Mats Hjelm

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 Schrodinger 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 mul...

Full band Monte Carlo simulation of electron transport in 6H-SiC

A study of electron transport in 6H-SiC is presented using a full band Monte Carlo simulation model. The Monte Carlo model uses four conduction bands obtained from a full potential band structure calculation based on the local density approximation to the density functional theory. Electron–phonon coupling constants are deduced by fitting the Monte Carlo simulation results to available experimental data for the mobility as a function of temperature. The saturation velocity perpendicular to the c axis is found to be near 2.0×107 cm/s, which is in good agreement with the experimental data available. In the c-axis direction the saturation velocity is much lower (4.5×106 cm/s). There are no direct experimental results available for the saturation velocity in the c-axis direction. A comparison between two-dimensional simulations of a 6H-SiC permeable base transistor, using transport parameters obtained from the Monte Carlo simulations, and experimental I–V characteristics confirms the low value. The physical m...

Monte Carlo simulation of electron transport in 2H-SiC using a three valley analytical conduction band model

A Monte Carlo study of the electron transport in 2H-SiC is presented. The study is based on a three valley analytical band model that has been derived from an ab initio band structure calculation. The scattering models have been extrapolated from recent Monte Carlo simulations of 4H-SiC and 6H-SiC. The low field mobility in the c-axis direction is higher than in 4H-SiC and 6H-SiC, while the mobility perpendicular to the c axis is similar. The saturation velocity at 300 K obtained from the Monte Carlo simulation is 2.3×107 cm/s for field applied in the c-axis direction and 1.9×107 cm/s for field applied perpendicular to the c-axis direction. The difference in mean energy as a function of electric field between 2H-SiC and 4H-SiC or 6H-SiC is large. The energy spectrum along the c axis in 2H-SiC is not discontinuous as in the case of 4H-SiC and 6H-SiC, which gives 2H-SiC a higher mean energy for electric fields applied in the c-axis direction. This indicates that the electron impact ionization coefficients s...

Monte Carlo simulation of the imaging properties of scintillator-coated X-ray pixel detectors

The spatial resolution of scintillator-coated X-ray pixel detectors is usually limited by the isotropic light spread in the scintillator. One way to overcome this limitation is to use a pixellated scintillating layer on top of the semiconductor pixel detector. Using advanced etching and filling techniques, arrays of CsI columns have been successfully fabricated and characterized. Each CsI waveguide matches one pixel of the semiconductor detector, limiting the spatial spread of light. Another concept considered in this study is to detect the light emitted from the scintillator by diodes formed in the silicon pore walls. There is so far no knowledge regarding the theoretical limits for these two approaches, which makes the evaluation of the fabrication process difficult. In this work we present numerical calculations of the signal-to-noise ratio (SNR) for detector designs based on scintillator-filled pores in silicon. The calculations are based on separate Monte Carlo (MC) simulations of X-ray absorption and light transport in scintillator waveguides. The resulting data are used in global MC simulations of flood exposures of the detector array, from which the SNR values are obtained. Results are presented for two scintillator materials, namely CsI(Tl) and GADOX.

Choice of wavefunction phases in the equations for electric-field-induced interband transitions

A set of equations for calculating the probability for electric-field-induced interband transitions in periodic crystals (Krieger and Iafrate 1986 Phys. Rev. B 33 5494) can be used in combination with the full band Monte Carlo method to study high-field electronic transport properties in semiconductors. However, when the equations are applied to realistic cases in which the underlying band structure is obtained from numerical band structure programmes, the equations are not directly solvable because of the indeterminacy of the phases of the band structure Bloch wavefunctions. Here we discuss this problem and present a method for choosing the phases of the Bloch functions in such a way that the equations yield physically correct interband transition probabilities.

Full band Monte Carlo study of high field transport in cubic phase silicon carbide

A full band Monte Carlo study of the electron transport in 3C–SiC is presented based on an ab initio band structure calculation using the local density approximation to the density functional theory. The scattering rates and impact ionization transition rates have been calculated numerically from the ab initio band structure using both energy dispersion and numerical wave functions. This approach reduces the number of empirical parameters needed to a minimum. The two empirical coupling constants used have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The peak velocity was found to be approximately 2.2×107 cm/s with a clear negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2–10 times stronger than the reported values for the hole initiated impact ionization.

The Effect of Different Transport Models in Simulation of High Frequency 4H-SiC and 6H-SiC Vertical MESFETs

A full band Monte Carlo (MC) study of the high frequency performance of a 4H-SiC short channel vertical MESFET is presented. The MC model used is based on data from a full potential band structure ...

Monte Carlo simulation of high field hole transport in 4H-SiC including band to band tunneling and optical interband transitions

The high field hole transport in 4H-SiC has been studied using a full band Monte Carlo (MC) simulation model that includes band to band tunneling and allows mixing of the band states during carrier ...

Monte Carlo study of hole mobility in Al-doped 4H–SiC

The ohmic transport of holes in p-type Al-doped 4H–SiC samples is investigated using a Monte Carlo tool based on a full-potential band structure. The temperature and doping dependence of the hole mobility and its anisotropy are calculated and discussed from a physical point of view, where we stress the importance of considering two-band conduction. Acoustic and optical phonon scattering, as well as ionized and neutral impurity scattering, have been considered. The Monte Carlo program also considers incomplete ionization of impurity atoms compatible with an Al ionization energy of 0.2 eV.

A comparison between different Monte Carlo models in simulation of hole transport in 4H-SiC

A Monte Carlo (MC) study of the hole transport in 4H-SiC is presented using three different MC models. The three models represent different approximation levels regarding band structure and scattering formulation. The most advanced model is a completely k-vector dependent full band model while the simplest model uses three analytical bands with energy dependent scattering rates. The intermediate MC model uses a full band structure calculated using a simple k·p formulation. A comparison between the models in terms of coupling constants, scattering rate, temperature dependent mobility and saturation velocity is presented.