C.M. Maziar

A computationally efficient model for inversion layer quantization effects in deep submicron N-channel MOSFETs

Successful scaling of MOS device feature size requires thinner gate oxides and higher levels of channel doping in order to simultaneously satisfy the need for high drive currents and minimal short-channel effects. However, in 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 electron motion perpendicular to the interface. This phenomenon is well known and begins to have an observable impact on room temperature deep submicron MOS device performance when compared to the traditional classical predictions which do not take into account these quantum mechanical effects. Thus, for accurate and efficient device simulations, these effects must be properly accounted for in todays widely used moment-based device simulators. This paper describes the development and implementation into PISCES of a new computationally efficient three-subband model that predicts both the quantum mechanical effects in electron inversion layers and the electron distribution within the inversion layer. In addition, a model recently proposed by van Dort et al. (1994) has been implemented in PISCES. By comparison with self-consistent calculations and previously published experimental data, these two different approaches for modeling the electron inversion layer quantization are shown to be adequate in order to both accurately and efficiently simulate many of the effects of quantization on the electrical characteristics of N-channel MOS transistors.

Physically-based models for effective mobility and local-field mobility of electrons in MOS inversion layers

Abstract A new physically-based semi-empirical equation for electron effective mobility in MOS inversion layers has been developed by accounting explicitly for surface roughness scattering and screened Coulomb scattering in addition to phonon scattering. The new semi-empirical model shows excellent agreement with experimentally measured effective mobility data from five different published sources for a wide range of effective transverse field, channel doping, fixed interface charge, longitudinal field and temperature. By accounting for screened Coulomb scattering due to doping impurities in the channel, our model describes very well the roll-off of effective mobility in the low field (threshold) region for a wide range of channel doping level ( N A = 3.9 × 10 15 −7.7 × 10 17 cm −3 ). We have also developed a local-field-dependent mobility model for electrons in the inversion layer for use in device simulators by applying the previously published method to this new semi-empirical equation for the effective mobility. The new local-field-dependent mobility model has been implemented in the PISCES 2-D device simulation program, and comparisons of calculated vs measured data show excellent agreement for I D − V G and I D − V D curves for different devices with L eff ranging from 0.5 to 1.2 μ.

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 new approach to verify and derive a transverse-field-dependent mobility model for electrons in MOS inversion layers

A modeling approach is described that extracts the functional dependence of carrier mobility on local transverse and longitudinal fields, channel doping, fixed interface charge, and temperature in MOS inversion and accumulation layers directly from the experimentally measured effective (or average) mobility. This approach does not require a priori detailed knowledge of the experimental variation of mobility within the inversion or accumulation layer, and it can be used to evaluate the validity of other models described in the literature. Also, an improved transverse-field dependent mobility model is presented for electrons in MOS inversion layers that was developed using this new modeling approach. This model has been implemented in the PISCES 2-D device simulation program. Comparisons of the calculated versus measured data show excellent agreement for I/sub D/-V/sub G/ and I/sub D/-V/sub D/ curves for devices with L/sub eff/=0.5 to 1.2 mu m. >

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.

Thermionic emission model of electron gate current in submicron NMOSFETs

A thermionic emission model based on a non-Maxwellian electron energy distribution function for the electron gate current in NMOSFETs is described. The model uses hydrodynamic equations to describe more correctly the electron transport and gate injection phenomena in submicron devices. A generalized analytical function is used to describe the high-energy tail of the electron energy distribution function. Coefficients of this generalized function are determined by comparing simulated gate currents with the experimental data. This model also includes the self-consistent calculation of the tunneling component of the gate current by using the WKB approximation, and by using a more accurate representation of the oxide barrier by including the image potential. Good agreement with gate currents over a wide range of bias conditions for three different technological sets of devices are demonstrated by using a single set of coefficients.

Simulation program suitable for hot carrier studies: An efficient multiband Monte Carlo model using both full and analytic band structure description for silicon

An efficient Monte Carlo model using both the full electronic band structure calculated from the empirical pseudopotential method and fitted, anisotropic, analytic bands is described. With the simulation program suitable for hot carrier studies, electron dynamics (i.e., scattering processes) are treated using the pseudopotential bands, where the accuracy of the band structure is most critical, while the electron kinetics (i.e., electron free flight between scattering and postscattering momentum selection) is treated using the fitted, analytic bands in order to greatly enhance the computational efficiency of the simulator. The analytic, multiband, multivalley model has 65 ellipsoidal, nonparabolic valleys and fits both the density of states and the electron E(k) dispersion relations of the pseudopotential band. The scattering ‘‘matrix elements’’ effect is also explored and an efficient model for this was developed and implemented for the first time in the framework of an analytic band. With this model, it ...

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.

Accounting for bandstructure effects in the hydrodynamic model : a first-order approach for silicon device simulation

Abstract This paper presents a non-parabolic formulation of the hydrodynamic model for Si device simulation. By adopting a first-order approach, non-parabolic band effects are included as simple corrections to the conventional hydrodynamic model. These corrections are shown to improve the accuracy of the hydrodynamic model under submicron device conditions. The first-order approach captures the important effects of non-parabolicity in the conservation relations by introducing a minimal number of approximations. Unlike previously reported non-parabolic models, only a single empirical constant is required in addition to the relaxation times. An advantage of the formulation presented here is that the average electron energy is retained as a state variable, and an electron temperature need not be defined. Advanced hot-carrier models based on average electron energy will benefit directly from the improved accuracy of the model.

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.