The formulation and calculation of quantum electron transport in double-gate metal oxide semi-conductor field effect transistors (DGMOSFETs) is presented based on multiband non-equilibrium Green's function (NEGF) formalism. In the formulation we employ the empirical sp 3 s * tight-binding approximation (TBA) with nearest neighbor coupling to obtain a realistic fullband structure. The multiband NEGF is performed self-consistently with the Poisson equation to acquire the effect of space charge density. We compare the non-parabolicity feature of bandstructure with that of parabolic effective mass model. Due to the difference in energy dispersion relation, we have found that the results of multiband simulations are quite different with those based on conventional effective mass model.

A New Two-Dimensional Analytical Model for Short-Channel Symmetrical Dual-Material Double-Gate Metal–Oxide–Semiconductor Field Effect Transistors

Based on resultant solution of a two-dimensional (2D) Poisson's equation in the silicon region, a new analytical model for short-channel fully depleted, symmetrical dual-material double-gate (SDMDG) metal–oxide–semiconductor field effect transistors (MOSFETs) has been developed. The SDMDG MOSFET exhibits significantly reduced short-channel effects (SCEs) when compared with the symmetrical double-gate (SDG) MOSFET due to the step potential profile at the interface between different gate materials. It is found that the threshold voltage roll-off can be effectively reduced using both the thin Si film and thin gate oxide. A considerable portion of the large workfunction of metal gate 1 (M1) when laterally merged with the small workfunction of metal gate 2 (M2) can efficiently suppress drain-induced barrier lowering (DIBL) and maintain the low threshold voltage degradation. In this work, not only a precise 2D analytical model of the surface potential and threshold voltage is presented, but also the minimum surface potential in M1 of the shorter channel device that brings about subthreshold swing degradation for the SDMDG MOSFET is discussed. The new model is verified to be in good agreement with numerical simulation results over a wide range of device parameters.

Self-consistent modeling of longitudinal quantum effects in nanoscale double-gate metal oxide semiconductor field effect transistors

Ultrathin double-gate silicon-on-insulator transistors are studied in the quantum coherent limit. By treating electron-electron interaction on the level of a mean field approach, the density matrix of the device becomes diagonal when expressed in a basis that results from imposing scattering boundary conditions at the terminals. The self-consistent scattering wave functions are computed using a multisubband scattering matrix formalism. This allows us to retain the full dimensionality of the wave functions and eliminates the need for the adiabatic decomposition of the Schrödinger equation. Subband mixing is fully taken into account and a piecewise analytical representation of the wave functions can significantly reduce the number of sampling positions along transport direction. By self-consistent simulations the size of source-to-drain tunneling as a function of gate length is demonstrated for different body thicknesses. A strong forward bias is shown to increase the tunnel current due to the thinning of the source-drain potential barrier. The effect of channel orientation on the tunnel current is also discussed.

New Fabrication Technology of Fin Field Effect Transistors Using Neutral-Beam Etching

Ultrathin-channel formation of a vertical-type double-gate metal oxide semiconductor field effect transistor using a low-energy neutral-beam etching (NBE) is proposed. The NBE can completely eliminate charge buildup and photon radiation damage from the plasma. By optimizing NBE conditions, rectangular vertical channels were fabricated with a SiO2 hard mask under low-energy NBE conditions.

Analytical Compact Model for Quantization in Undoped Double-Gate Metal Oxide Semiconductor Field Effect Transistors and Its Impact on Quasi-Ballistic Current

In this paper we present an original analytical model for quantization in undoped double-gate metal oxide semiconductor field effect transistors (MOSFET). This analytical evaluation of energy levels replaces advantageously usual time-consuming Poisson–Schrödinger numerical simulations. This model, based on an improvement in the standard quantum perturbation theory, exploits efficiently the symmetry of the structure and allows a very good estimation of quantum mechanical effects. This has been successfully validated by comparison with Poisson–Schrödinger simulations (few percents of discrepancy) from node HP 90 to node HP 22 of the 2003 ITRS roadmap. Following Lundstrom's approach, the impact of quantization on quasi-ballistic current has also been investigated. This model has been used for the evaluation of the quasi-ballistic current along the ITRS roadmap, showing a significant increase in drain current compared with the ITRS drift-diffusion-based predictions (except for node 22 nm, where the “ballistic enhancement factor” has been taken equal to 1.2 in ITRS estimations).

Frequency characteristics of sub-100 nm double-gate MOSFET for suppressing short channel effects

We have investigated threshold voltage for each gate length in order to find the optimum value for side gate length and side gate voltage. We find clear on–off stages according to the bias conditions of both gates when side gate voltage is above 2 V and side gate length is above 70 nm. We have obtained the minimum short channel effect for a double-gate metal oxide semiconductor field effect transistor (DG MOSFET). We know that the optimum side gate voltage for each side gate length is about 2 V in the case that the main gate length is 70 nm. The cut-off frequency value is 41.4 GHz at LSG = 70 nm. As a result, the DG MOSFET has excellent electrical characteristics and very high cut-off frequency.

Implementation and characterization of the double-gate mosfet using lateral solid-phase epitaxy

In this paper, a simple high performance double-gate metal oxide semiconductor field effect transistor (MOSFET) using lateral solid-phase epitaxy (LSPE) is experimentally demonstrated and characterized. The thin channel of the double-gate MOSFET was obtained using the high quality LSPE crystallized layer. The fabricated double-gate MOSFET provides good current drive capability and steep subthreshold slope, and they are approximately 350 /spl mu/A//spl mu/m (@ V/sub ds/ = 2.5 V and V/sub gs/ - V/sub T/ = 2.5 V) and 78 mV/dec for the devices with 0.5 /spl mu/m channel length. Compared to the conventional single-gate transistor, the double-gate NMOSFET fabricated on the LSPE layer has better V/sub T/ roll-off characteristics, DIBL effect, and 1.72 times higher current drive. The peak effective electron mobility of the LSPE crystallized layer is approximately 382 cm/sup -2//V.s.

Investigation of Gate-Induced Drain Leakage (GIDL) Current in Thin Body Devices: Single-Gate Ultra-Thin Body, Symmetrical Double-Gate, and Asymmetrical Double-Gate MOSFETs

Gate-induced drain leakage (GIDL) current is investigated in single-gate (SG) ultra-thin body field effect transistor (FET), symmetrical double-gate (DG) FinFET, and asymmetrical DG metal oxide semiconductor field effect transistor (MOSFET) devices. Measured reductions in GIDL current for SG and DG thin-body devices are reported for the first time. The thin-body devices exhibit much lower GIDL current than bulk-Si MOSFETs, and the GIDL is found to decrease with decreasing body thickness. These results can be explained by the reduction in transverse electric field at the surface of the drain and the increase in transverse effective mass with decreasing body thickness.

A compact scattering model for the nanoscale double-gate MOSFET

An analytically compact model for the nanoscale double gate metal-oxide semiconductor field effect transistor (MOSFET) based on McKelvey's flux theory is developed. The model is continuous above and below threshold and from the linear to saturation regions. Most importantly, it describes nanoscale MOSFETs from the diffusive to ballistic regimes. In addition to its use in exploring the limits and circuit applications of double gate MOSFETs, the model also serves as an example of how semiclassical scattering theory can be used to develop physically sound models for nanoscale transistors.

A spacer patterning technology for nanoscale CMOS

A spacer patterning technology using a sacrificial layer and a chemical vapor deposition (CVD) spacer layer has been developed, and is demonstrated to achieve sub-7 nm structures with conventional dry etching. The minimum-sized features are defined not by the photolithography but by the CVD film thickness. Therefore, this technology yields critical dimension (CD) variations of minimum-sized features much smaller than that achieved by optical or e-beam lithography. In addition, it also provides a doubling of device density for a given lithography pitch. This method is used to pattern silicon fins for double-gate metal-oxide semiconductor field effect transistors (MOSFETs) (FinFETs) and gate electrode structures for ultrathin body MOSFETs. Process details are presented.