Maziar Farahmand

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Electron transport characteristics of GaN for high temperature device modeling

Monte Carlo simulations of electron transport based upon an analytical representation of the lowest conduction bands of bulk, wurtzite phase GaN are used to develop a set of transport parameters for devices with electron conduction in GaN. Analytic expressions for spherical, nonparabolic conduction band valleys at the Γ, U, M, and K symmetry points of the Brillouin zone are matched to experimental effective mass data and to a pseudopotential band structure. The low-field electron drift mobility is calculated for temperatures in the range of 300–600 K and for ionized impurity concentrations between 1016 and 1018 cm−3. Compensation effects on the mobility are also examined. Electron drift velocities for fields up to 500 kV/cm are calculated for the above temperature range. To aid GaN device modeling, the drift mobility dependences on ambient temperature, donor concentration, and compensation ratio are expressed in analytic form with parameters determined from the Monte Carlo results. Analytic forms are also...

Monte Carlo calculation of electron transport properties of bulk AlN

The Monte Carlo method is used to simulate electron transport in bulk, wurtzite phase AlN using a three valley analytical band structure. Spherical, nonparabolic conduction band valleys at the Γ, K, and U symmetry points of the Brillouin zone are fitted to a first-principles band structure. The electron drift mobility is calculated as a function of temperature and ionized donor concentration in the ranges of 300–600 K and 1016–1018 cm−3, respectively. The effect of compensation on ionized impurity scattering and the associated change in the mobility are considered. The simulated electron steady-state drift velocity and valley occupancy for electric fields up to 600 kV/cm are presented for 300, 450, and 600 K. Our calculations predict that AlN will exhibit a much smaller negative differential mobility effect than GaN, and that the drift velocity versus electric field curve will show a very broad peak.

Materials theory based modeling of wide band gap semiconductors: from basic properties to devices

Abstract In this paper we present a general methodology, materials theory based modeling, for predicting device performance in technologically immature materials that can proceed relatively independently of experiment. The models incorporated within this general approach extend from a fundamental physics based, microscopic analysis to macroscopic, engineering based device models. Using this scheme, we have investigated the transport and breakdown properties of several emerging wide band gap semiconductor materials, i.e. GaN, InN, 3C-SiC, and 4H-SiC. The carrier drift velocities, mobilities, and impact ionization coefficients for these materials can be predicted using the materials theory based modeling method. Using these results, device level simulations can then be made. Here we report Monte Carlo and selfconsistent charge control modeling of GaN based devices. Comparison to experimental measurements is made when possible. Good agreement between the selfconsistent charge control model calculations and experiment is obtained. Some of the issues pertinent to heterostructure bipolar transistor modeling of GaN are discussed.

Full band Monte Carlo simulation of zincblende GaN MESFET's including realistic impact ionization rates

In this paper, we present the first theoretical study of the breakdown properties of zincblende phase GaN MESFET devices. The calculations are made using a full band, ensemble Monte Carlo simulation that includes a numerical formulation of the impact ionization transition rates. The breakdown voltage, transconductance and cutoff frequency are calculated for the GaN MESFET under two different conditions, with and without semiconductor-oxide interface states. Uniform surface depletion regions model the effect of the interface states. It is found that the breakdown voltage of the zincblende GaN MESFET is less dependent upon the surface depletion conditions than a corresponding GaAs MESFET. It is also found that the drain current increases more gradually with increasing drain-source voltage at the onset of breakdown and that the breakdown voltage of the zincblende GaN MESFET is predicted to be several times larger than that of a comparable GaAs MESFET. The maximum current gain cutoff frequency of a 0.1 /spl mu/m gate length GaN MESFET is calculated to be 230 and 220 GHz, for the non-surface-depleted and the surface depleted devices respectively.

Monte Carlo simulation of noncubic symmetry semiconducting materials and devices

In this paper, we discuss the complexities that arise in Monte Carlo based modeling of noncubic symmetry semiconductors and their related devices. We have identified three general issues, band structure, scattering mechanisms, and band intersections that require some modification of the Monte Carlo simulator from that for cubic symmetry. Owing to the increased size and number of atoms per unit cell, the band structure is far more complex in noncubic than in zincblende phase semiconductors. This added complexity is reflected by the greater number of bands, smaller Brillouin zone and concomitant increase in the number of band intersections. We present strategies for modeling the effects of band intersections on the carrier dynamics using the Monte Carlo method. It is found that the band intersection points greatly affect the carrier transport, most dramatically in the determination of the impact ionization and breakdown properties of devices and bulk material. Excellent agreement with experimental measurements of the impact ionization coefficients is obtained only when treatment of the band intersections is included within the model.

Comparison between wurtzite phase and zincblende phase GaN MESFETs using a full band Monte Carlo simulation

Cutoff frequency, breakdown voltage, and the transconductance of wurtzite and zincblende phase GaN MESFETs have been calculated using a self-consistent, full band Monte Carlo simulation. The effect of interface states on the device performance is modeled by including uniformly depleted regions at the device surface under the passivation layers. It is found that the drain current increases gradually with increasing drain-source voltage at the onset of breakdown for both phases. The calculated breakdown voltage for the wurtzite device is considerably higher than the breakdown voltage calculated for the zincblende device. On the other hand, the zincblende device is calculated to have higher transconductance and cutoff frequency than the wurtzite device. The higher breakdown voltage of the wurtzite phase device is attributed to the higher density of electronic states for this phase compared to the zincblende phase. The higher cutoff frequency and transconductance of the zincblende phase device is apparently due to the greater electron velocity overshoot for this phase compared to that for the wurtzite phase. The maximum cutoff frequency and transconductance of a 0.1 /spl mu/m gate-length zincblende GaN MESFET are calculated to be 220 GHz and 210 mS/mm, respectively. The corresponding quantities for the wurtzite GaN device are calculated to be 160 GHz and 158 mS/mm.

Monte Carlo Calculation Of High- And Low-Field Al x Ga 1−x N Electron Transport Characteristics

The Monte Carlo technique is used to simulate electron transport in bulk, wurtzite phase Al x Ga 1−x N. A multi-valley analytical band model consisting of five spherical, non-parabolic conduction band valleys at the Γ, U, M, and K symmetry points of the Brillouin zone is matched to band structures of GaN and AlN. Parameters for the Al x Ga 1−x N alloy are obtained by linear interpolation. The Monte Carlo simulations are performed for ambient temperatures in the range of 300K to 600K. Scattering mechanisms taken into account include ionized impurity scattering and alloy scattering, in addition to deformation potential scattering (intra- and inter-valley), and polar optical phonon scattering. We present results for the electron steady-state drift velocity and the valley occupancy for electric fields up to 500 kV/cm. Low-field drift mobilities are extracted from the Monte Carlo calculations as functions of the electron concentration, of the ambient temperature, and of the alloy composition.

Simulation of carrier transport in wide band gap semiconductors

Dept. of Electrical Engineering, Boston University, Boston, MA. 02215. Movaz Networks, 5445 Triangle Parkway, Norcross, GA 30092 Dipartimento di Elettronica, Politecnico di Torino, corso Duca degli Abruzzi 24,110129 Torino, Italy Dept. of Information Technology, Mid-Sweden University, S-851 70 Sundsvall, Sweden. School of Electrical and Computer Engineering, Georgia Tech, Atlanta, GA, 303320250 Dept. of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455.

Monte Carlo modeling of wurtzite and 4H phase semiconducting materials

We present a discussion of the complexities encountered in particle simulation models for noncubic symmetry semiconductors, focusing on the wurtzite and 4H polytypes of GaN and SiC. We have identified three general issues, band structure, scattering mechanisms, and band intersections, which in our opinion, constitute the most important modifications to conventional Monte Carlo simulators for cubic symmetry semiconductors. It is found that the band intersection points present the greatest modeling challenge. We discuss the effect of band intersections on the transport dynamics and our initial attempts at treating transport near these points. Comparison to experimental data is also made.