J. D. Albrecht

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...

Ensemble Monte Carlo study of electron transport in wurtzite InN

Electronic transport in wurtzite phase InN is studied using an ensemble Monte Carlo method. The model includes the full details of the first five conduction bands derived from the pseudopotential method and a numerically calculated impact ionization transition rate using a wave-vector- dependent dielectric function. Calculated results for electron transport at both low and high electric field are presented and compared with available results from simpler methods. The dependence of the relevant transport properties on the parameters is discussed, in particular in regards to the uncertainties in the band structure and coupling constants. It is found that at a field of 65 kV/cm that the peak electron drift velocity is 4.2×107 cm/s. The peak velocity in InN is substantially higher than in GaN. The velocity field curve presents a noticeable anisotropy with respect to field direction. The peak velocity decreases to 3.4×107 cm/s for a field of 70 kV/cm in the direction perpendicular to the basal plane. The elect...

High field electron transport properties of bulk ZnO

The Monte Carlo method is used to simulate electron transport for electric field strengths up to 350 kV/cm in bulk, wurtzite structure ZnO. The relevant parts of the conduction bands of a first-principles band structure are approximated by spherically symmetric, nonparabolic valleys located at the Γ and Umin symmetry points of the Brillouin zone. It is shown that the analytic expressions represent the band structure and the density of states well over a range of nearly 5 eV from the bottom of the conduction band. The simulated electron steady-state drift velocity versus electric field characteristics are calculated for lattice temperatures of 300, 450, and 600 K. For room temperature, drift velocities higher than 3×107 cm/s are reached at fields near 250 kV/cm. Examination of the electron energy distributions shows that the strong decrease of the differential mobility with increasing electric field in the field range studied is to be associated with the pronounced nonparabolicity of the central valley and...

AlGaN/GaN heterostructure field-effect transistor model including thermal effects

A new AlGaN/GaN heterostructure field-effect transistor (HFET) model, in the framework of the gradual channel approximation and based on Monte Carlo simulations of the electron transport properties, is presented. The effects on the dc HFET output characteristics arising from contact resistances, from the ungated access channels between the gate and the source and between the gate and the drain, and from self-heating are analyzed. By examining the channel potential, the ungated regions are shown to have nonlinear characteristics. The solution method uses implicit analytical relationships for the current in the gated and ungated segments of the channel that are connected by matching boundary conditions. Thermal effects on the transport parameters associated with self-heating are included self-consistently. The model results are in very good agreement with experimental data from AlGaN/GaN HFETs fabricated on sapphire substrates. The model also identifies several device design parameters that need to be adjusted to obtain optimized performance in terms of output current and transconductance.

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.

Piezoelectric effect in (001)‐ and (111)‐oriented double‐barrier resonant tunneling devices

We show experimentally that current–voltage characteristics of double‐barrier resonant tunneling devices (DBRTDs) can be modified by internal polarization fields due to the piezoelectric effect induced by external uniaxial stresses. Electric polarization fields, perpendicular to the interfaces, arise in DBRTDs grown on (001)‐oriented substrates under uniaxial, compressive stresses parallel to the (110) or (110) crystal orientations, and in DBRTDs grown on (111)B‐oriented substrates under stress parallel to (111) crystal orientation. The voltages at which the resonant tunneling current peaks occur (peak voltages) are sensitive to the polarization fields induced by external stresses. The peak voltages can shift to more positive voltages or more negative voltages depending on the directions of applied stresses. We measured current–voltage characteristics of AlAs/GaAs/AlAs double‐barrier resonant tunneling structures as a function of external stresses at 77 K. Uniaxial stress was applied parallel to the (110...

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.

Piezoelectric effects in (001)‐oriented double barrier resonant tunneling structures

We show that piezoelectric effects can give rise to internal electric fields that modify the current–voltage characteristics of double barrier resonant tunneling devices, if suitable stresses are applied. Electric polarization fields in the direction perpendicular to the interfaces arise in heterostructures on (001)‐oriented substrates under uniaxial stress parallel to the (110) or (110) orientations. We measured current–voltage characteristics of (001)‐oriented AlAs/GaAs/AlAs double barrier structures as a function of uniaxial external stress applied parallel to the (110) and the (110) orientations. The substrate contact was grounded in all measurements. Under (110) stress, the resonance peaks shift to more positive voltages for both positive and negative bias. For (110) stress, the peaks shift toward more negative voltages. We also calculated the current–voltage characteristics of resonant tunneling structures under uniaxial stress, taking into account stress effects on the band alignment and piezoel...