Impact Ionization Of Carriers Research Articles

A simple Monte Carlo model for performance optimization of single photon avalanche diode

Single photon avalanche diode (SPAD) has the advantage of high internal gain, which is widely used in fluorescence detection and quantum communication. The high internal gain of the device is mainly due to avalanche multiplier effect. Therefore, it is of great significance to study avalanche multiplication effect for the design and performance optimization of SPAD devices. In this paper, a Simple Monte Carlo (SMC) model is implemented based on MATLAB, which includes interval phonon scattering and impact ionization of carriers. In this model, the electric field at different reverse bias voltages is extracted based on Technology-Computer- Aided- Design (TCAD). The model simulates the energy, velocity and impact ionization of carriers in the depletion region. The model has been verified on SPAD devices fabricated by 0.18um Bipolar - Complementary Metal Oxide Semiconductor - Diffused Metal Oxide Semiconductor (BCD) process. The SMC model simulates the dependence of the avalanche multiplication gain and excess noise factor of the SPAD device on the reverse bias voltage. In addition, the SMC model estimates the current–voltage (IV) characteristics and avalanche breakdown voltage of the device. In conclusion, the SMC model can provide guidance for the design and performance optimization of SPAD devices.

Unipolar carrier multiplication high-gain and low-noise AlGaN ultraviolet avalanche photodiode with periodically stacked structure

Highly sensitive avalanche photodetector (APD) has become a promising candidate for detecting extremely weak target signals. However, the impact ionization multiplication simultaneously triggered by electrons and holes will lead to large excess noise, thus significantly influencing device avalanche performance. Herein, we propose a distinctive AlGaN-based ultraviolet avalanche photodiode with AlN/Al0.2Ga0.8N periodically stacked multiplication region. The higher effective masses and density of states in valence band renders holes limited in the quantum-well region, where thermalization plays a dominant role during carrier transport process. On the contrary, in the atomic-scale AlN/AlGaN stacked structure with a periodic thickness of 10 nm, the electron mutualization motion is conductive to electron obtaining sufficient energy to induce impact ionization. Hence, the mechanism of unipolar carrier induced avalanche multiplication effectively reduces device noise and improving multiplication gain. Meanwhile, the high electric field intensity and tilted energy band in the AlGaN/AlN periodically stacked region significantly contribute to the carrier impact ionization. Consequently, the device achieves a superior avalanche gain of more than 105 at 74 V reverse bias. It is envisioned that the unipolar carrier triggering avalanche events offers a viable route to build high-performance APDs.

Dynamical rate equation model for femtosecond laser-induced breakdown in dielectrics

Experimental and theoretical studies of laser-induced breakdown in dielectrics provide conflicting conclusions about the possibility to trigger ionization avalanche on the subpicosecond time scale and the relative importance of carrier-impact ionization over field ionization. On the one hand, current models based on a single ionization-rate equation do not account for the gradual heating of the charge carriers, which, for short laser pulses, might not be sufficient to start an avalanche. On the other hand, kinetic models based on microscopic collision probabilities have led to variable outcomes that do not necessarily match experimental observations as a whole. In this paper, we present a rate-equation model that accounts for the avalanche process phenomenologically by using an auxiliary differential equation to track the gradual heating of the charge carriers and define the collisional impact rate dynamically. The computational simplicity of this dynamical rate-equation model offers the flexibility to extract effective values from experimental data. This is demonstrated by matching the experimental scaling trends for the laser-induced damage threshold of several dielectric materials for pulse durations ranging from a few fs to a few ps. Through numerical analysis, we show that the proposed model gives results comparable to those obtained with multiple rate equations and identify potential advantages for the development of large-scale, three-dimensional electromagnetic methods for the modeling of laser-induced breakdown in transparent media.

Characteristics of Avalanche Charge Domain in High-Power GaAs Devices

The formation of avalanche charge domain in high-power GaAs device has been studied numerically. According to the numerical simulation results of the distribution of electric field in the bulk of the device, the variation rules of peak electric field within the domain and the domain width with the bias electric field, the carrier concentration, and the device length are obtained, respectively, and by the comparison of the variation rules with and without considering carrier impact ionization, the effect of the carrier impact ionization on the peak electric field and the domain width have been obtained. It shows that 1) the peak electric field is mainly determined by the bias electric field, the carrier concentration and the device length, and the domain width is mainly determined by the carrier concentration and the device length; 2) the carrier impact ionization and avalanche multiplication have a significant effect on the peak electric field and domain width; and 3) the value of the peak electric field and domain width are closely related to the position of domain in the direction of domain motion. Based on the variation of peak electric field and domain width, the formation of avalanche charge domain is discussed. Our results can deepen the understanding of breakover mechanism of GaAs devices and contribute to the optimization of GaAs devices performance.

Analysis of Dependence of Breakdown Voltage on Gate–Drain Distance in AlGaN/GaN HEMTs With High-k Passivation Layer

A 2-D analysis of OFF-state breakdown characteristics of AlGaN/GaN HEMTs with a high- k passivation layer is performed as a function of gate-to-drain distance L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> . The relative permittivity of the passivation layer ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> is changed from 1 to 60, and L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> is changed from 1.5 to 10 μ\textm. It is shown that, in all cases with different L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> , the breakdown voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> increases as ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> increases. When a deep-acceptor density in an Fe-doped buffer layer N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DA</sub> is 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17 </sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and the gate length is 0.3 μm, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is determined by buffer leakage current at ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ≥ 30 before impact ionization dominates. Hence, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is similar at L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> = 3-10 μm, and the increase rate in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> from L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> = 1.5 μm is about 50% even at ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> = 60. However, when N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DA</sub> is 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is determined by impact ionization of carriers even at ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ≥ 30 because the buffer leakage current is reduced. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> becomes about 500, 930, 1360, and 1650 V for L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> = 1.5, 3, 5, and 7 μm, respectively, at ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> = 60. These voltages correspond to gate-to-drain average electric fields of about 3.3, 3.1, 2.7, and 2.3 MV/cm, respectively. Particularly, for short L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> , the electric field profiles between the gate and the drain are rather uniform. However, in the case of L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> = 10 μm, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is about the same as that (1650 V) of L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> = 7 μm, suggesting that the electric field at the drain edge of the gate becomes a critical value before the high electric field region extends to the drain enough. This may be a limitation to increase V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> by using a high- k passivation layer in this case. However, it can be said that, to improve V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> further at long L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> , such as 10 μm, the combination of field plate or using a higher ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> material may be effective because both of them decrease the electric field at the drain edge of the gate.

Constraints on low-mass, relic dark matter candidates from a surface-operated SuperCDMS single-charge sensitive detector

This article presents an analysis and the resulting limits on light dark matter inelastically scattering off of electrons, and on dark photon and axion-like particle absorption, using a second-generation SuperCDMS high-voltage eV-resolution detector. The 0.93 gram Si detector achieved a 3 eV phonon energy resolution; for a detector bias of 100 V, this corresponds to a charge resolution of 3% of a single electron-hole pair. The energy spectrum is reported from a blind analysis with 1.2 gram-days of exposure acquired in an above-ground laboratory. With charge carrier trapping and impact ionization effects incorporated into the dark matter signal models, the dark matter-electron cross section $\bar{\sigma}_{e}$ is constrained for dark matter masses from 0.5--$10^{4} $MeV$/c^{2}$; in the mass range from 1.2--50 eV$/c^{2}$ the dark photon kinetic mixing parameter $\varepsilon$ and the axioelectric coupling constant $g_{ae}$ are constrained. The minimum 90% confidence-level upper limits within the above mentioned mass ranges are $\bar{\sigma}_{e}\,=\,8.7\times10^{-34}$ cm$^{2}$, $\varepsilon\,=\,3.3\times10^{-14}$, and $g_{ae}\,=\,1.0\times10^{-9}$.

Defect Assisted Carrier Multiplication in Amorphous Silicon

Recent studies show disordered materials such as amorphous silicon (a-Si), in spite of their low mobility, can efficiently amplify photocurrent via carrier multiplication process. Detectors with a thin (~40nm) a-Si multiplication layer has demonstrated single photon sensitivity at high speed under room temperature. It is believed that the abundance of bandtail states that cause low mobility of the amorphous materials actually contribute to the detector’s superior performance because carrier impact ionization involving these states, which can be modeled by donor-acceptor pairs (DAPs), relax the k-selection rule of the many-body carrier multiplication process. Our paper presents a theoretical framework to calculate the carrier multiplication process in a-Si or other disordered materials involving DAPs. Our analysis also establishes the relations between detector characteristics and key parameters such as the density of band tail states, layer thickness, and applied electric field. DAP assisted carrier multiplication rate is computed first. Carrier multiplication coefficients for electrons and holes under given applied field are then calculated using a trial distribution function that satisfies both the continuity equation and the energy balance equation. Our analysis shows that thinner a-Si gives rise to higher multiplication efficiency than thicker a-Si because of its higher carrier kinetic energy when the layer thickness is shorter than the length of energy relaxation by phonon scattering. Using the calculated carrier multiplication coefficients, voltage dependent gain of the device is computed and the results agree well with the measured results of a-Si cycling excitation process (CEP) detectors.

A Physical-Based Analytical Model for the Kink Current of Polycrystalline Silicon TFTs

A physical-based analytical kink current model for polycrystalline -silicon (poly-Si) thin-film transistors (TFTs) has been proposed. Two important mechanisms for the kink current, namely carrier impact ionization and the parasitic bipolar junction transistor effect, are physically included in the model without introducing any artificial parameters. The proposed kink current model is fully compatible with existing ON-state drain current models of poly-Si TFTs. Model parameter extraction procedure is presented, which is based on a group of output characteristics of poly-Si TFTs with different channel lengths. The model prediction straightforwardly calculated with a set of extracted parameters is verified by excellent agreement with experimental output characteristics measured from TFTs over a range of channel lengths and gate voltages.

The Influence of Structural Defects in Silicon on the Formation of Photosensitive Mn4Si7–Si❬Mn❭–Mn4Si7 and Mn4Si7–Si❬Mn❭–M Heterostructures

The effect of the amorphous transition layer at the interface between Mn4Si7 and silicon doped with manganese on the photoelectric properties of heterostructures is considered. It is found that the precipitated Mn atoms on the silicon surface are grouped at high temperatures, forming drops of liquid manganese, which dissolve the near-surface layer of silicon, forming a liquid solution-melt of Mn with Si. As the solution solidifies, Mn4Si7 forms, and the Si–Si bonds under the silicide break due to intense diffusion of Si atoms; an elastically deformed Si region forms, which predetermines the evolution of the formation of photoelectric phenomena in the Mn4Si7–Si❬Mn❭–Mn4Si7 and Mn4Si7–Si❬Mn❭–M heterostructures. The microstructure and chemical composition of doped Si❬Mn❭ samples were studied by means of scanning electron microscopy and X-ray energy dispersive spectrometry using a Quanta 200-3D microscope, and the interface structure of the higher manganese silicide (HMS) and Si❬Mn❭ layer at the nanoscale was refined using the Fourier transform of local zones of high-resolution electron-microscopic images. It is concluded that in the process of diffusion doping of silicon with manganese, broken layers on the surface of the crystal deepen the embedding of manganese atoms, facilitate adsorption, dissolution, and diffusion of Mn in the volume of Si, and also enable the formation of an amorphous layer at the interface of higher manganese silicide and the Si❬Mn❭ layer. The presence of an amorphous transition layer facilitates the process of impact ionization of current carriers upon application of external voltage, as well as the formation of photoelectric phenomena: infrared quenching, temperature quenching, high photosensitivity, and long-term relaxation of residual conductivity.

Temperature dependence of single-event transient response in devices with selective-buried-oxide structure

The silicon-on-insulator (SOI) device has been found to possess low leakage current and high operation speed due to reduced internal capacitances. The sensitive volume for charge collection in SOI device is smaller than that in bulk-silicon device, which improves the ability of SOI devices to resist single-event effect (SEE). In spite of these benefits, the SOI device has certain undesirable effects such as the kink effect. To mitigate the kink effect, selective-buried-oxide (SELBOX) SOI structure has been introduced. Space-borne electronic circuits based on SOI technology recently have been used in high radiation and extreme temperature environments. However, temperature affects internal carrier transport process and impact ionization process, which makes single-event transient (SET) pulse widths increased. Most of previous researches regarding temperature dependence of SEE were for SOI floating-body devices. But the influence of operating temperature on SEE of SELBOX SOI devices are yet unclear. In this paper, an SOI floating-body device and a SELBOX SOI device under 90 nm process are established by three-dimensional device simulation, and then temperature dependence of SET response in partially depleted SOI inverter chains is studied by a mixed-mode approach over a temperature range from 200 K to 450 K. Simulation results show that the N-type SELBOX SOI device has a better ability to resist SEE than the floating-body device, while the P-type SELBOX SOI device has the same ability to resist SEE at high linear energy transfer value as the floating-body device. And temperature dependence analysis of charge collection indicates that there is only drift-diffusion process in the N-type SELBOX SOI device. The amount of charge collection in the N-type SELBOX SOI device almost does not change with the increase of temperature. In addition, both the P-type SELBOX SOI device and the P-type floating-body device have a bipolar amplification process. With the increase of temperature, the bipolar amplification process in the substrate turns more serious. However, it suppresses the bipolar amplification process of the source because of SELBOX structure, so that the amount of charge collection is reduced in the drain significantly. According to our simulation results, compared with the floating-body device, the SELBOX SOI device can very well suppress the influence of temperature on SET pulse.

Simulation of Low Gain Avalanche Detector characteristics based on the concept of negative feedback in irradiated silicon detectors with carrier impact ionization

In this study the main characteristics of silicon Low Gain Avalanche Detectors (LGAD), the dependencies of the collected charge versus bias voltage and fluence, are calculated to fit experimental data. The calculations are based on two previously developed Ioffe Institute models of radiation degradation in Si detectors: 1) a model of two effective energy levels of radiation-induced defects, and 2) a mechanism of internal negative feedback responsible for the gain degradation in irradiated Si detectors originating from the avalanche multiplication at the detector junction. The combination of these models describes well the properties of irradiated p-i-n detectors in a wide range of fluences. For simulating the LGAD characteristics the models are adapted to its n+-pbi-p-p+ structure, where the built-in boron-doped layer pbi produces high electric field sufficient for carrier impact ionization. It is shown that the developed models give adequate quantitative description of the experimental results for the LGADs up to the fluence of 2×1015 n/cm2 including the detector pulse response; however, additional boron removal from the pbi layer is required to have the best correlation with the experimental data. Similar to the physical model developed for silicon strip detectors operated at high voltage, the results are interpreted in terms of the internal negative feedback mechanism. It is shown that in irradiated LGADs this feedback leads to the transfer of a significant fraction of the potential drop from the built-in layer toward the p+ contact. It initiates two negative effects, which both cause the gain degradation with irradiation: the lowering of the electric field in the n+-pbi region that reduces the multiplication probability, and the increase of the collection time and trapping-related charge losses.

Uniform and Reliable GaN &lt;italic&gt;p-i-n&lt;/italic&gt; Ultraviolet Avalanche Photodiode Arrays

GaN p-i-n ultraviolet avalanche photodiodes (UV-APDs) were fabricated from epitaxial structures grown on low-dislocation-density free-standing GaN substrates to form $4\times 4$ UV-APD arrays with a device size of $75\times 75~\mu \text{m}^{{ {2}}}$ . The devices in the UV-APD array showed a uniform and reliable distribution of breakdown voltage ( $V_{\text {BR}}$ ) and leakage current density. The average $V_{\text {BR}}$ of the 16 devices in one of the UV-APD arrays was 96±0.6 V, and the average dark current density ( $J_{R\_{}{\text {Dark}}}$ ) and photocurrent density ( $J_{R\_{}{\text {Photo}}}$ ) were measured to be (6.5±1.8) $\times 10^{-7}$ and (5.7±1.1) $\times 10^{{\text {-6}}}$ A/cm $^{{{2}}}$ at the reverse bias voltage of $V_{R}=48$ V (50% of the average onset point of $V_{\text {BR}}$ ), respectively. The reliable device performance was confirmed by performing multiple reverse bias $I$ – $V$ scans for the selected devices in the UV-APD array. We also observed the significantly enhanced spectral responsivity from the 142 to 5485 mA/W due to the strong carrier impact ionization at high reverse bias.

Effect of temperature on avalanche region width and DC to RF conversion efficiency of the p+nn−n+ 4H–SiC impact avalanche transit time diodes

The effect of temperature on avalanche region width and DC to RF conversion efficiency of the p+nn−n+ impact ionization avalanche transit time diodes based on 4H–SiC material for 140 GHz design frequency has been investigated by means of the MEDICI simulation platform. It is shown that the avalanche region width of the IMPATT diodes working at the same operating current densities first decreases and then increases with the increasing temperature. The DC to RF conversion efficiency of the IMPATT diodes first increases and then decreases with the increasing temperature. The increase in ionization rate of impurities with increasing temperature and the decrease in the carrier impact ionization rate with increasing temperature are responsible for the results. In addition, the expansion of avalanche region and the degradation of DC to RF conversion efficiency at lower temperature are more pronounced in lower p+ region doping concentration diode than that in higher p+ region doping concentration.

Effects of buffer leakage current on breakdown characteristics in AlGaN/GaN HEMTs with a high-k passivation layer

A two-dimensional analysis of off-state breakdown characteristics in AlGaN/GaN HEMTs is performed by considering a deep donor and a deep acceptor in a buffer layer, with the relative permittivity of passivation layer εr as a parameter. The analysis is made with and without impact ionization of carriers to study how the buffer leakage current affects the breakdown performance. It is shown that when εr is low, the breakdown voltage is determined by the impact ionization of carriers, and when εr becomes high, it is determined by the buffer leakage current. This buffer leakage current decreases as εr increases because the electric field at the drain edge of the gate is weakened, and hence the breakdown voltage increases as εr increases. It is also shown that when the gate voltage is more negative, the breakdown voltage in the high εr region becomes higher because the buffer leakage current becomes smaller.

A numerical study of low- and high-field carrier transport properties in In0.18Al0.82N lattice-matched to GaN

This work presents a numerical study of the carrier transport properties of In0.18Al0.82N lattice-matched to GaN. Using a full-band Monte-Carlo model, we have evaluated the low- and high-field transport coefficients of this alloy. We have computed the carrier mobilities as a function of temperature for different doping concentrations and areal dislocation densities. Furthermore, we have evaluated the electron and hole drift velocities with and without considering the effect of the alloy scattering. Finally, we have computed the carrier impact ionization coefficients for transport along both the Γ−A and Γ−M crystallographic directions.

Analysis of buffer-impurity and field-plate effects on breakdown characteristics in small-sized AlGaN/GaN high electron mobility transistors

The two-dimensional analysis of breakdown characteristics in field-plate AlGaN/GaN high electron mobility transistors with a relatively short gate length and short gate-to-drain distances is performed by considering a deep donor and a deep acceptor in a buffer layer. It is shown that when the acceptor density in the buffer layer is high, the breakdown voltage is determined by the impact ionization of carriers, and it can decrease with increasing the field-plate length. This is because the distance between the field-plate edge and the drain becomes very short and the electric field there becomes very high. On the other hand, when the acceptor density in the buffer layer is relatively low, the buffer leakage current becomes very large and this can determine the breakdown voltage, which becomes very low. In this case, the breakdown voltage increases with increasing the field-plate length.

A numerical study of carrier impact ionization in AlxGa1−xN

Using a full-band Monte Carlo model we have computed the carrier impact ionization coefficients in AlxGa1−xN for seven alloy compositions between x = 0 (GaN) and x = 1.0 (AlN). We have found that holes dominate the impact ionization process for compositions below 50%, while electrons dominate for larger aluminum contents. The model also predicts that, due to the particular features of the AlxGa1−xN valence band structure, holes impact ionization processes are effectively negligible for aluminum composition above 60%. Furthermore, we find that the electron-alloy scattering significantly reduces the electron ionization coefficients leading to the dominant behavior of holes in for an aluminum composition below 50%.

Improvement in breakdown characteristics of 4H-SiC MESFET with a gate-drain surface epi-layer and optimization of the structure

A novel SiC MESFET structure with a p-type surface epi-layer is proposed and 4H-SiC MESFET models are presented which precisely describe the working mechanism of the device. Considering carrier velocity saturation, impact ionization and electric field modulation, the effect on distribution of electric field is analyzed. Also, the output current (Ids) and breakdown voltage (VB) dependences on the dimensions of the p-type epi-layer are studied based on abrupt junction approximation. The high electric field peak at the gate edge is suppressed by introducing a new electric field peak at the drain side, and the built-in field of p-n junction formed along channel surface further weakens the electric field peaks, leading to smoother distribution of electric field. By comparison with the conventional and the fieldplated MESFETs, it is shown that the proposed structure greatly improves the characteristic of SiC MESFET. In addition, the optimized dimensions are obtained and the results show that VB is greatly increased by 33% with Ids unchanged (less than 3%) when the thickness and the doping concentration of the surface epi-layer are chosen as 0.12 upm and 5 1015 cm-3, respectively.

Threshold conductivity switching in sulfurized antimony selenide nanowires

We report reversible switching between Ohmic and negative differential resistance states at a threshold voltage in sub-100-nm diameter sulfurized antimony selenide nanowires. We show that threshold switching in our nanowires arises due to high non-equilibrium free carrier concentrations resulting from impact ionization of carriers from defect states traceable to sulfurization and surface dangling bonds. Threshold switching is suppressed because of inhibited carrier generation at air-passivated defect states or at high temperatures due to thermally induced carrier depletion from deep states which preempts impact ionization. Such non-linear phenomena would be important for designing phase-change memories, thermoelectric devices, and sensors using pnictogen chalcogenide nanowires.

First-principles investigation of carrier Auger lifetime and impact ionization rate in narrow-gap superlattices

We investigate theoretically the technologically essential Auger recombination lifetime in narrow-gap semiconductor superlattices by means of a completely first-principles formalism, based on accurate energy bands and wave functions provided by the full-potential linearized augmented plane wave scheme. The minority carrier Auger lifetimes are determined by two correlated approaches: (1) direct evaluation in Fermi's golden rule, and (2) indirect evaluation, based on a detailed balance formulation relating Auger recombination and its inverse process, impact ionization, in a unified framework. Lifetimes determined by the direct and indirect methods for n -doped HgTe/CdTe and InAs/InxGa1－xSb superlattices exhibit excellent consistency with experimentally measured values. This justifies the computational formalism as a new sensitive tool in performance optimization of the synthetic narrow-gap semiconductor superlattice systems.