W.-K. Shih

Modeling gate leakage current in nMOS structures due to tunneling through an ultra-thin oxide

Abstract For the first time, the tunneling current in silicon nMOS structures with ultra-thin gate oxides has been studied both by numerically solving Schrodingers equation and by using the WKB approximation, which explicitly includes the size quantization effects in the inversion layers. The numerical solution employs first-order perturbation within the one-band effective-mass approximation to calculate the lifetime of an inversion-layer quasi-bound state. The good agreement in the tunneling currents estimated with these two methods justifies the use of the WKB approximation in the direct tunneling regime. The range of validity of the WKB approximation is also discussed.

A pseudo-lucky electron model for simulation of electron gate current in submicron NMOSFET's

An energy parameterized pseudo-lucky electron model for simulation of gate current in submicron MOSFETs is presented in this paper. The model uses hydrodynamic equations to describe more correctly the carrier energy dependence of the gate injection phenomenon. The proposed model is based on the exponential form of the conventional lucky electron gate current model. Unlike the conventional lucky electron model, which is based on the local electric fields in the device, the proposed model accounts for nonlocal effects resulting from the large variations in the electric field in submicron MOSFETs. This is achieved by formulating the lucky electron model in terms of an effective-electric field that is obtained by using the computed average carrier energy in the device and the energy versus field relation obtained from uniform-field Monte Carlo simulations. Good agreement with gate currents over a wide range of bias conditions for three sets of devices is demonstrated.

Computationally efficient models for quantization effects in MOS electron and hole accumulation layers

In this paper, models appropriate for device simulators are developed which account for the quantum mechanical nature of accumulated regions. Accumulation layer quantization is important in deep submicron (/spl les/0.25 /spl mu/m) MOS devices in the overlapped source/drain extension regions, in accumulation mode SOI devices, and in buried-channel PMOS structures. Computationally efficient models suitable for routine device simulation are presented that predict the reduction of the accumulated net electron (hole) sheet charge when quantization of the electron (hole) accumulation region is accounted for. The results of comparisons with self-consistent simulations support the validity of these models. In addition, simulation results will be shown which illustrate that when inversion layer quantum mechanical effects are modeled, it is also necessary to account for accumulation layer quantum mechanical effects in order to obtain more physically accurate as well as numerically stable solutions.

Models for electron and hole mobilities in MOS accumulation layers

We present new physically based effective mobility models for both electrons and holes in MOS accumulation layers. These models take into account carrier-carrier scattering, in addition to surface roughness scattering, phonon and fixed interface charge scattering, and screened Coulomb scattering. The newly developed effective mobility models show excellent agreement with experimental data over the range 1/spl times/10/sup 16/-4/spl times/10/sup 17/ cm/sup -3/ for which experimental data are available. Local-field dependent mobility models have also been developed for both electrons and holes, and they have been implemented in the two-dimensional (2-D) device simulators, PISCES and MINIMOS, thus providing for more accurate prediction of the terminal characteristics in deep submicron CMOS devices. In addition, transition region mobility models have been developed to account for the transition in the mobility in going from the accumulation layer in the gate-to-source overlap region to the inversion layer region in the channel.

A physically-based model for quantization effects in hole inversion layers

As MOS devices have been successfully scaled to smaller feature sizes, thinner gate oxides and higher levels of channel doping have been used in order to simultaneously satisfy the need for high drive currents and minimal short-channel effects. With the onset and development of deep submicron (/spl les/0.25 /spl mu/m gate length) technology, the combination of the extremely thin gate oxides (t/sub ox//spl les/10 nm) and high channel doping levels (/spl ges/10/sup 17/ cm/sup -3/) results in transverse electric fields at the Si/SiO/sub 2/ interface that are sufficiently large, even near threshold, to quantize the motion of inversion layer carriers near the interface. The effects of quantization are well known and begin to impact the electrical characteristics of the deep submicron devices at room temperature when compared to the traditional classical predictions which do not take into account these quantum mechanical (QM) effects. For accurate device simulations, quantization effects must be properly accounted for in todays widely used moment-based device simulators. This paper describes a new computationally efficient three-subband model that predicts the effects of quantization on the terminal characteristics in addition to the spatial distribution of holes within the inversion layer. The predictions of this newly developed model agree very well with both the predictions of a self-consistent Schrodinger-Poisson solver and experimental measurements of QM effects in MOS devices.

A simple model for quantum mechanical effects in hole inversion layers in silicon PMOS devices

The effects of quantization of the inversion layer of MOSFET devices is an area of increasing importance as technology is aggressively scaled below 0.25 /spl mu/m. Although electron inversion layers have attracted considerable interest, very little work has been reported for holes. This paper describes the implementation and results of a simple, computationally efficient model, appropriate for device simulators, for predicting the effects of hole inversion layer quantization. This model compares very favorably with experimental results and the predictions of a full-band, self-consistent Schrodinger-Poisson solver.

An improved technique and experimental results for the extraction of electron and hole mobilities in MOS accumulation layers

For the first time, experimental results are presented for electron and hole mobilities in the electron and hole accumulation layers of a MOSFET for a wide range of doping concentrations. Also presented is an improved methodology that has been developed in order to enable more accurate extraction of the accumulation layer mobility. The measured accumulation layer mobility for both electrons and holes is observed to follow a universal behavior at high transverse electric fields, similar to that observed for minority carriers in MOS inversion layers. At low to moderate transverse fields, the effective carrier mobility values are greater than the bulk mobility values for the highest doping levels. This is due to screening by accumulated carriers of the ionized impurity scattering by accumulated carriers, which dominates at higher doping concentrations. For lower doping levels, surface phonon scattering is dominant at low to moderate transverse fields so that the carrier mobility is below the bulk mobility value.

An experimental study of the effect of quantization on the effective electrical oxide thickness in MOS electron and hole accumulation layers in heavily doped Si

This work presents for the first time experimental results for the extraction of the increase in the effective electrical oxide thickness (/spl Delta/t/sub ox/=t/sub ox,expt/-t/sub ox,physical/) in MOS accumulation layers with heavily doped substrates due to quantum mechanical (QM) effects, using experimentally measured MOS capacitance-voltage (C-V) characteristics and experimentally verified fullband self-consistent calculations. In addition, the fullband self-consistent simulations have been extended to accumulation regions, and the experimental results for the accumulation region have been compared with simulations. It has been shown that at moderate to high doping levels, /spl Delta/t/sub ox/ is as much as 0.4 to 0.5 nm for both electrons and holes, whereas for very high doping levels (>1/spl times/10/sup 19/ cm/sup -3/) /spl Delta/t/sub ox/ approaches zero. Thus, the experimental accumulation capacitance is predicted sufficiently well by the classical analysis itself.

Hydrodynamic (HD) Simulation of N-Channel MOSFET'swith a Computationally Efficient Inversion LayerQuantization Model

A quantum mechanical treatment of electron inversion layers is incorporated in the hydrodynamic (HD) transport model used in UT-MiniMOS. A physically based, yet computationally efficient, three-subband model is implemented in the HD simulation tool. The three-subband model, which is based upon solutions to Schrodingers equation, has the important advantage of more accurately predicting the distribution of electrons in the inversion layers than does more conventional classical models. A more simplified quantum mechanical model with carrier heating effects included has also been developed. Terminal currents are calculated using these quantum mechanical models and the comparison with results from classical calculations indicates the importance of quantum mechanical effects in the deep submicron device simulations.

Study of Electron Velocity Overshoot in NMOS Inversion Layers

Non-local electron transport in nMOSFET inversion layers has been studied by Monte Carlo (MC) simulations. Inversion layer quantization has been explicitly included in the calculation of density of states and scattering rate for low-energy electrons while bulk band structure is used to describe the transport of more energetic electrons. For uniform, high-lateral field conditions, the effects of quantization are less pronounced due to the depopulation of electrons in the lower-lying subbands. On the other hand, Monte Carlo results for carrier transport in spatially varying lateral fields (such as those in the inversion layer of MOSFETs) clearly indicate that depopulation of the low-lying subbands is less evident in the non-local transport regime. Quasi-2D simulations have shown that, at high transverse effective field, the inclusion of a quantization domain does have an impact on the calculated spatial velocity transient.