Full-band Monte Carlo Simulations Research Articles

Properties of wurtzite GaN MESFETs studied by two-dimensional full band Monte Carlo approach

The properties of a GaN Metal–Semiconductor Field Effect Transistor were studied based on two-dimensional full band Monte Carlo simulations. The dependences of the distributions of the electric potential, electron concentration, electric field, drift velocity, and average electron energy on the gate–source voltage (VGS) and drain–source voltage (VDS) were obtained, then the characteristics of the drain current (IDS) versus the drain–source voltage (VDS) and the transconductance (Gm) versus VGS characteristics were determined. At VDS=15V and VGS=0V,IDS is 5.03 A/cm, which is higher value. The Gm−VGS curve shows bell shaped and the maximum Gm is 112 ms/mm at VDS=15V and VGS=1.5V. The current gain cutoff frequency (fT) is 98GHz at VDS=15V and VGS=0V.

Comparison of Double-Gate MOSFETs and FinFETs with Monte Carlo Simulation

A full-band Monte Carlo simulator has been used to analyze and compare the performance of n-channel double-gate MOSFETs and FinFETs. Size quantization effects were accounted for by using a quantum correction based on Schrodinger equation. FinFETs are a variation of typical double-gate devices with the gate surrounding the channel on three sides. From our simulations, we observed that the quantization effects in double-gate devices are less significant as compared to bulk MOSFETs. The total sheet charge density drops only slightly as the depletion of charge at the interface is counterbalanced by the increased volume inversion effect. We also observed an appreciable drop in average velocity distribution when quantum corrections were applied. For FinFETs, the fin extension lengths on either side of the gate affect the device performance significantly. These underlap regions have low carrier concentration and behave as large resistors. The current drops non-linearly with increasing fin extension lengths.

Monte Carlo simulations of double-gate MOSFETs

A fullband Monte Carlo simulator has been used to analyze the performance of scaled n-channel double-gate (DG) MOSFETs. Size quantization in the channel is accounted for by using a quantum correction based on Schrodinger equation. Scattering induced by the oxide interface is included with a model calibrated with measurements for bulk devices. The detailed self-consistent treatment of quantum effects leads to several interesting observations. We observe that the sheet charge in DG devices does not decrease as much as expected in bulk devices when quantum-mechanical effects are included. The average carrier velocity in the channel is also somewhat reduced by quantum effects, as a second-order effect. In the test cases studied here, application of quantum effects causes a reduction in simulated current not exceeding 15%. In a DG structure, quantum effects tend to concentrate the charge density in the center of the channel, where transverse fields are lower. Because of this, interface scattering appears to be less pronounced when quantum effects are included.

Monte Carlo, Hydrodynamic and Drift-Diffusion Simulation of Scaled Double-Gate MOSFETs

Double-gate MOSFETs with gate lengths of 50 and 25 nm are theoretically analyzed by drift-diffusion (DD), hydrodynamic (HD) and self-consistent full-band Monte Carlo (MC) simulation. The underestimation of the on-current Ion by DD is found to be stronger than the overestimation by HD. The main differences to the case of bulk MOSFETs are: (i) not only the velocities in the source-side of the channel, but also the sheet densities vary appreciably between the different transport models, (ii) current conservation leads to strong non-equilibrium in the highly doped source region with high velocities and electric fields and (iii) surface roughness appears to become more effective for reduced silicon film thicknesses which might jeopardize the performance enhancement upon further scaling.

Full Band Monte Carlo Study for Two-Dimensional Hole Transport in Strained Si p-MOSFETs

Transport properties of the two-dimensional hole gas in inversion layer of strained Si/SiGe p-MOSFETs are investigated using the full-band Monte Carlo simulator based on the nonlocal pseudopotential calculation. The hole mobility is significantly enhanced by the strain in the case of Ge content of ≥20%. Moreover, we also present the high-field transport characteristics of 2D holes. In contrast to the low-field mobility, the hole saturation velocity does not significantly enhanced by the strain.

Scaling and strain dependence of nanoscale strained-Si p-MOSFET performance

Self-consistent fullband Monte Carlo simulations based on nonlocal empirical pseudopotential band structures including spin-orbit splitting are employed to estimate the on-current in nanoscale strained-Si p-MOSFETs. Effective gate lengths from L/sub eff/ = 75 nm down to L/sub eff/ = 25 nm and strain levels corresponding to germanium contents of up to x = 0.4 in the relaxed Si/sub 1-x/Ge/sub x/ substrate are considered. It is found that the on-current continuously increases for growing substrate germanium contents. The strain-induced performance enhancement moderately decreases with scaling, but the improvement at L/sub eff/ = 25 nm still attains 20% for x = 0.4. In contrast to strained-Si n-MOSFETs, increasing the substrate germanium content beyond x = 0.2 is essential for p-MOSFET performance improvement by strain in the sub 0.1 /spl mu/m regime. However, even for x = 0.4 the on-current in a strained-Si p-MOSFET is still smaller than in a corresponding unstrained-Si n-MOSFET.

Comparison of zincblende-phase GaN, cubic-phase SiC, and GaAs mesfets using a full-band monte carlo simulator

We present a theoretical study of metal-semiconductor field-effect transistor (MESFET) devices for three different materials: zincblende-phase gallium nitride (ZB-GaN), cubic-phase silicon carbide (3C-SiC) and gallium arsenide (GaAs). The dc breakdown voltage of comparable MESFETs made with the two wide bandgap materials, ZB-GaN and 3C-SiC are compared to that made with the well studied material, GaAs. In this way, the GaAs calculations serve as a control, enabling an accurate comparison of the device behaviors. The simulations are performed with a new, generalized, self-consistent, full-band Monte Carlo simulator. The new simulator includes fully numerical scattering rates and a fully numerical, overlap-based final-state selection process. A 0.1 /spl mu/m gate-length MESFET is used for all of the simulations, and rectangular wells of lightly doped material are used to model interface states. The calculated dc breakdown voltages of the ZB-GaN, 3C-SiC, and GaAs MESFETs are 18, 16, and 5 V respectively. The previously estimated factor-of-four difference between the breakdown voltage of ZB-GaN and GaAs devices is verified.

CHISEL programming operation of scaled nor flash EEPROMs-effect of voltage scaling, device scaling and technological parameters

The impact of programming biases, device scaling and variation of technological parameters on channel initiated secondary electron (CHISEL) programming performance of scaled NOR Flash electrically erasable programmable read-only memories (EEPROMs) is studied in detail. It is shown that CHISEL operation offers faster programming for all bias conditions and remains highly efficient at lower biases compared to conventional channel hot electron (CHE) operation. The physical mechanism responsible for this behavior is explained using full band Monte Carlo simulations. CHISEL programming efficiency is shown to degrade with device scaling, and various technological parameter optimization schemes required for its improvement are explored. The resulting increase in drain disturbs is also studied and the impact of technological parameter optimization on the programming performance versus drain disturb tradeoff is analyzed. It is shown that by judicious choice of technological parameters the advantage of CHISEL programming can be maintained for deeply scaled electrically erasable programmable read-only memory (EEPROM) cells.

HYDRODYNAMIC MODELING OF RF NOISE FOR SILICON-BASED DEVICES

A 2D hydrodynamic model based on modified Langevin forces for terminal current noise in the RF range is presented for Si and SiGe devices, where all transport and noise parameters are generated by full-band Monte Carlo simulations under bulk conditions and stored in lookup tables. Since these tables have to be built only once, the accuracy of the noise model is improved without increasing the CPU time compared to models based on analytical expressions for the parameters. The accuracy of the noise model is assessed by comparison with the Monte Carlo device model and good agreement of both models is found for diffusion and generation noise. The terminal current noise of a realistic SiGe HBT is investigated and it is found that hole diffusion noise has a strong impact on the collector current noise. The limitations of the thermodynamic model, a compact model for noise, are explored by comparison with the hydrodynamic model.

Model of hole-initiated impact-ionization rate in α-quartz for a full band Monte Carlo simulation

The hole-initiated impact-ionization rate in α-quartz was investigated using both an energy band structure and the corresponding pseudowave functions, which are derived from a self-consistent nonlocal pseudopotential method. The ionization probability is calculated formally from Fermi’s golden rule in a straightforward way. The matrix elements for the ionization transition were evaluated from the band structure and the corresponding pseudowave functions. The matrix elements include both direct and the exchange terms with umklapp terms associated with the periodic part of the Bloch function. The hole-initiated impact-ionization rate is discussed with emphasis on both the anisotropy (wave-vector dependence of the primary hole) associated with the band structure and the contribution of the umklapp process to the impact-ionization rate. In contrast to the Keldysh formula, which has a power exponent of 2, the computed impact-ionization rates are fitted to an analytical formula that has a power exponent of 6.7, which originates from the complexity of the α-quartz band structure. We have found that primary holes that exist in the eighth and ninth valence bands labeled from the lowest valence band in ascending order especially contribute to the subthreshold region of ionization. In addition, we show the average energies of the secondary holes and electrons generated at the moment of transition as a function of the initial hole energy.

Full-band Monte Carlo simulations of high-field electron transport in GaAs and ZnS

We study the high-field transport in GaAs and ZnS based on ab initio band structures determined within density-functional theory using an exact exchange formalism with a local-density approximation for correlations. The transport properties are gained from ensemble Monte Carlo simulations where all relevant scattering mechanisms are considered including a realistic impact ionization rate. Important results are shown for the drift velocity, the mean kinetic energy, and the valley occupation.

Monte Carlo simulation and measurement of nanoscale n-MOSFETs

The output characteristics of state-of-the-art n-MOSFETs with effective channel lengths of 40 and 60 nm have been measured and compared with full-band Monte Carlo simulations. The device structures are obtained by process simulation based on comprehensive secondary ion mass spectroscopy and capacitance-voltage measurements. Good agreement between the measured output characteristics and the full-band Monte Carlo simulations is found without any fitting of parameters and the on-currents are reproduced within 4%. The analysis of the velocity profiles along the channel confirms that the on-current is determined by the drift velocity in the source side of the channel. Analytic-band Monte Carlo simulations are found to involve an overestimation of the drain current in the nonlinear regime which becomes larger for increasing drain voltage and decreasing gate length. The discrepancy originates from a higher nonlinear drift velocity and a higher overshoot peak in bulk silicon which is due to differences in the band structures above 100 meV. The comparison between analytic-band and full-band Monte Carlo simulation therefore shows that the source-side velocity in the on-state is influenced by nonlinear and quasiballistic transport.

Scaling of strained-si n-MOSFETs into the ballistic regime and associated anisotropic effects

The dependence of the strain-induced on-current improvement in n-MOSFETs on scaling and the crystallographic orientation of the channel is investigated by self-consistent full-band Monte Carlo simulation. For a channel orientation along the <110> direction, the enhancement decreases weakly from almost 40% to 30% as the effective gate length is reduced from 75 to 25 nm. For the <100> direction, the improvement is about 10% higher. The anisotropy of the drain current, which vanishes for small drain voltages, is attributed to the different band curvatures above 100 meV. This feature appears to be crucial for quasi-ballistic transport of the electrons in the high longitudinal field as they enter the source-side of the channel.

Monte Carlo simulation of the transient response of single photon absorption in X-ray pixel detectors

A Monte Carlo study of the transient response of single photon absorption in X-ray pixel detectors is presented. The simulation results have been combined with Monte Carlo simulation of the X-ray photon transport and absorption, and used to estimate the image properties of a detector system, including the pixel array and readout electronics. The study includes several different simulation challenges, such as full band Monte Carlo simulation of charge transport in large devices ( 300 μm×100 μm ), modelling of three-dimensional electrostatic effects using cylindrical coordinates, Monte Carlo simulation of photon transport and absorption, and a system level Monte Carlo simulation of the entire pixel detector and readout.

Monte Carlo study of high-field carrier transport in 4H-SiC including band-to-band tunneling

A full-band ensemble Monte Carlo simulation has been used to study the high-field carrier transport properties of 4H-SiC. The complicated band structure of 4H-SiC requires the consideration of band-to-band tunneling at high electric fields. We have used two models for the band-to-band tunneling; one is based on the overlap test and the other on the solution of the multiband Schrödinger equations. The latter simulations have only been performed for holes in the c-axis direction, since the computer capacity requirement are exceedingly high. Impact-ionization transition rates and phonon scattering rates have been calculated numerically directly from the full band structure. Coupling constants for the phonon interaction have been deduced by fitting of the simulated low-field mobility as a function of lattice temperature to experimental data. Secondary hot electrons generated as a consequence of hole-initiated impact ionization are considered in the study for both models of band-to-band tunneling. When the multiband Schrödinger equation model is used for holes in the c-axis direction, a significant change in the electron energy distribution is found, since the hole impact-ionization rate is very much increased with this model. The secondary electrons increase the average energy of the electron distribution leading to a significant increase in the electron-initiated impact-ionization coefficients. Our simulation results clearly show that both electrons and holes have to be considered in order to understand electron-initiated impact ionization in 4H-SiC.

Simulation of photon and charge transport in X-ray imaging semiconductor sensors

A fully stochastic model for the imaging properties of X-ray silicon pixel detectors is presented. Both integrating and photon counting configurations have been considered, as well as scintillator-coated structures. The model is based on three levels of Monte Carlo simulations; photon transport and absorption using MCNP, full band Monte Carlo simulation of charge transport and system level Monte Carlo simulation of the imaging performance of the detector system. In the case of scintillator-coated detectors, the light scattering in the detector layers has been simulated using a Monte Carlo method. The image resolution was found to be much lower in scintillator-coated systems due to large light spread in thick scintillator layers. A comparison between integrating and photon counting readout methods shows that the image resolution can be slightly enhanced using a photon-counting readout. In addition, the proposed model has been used to study charge-sharing effects on the energy resolution in photon counting detectors. The simulation shows that charge-sharing effects are pronounced in pixel detectors with a pixel size below 170×170 μm 2. A pixel size of 50×50 μm 2 gives a highly distorted energy spectrum due to charge sharing. This negative effect can only be resolved by introducing advanced counting schemes, where neighbouring pixels communicate in order to resolve the charge sharing.

Numerical Simulation of Field Effect Transistors in 4H- and 6H-SiC

Silicon Carbide is a very interesting semiconductor material for high temperature, high frequency, and high power applications. The main reasons are its high saturation velocity, large thermal conductivity, high Schottky barriers, and high breakdown voltages. High quality 4H-SiC and 6H-SiC polytype substrates and epitaxial layers are commercially available today. An additional advantage of SiC is the native oxide that allows fabrication of MOS devices. A large effort has been devoted towards the development of high performance devices in SiC. The largest success has been for unipolar devices like Schottky diodes and different kinds of MESFETs. MOSFETs have also been fabricated in both 4H- and 6H-SiC. Unfortunately, the MOSFET performance was found to be much worse than expected, due to a very low surface mobility. Nevertheless, the technology developed is very interesting and includes possible large scale integration of digital circuits operating at very high temperatures. In this work we present numerical simulations of the device performance of different Field Effect Transistors (FETs). Both full band Monte Carlo simulations and macroscopic modeling using the drift-diffusion approach have been utilized in this work. The Monte Carlo simulations have been used to extract transport parameters and to evaluate the macroscopic models in a device configuration.

Simulations of Submicron MOSFETs in 2H-, 4H- and 6H-SiC

In this paper, we present numerical studies of the high frequency performance of a submicron MOSFET in 2H-, 4H- and 6H-SiC. The studies are based on simulations where commercial two-dimensional drift-diffusion and hydrodynamic carrier transport models have been used. The results have been compared with those obtained from full band Monte Carlo simulations. The Monte Carlo carrier transport model is based on data from a full potential band structure calculation using the Local Density Approximation to the Density Functional Theory. In 6H-SiC the bulk transport properties in the direction perpendicular to the c-axis, are slightly lower than in 2H- and 4H-SiC. However, in the direction parallel to the c-axis the transport properties are considerably less favourable than in the other two polytypes. The effects of these differences, on surface mobility, device performance and carrier energy, have been studied.

Luminescence from p-type GaAs crystals under intense mid-infrared irradiation

The band-gap luminescence from p-type GaAs crystals is observed with the intense mid-infrared (IR) radiation beam being directed to the samples. The FEL wavelength and power dependence of the luminescence intensity is found to be consistent with the results of a full band Monte Carlo simulation of the hole motion under the intense mid-IR FEL beam.

Full Band Monte Carlo Simulation of Wurtzite AlGaN/GaN MODFETs

We present full band Monte Carlo simulations of a wurtzite Al0.15Ga0.85N/GaN modulation-doped field-effect transistor (MODFET). We found that without inclusion of the piezoelectric effect, the electron concentrations in the channel are much lower than obtained from experimental data. The calculated Ids-Vds curves show a strong negative differential resistance, which is a feature observed in experimental devices. Self-heating effects are usually believed to be the main cause of the negative differential resistance. Our simulations do not include self-heating, and this would indicate that at least part of what is observed is also caused by the drift-velocity behavior vs. electric field of the narrow conduction channel. For a 0.2 μm gate MODFET, the simulations yield a maximum trans-conductance Gm ≈ 250 mS/mm with VG = 1.0 V and Vds = 5.0 V. When VG = 0.0 V and Vds = 8.0 V, we obtain a maximum cutoff frequency fT = 180 GHz with Id = 1159 mA/mm.