Avalanche Generation Research Articles

Viscoelastic Slider Blocks as a Model for a Seismogenic Fault.

In this work, a model is proposed to examine the role of viscoelasticity in the generation of simulated earthquake-like events. This model serves to investigate how nonlinear processes in the Earth's crust affect the triggering and decay patterns of earthquake sequences. These synthetic earthquake events are numerically simulated using a slider-block model containing viscoelastic standard linear solid (SLS) elements to reproduce the dynamics of an earthquake fault. The simulated system exhibits elements of self-organized criticality, and results in the generation of avalanches that behave similarly to naturally occurring seismic events. The model behavior is analyzed using the Epidemic-Type Aftershock Sequence (ETAS) model, which suitably represents the observed triggering and decay patterns; however, parameter estimates deviate from those resulting from natural aftershock sequences. Simulated aftershock sequences from this model are characterized by slightly larger p-values, indicating a faster-than-normal decay of aftershock rates within the system. The ETAS fit, along with realistic simulated frequency-size distributions, supports the inclusion of viscoelastic rheology to model the seismogenic fault dynamics.

Impact of Dimensions and Doping on the Breakdown Voltage of a Trench 4H-SiC Vertical JFET

In this paper we study the feasibility of the design/fabrication of a vertical trench 4H-SiC Junction Field Effect Transistor (JFET), assuming realistic constraints of the depth of the P+ implantation. The P+ doping profile is obtained using a Monte Carlo implantation simulation. The calculation used a drift-diffusion approach. The JFET aims to achieve a threshold voltage of-3V. We found that this constraint in concomitance with the proposed structure limits the breakdown voltage to approximately 200V. This is the result of a premature breakdown induced by short channel effects, namely Drain Induced Barrier Lowering (DIBL). However, a negative increase in the gate bias represses this short channel effect and improves the breakdown voltage to roughly 1800V. At this gate bias, the breakdown is induced by reaching the critical field strength of 4H-SiC at the gate P+/N junction, which causes avalanche generation of carriers. In addition, we have calculated the dependence of the threshold voltage on the drift doping and pillar width. This work also shows the vulnerability of the design to random fluctuation in the doping profile.

Development and test of innovative Low-Gain Avalanche Diodes for particle tracking in 4 dimensions

The MIUR PRIN 4DInSiDe collaboration aims at developing the next generation of 4D (i.e., position and time) silicon detectors based on Low-Gain Avalanche Diodes (LGAD) that guarantee to operate efficiently in the future high-energy physics experiments. To this purpose, different areas of research have been identified, involving the development, design, fabrication and test of radiation-hard devices. This research has been enabled thanks to ad-hoc advanced TCAD modelling of LGAD devices, accounting for both technological issues as well as physical aspects, e.g. different avalanche generation models and combined surface and bulk radiation damage effects modelling. In this contribution, it is reviewed the progress and the relevant detector developments obtained during the research activities in the framework of the 4DInSiDe project. • TCAD modelling for the design of radiation-hard LGAD sensors for 4D tracking. • Gain layer compensation, (p ＋ - and n ＋ -doping) to preserve the gain at high fluences. • New design approach to resistive read-out sensors: DC-coupled RSD. • DC-RSD employs a direct coupling of the resistive layer to the read-out pads. • DC-coupled low resistivity strips between read-out pads to improve the resolution.

Direct comparison of silicon carbide and silicon diode avalanche shaper in multi-pulse applications

This paper compares the pulse shaping characteristics of SiC and Si Diode Avalanche Shaper (DAS) under quasi-uniform triggering with the help of Technology Computer-Aided Design (TCAD) software. Under the same static breakdown voltage (VSB), the opening speed of SiC DAS does not show obvious advantages over that of Si DAS. With the same device structure, SiC DAS has a higher breakdown voltage than Si DAS, so it has a faster dV/dt in the triggering process. In addition, compared to Si, SiC DAS delayed breakdown voltage (VDB) is less affected by the input pulse rise rate. This is because the SiC DAS avalanche generation rate is more sensitive to the device voltage in the vicinity of VDB. Then, the multi-pulse operating characteristics of SiC and Si DAS with the same VSB were compared by continuously triggering the SiC and Si DAS 50 times at the 1 MHz frequency. It is revealed that the VDB of the SiC DAS is low but has excellent multi-pulse consistency. At the same time, under multi-pulse operation, the temperature rise speed of SiC DAS is slow, and the temperature distribution in the device is more uniform. It can be concluded that SiC DAS has significant advantages in high voltage and multi-pulse shaping.

Implications of Various Charge Sources in AlGaN/GaN Epi-Stack on the Drain & Gate Connected Field Plate Design in HEMTs

We have established that a design strategy for drain and gate connected field plates should be adopted while keeping in mind the interplay of various charge sources across AlGaN/GaN epi-stack, which governs the electric field distribution across HEMT. The investigations in this work are carried out for Schottky, MIS and p-GaN gate stacks while accounting for possible GaN buffer types (Fe-doped and C-doped). The role of gate and drain field plates was found to be different in the Fe-doped buffer compared to the C-doped buffer. More than suppression of avalanche generation, mitigation of gate injection by shifting the peak electric field position away from the gate edge was found to be the dominant cause of breakdown voltage improvement when field plates were adopted. In a few cases, however, the widening of the depletion region near the gate or dominance of the buffer field was the reason for breakdown voltage improvement with a gate field plate. On the other hand, the drain field plate was found to be effective only for lower polarization % and lower surface trap concentration. The role of buffer trap parameters, surface/passivation trap concentration, interface trap concentration at the gate, and passivation thickness in defining the optimum field plate strategy are discussed.

Ultrafast synthesis and densification of ZrO2 doped KNN ceramics by reactive flash sintering

AbstractFabrication of dense KNN‐based lead‐free piezoelectric ceramics at low temperatures in short time through a cost‐effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO3‐NaNbO3 into (K,Na)NbO3 and densification occur simultaneously during the flash event. Most importantly, ZrO2 doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash‐sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO2 doped and undoped KNN ceramics show a comparable coercive field Ec, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid‐state reaction and densification rates.

Interplay of Various Charge Sources in AlGaN/GaN Epi-Stack Governing HEMT Breakdown

In this work, we have revealed the interplay of various charge sources (surface, polarization, and buffer) and their relative concentrations across the AlGaN/GaN epi-stack governing the electric field distribution and the breakdown mechanism in high electron mobility transistors (HEMTs). The investigations are carried out for Schottky, metal-insulator-semiconductor (MIS), and p-GaN gate stacks while accounting for possible GaN buffer types (Fe-doped and C-doped). A strong correlation between 3-terminal (3T) breakdown voltage, gate design, and relative concentration of various charges was found. % increase in 3T breakdown voltage when physical dimensions were doubled was also found to be strongly correlated with the relative concentration of various charges for both Fe- and C-doped buffer HEMTs. On the other hand, the 2T (source–drain, without gate, and GaN channel below gate) breakdown voltage was independent of all other parameters except buffer properties and physical dimensions. Physical insights are developed to explain the dependence of electric field distribution, carrier injection, and HEMT breakdown on the surface states, polarization charge, and buffer traps, as well as their relative concentrations for both the buffer types and all three gate types. These insights will help to design efficient surface passivation schemes and resolve ambiguities, often observed in experiments, in terms of location of peak electric field (drain side, or gate side or both) as well as OFF-state conduction and breakdown mechanism (gate injection, or punchthrough, or parasitic conduction through buffer or avalanche generation).

Predicting Extreme Solar Flare Events Using Lu and Hamilton Avalanche Model

Solar flares are the most powerful events in the solar atmosphere, releasing a huge amount of energy in a few minutes. Any progress in predicting when a flare of a big magnitude will occur is extremely important to evaluate the risk related to space weather. The Lu and Hamilton (Astrophys. J. Lett. 380, L89, 1991) self-organized criticality (SOC) model for solar flares is the one most conspicuous amongst the several avalanche models for flares that have been developed in the last 30 years. It has been very successful in reproducing some of the characteristic features of observed flares (e.g. probability density function of flare energy) and in the last years has been explored as a way of predicting extreme flaring events. In this work, we study the predicting capabilities of Lu and Hamilton model by assessing the proximity to stability of the 2D lattice and studying the influence of the lattice structure in the generation of large avalanches. We find that the mean value of the lattice nodes bears enough information to predict large avalanches in more than half of the cases, making it a reliable precursor for forecasting purposes.

Runaway dynamics in the DT phase of ITER operations in the presence of massive material injection

A runaway avalanche can result in a conversion of the initial plasma current into a relativistic electron beam in high-current tokamak disruptions. We investigate the effect of massive material injection of deuterium–noble gas mixtures on the coupled dynamics of runaway generation, resistive diffusion of the electric field and temperature evolution during disruptions in the deuterium–tritium phase of ITER operations. We explore the dynamics over a wide range of injected concentrations and find substantial runaway currents, unless the current quench time is intolerably long. The reason is that the cooling associated with the injected material leads to high induced electric fields that, in combination with a significant recombination of hydrogen isotopes, leads to a large avalanche generation. Balancing Ohmic heating and radiation losses provides qualitative insights into the dynamics; however, an accurate modelling of the temperature evolution based on energy balance appears crucial for quantitative predictions.

New Insight into Single-Event Radiation Failure Mechanisms in Silicon Carbide Power Schottky Diodes and MOSFETs

Ion-induced leakage current degradation, and single-event burnout may be manifestestations of the same device mechanisms in both silicon carbide power diodes and MOSFETs. In all cases there is a migration of the electrical field from the front body-drain interface to the back epi-drain n+ interface, with a peak exceeding the critical electric field of silicon carbide, causing avalanche generation which enables high short-duration power densities during an approximate 20 psec window after the ion strike. The degradation effect in JBS SiC diodes seems to be independent of the length of the epitaxial region for different voltage-rated diodes.

Atomic Scale Dynamics Drive Brain-like Avalanches in Percolating Nanostructured Networks.

Self-assembled networks of nanoparticles and nanowires have recently emerged as promising systems for brain-like computation. Here, we focus on percolating networks of nanoparticles which exhibit brain-like dynamics. We use a combination of experiments and simulations to show that the brain-like network dynamics emerge from atomic-scale switching dynamics inside tunnel gaps that are distributed throughout the network. The atomic-scale dynamics emulate leaky integrate and fire (LIF) mechanisms in biological neurons, leading to the generation of critical avalanches of signals. These avalanches are quantitatively the same as those observed in cortical tissue and are signatures of the correlations that are required for computation. We show that the avalanches are associated with dynamical restructuring of the networks which self-tune to balanced states consistent with self-organized criticality. Our simulations allow visualization of the network states and detailed mechanisms of signal propagation.

Dissipation of runaway current by massive gas injection on J-TEXT

Plasma disruption is one of the major challenges for ITER. A large fraction of runaway current may be formed due to the avalanche generation of runaway electrons (REs) in disruptions. Current researches find that the generation of REs may be hard to totally suppress during the disruptions, and lead to the formation of large runaway current in future fusion devices. Runaway currents carry huge magnetic and kinetic energies that need to be dissipated safely. Runaway current dissipation by high-Z impurities has been performed on J-TEXT. Runaway currents are formed by an injection of ∼1019 argon atoms, and then large quantities of argon or krypton impurities are injected by the massive gas injection valve to dissipate the runaway current during the runaway current plateau phase. The dissipation efficiency increases with the increase of injected impurity quantity. When the injected impurity quantity exceeds 2 × 1021, the dissipation efficiency becomes saturated. Up to a 28 MA s−1 runaway current dissipation rate and a 15% energy dissipation rate can be achieved. Analysis shows that the saturation of dissipation efficiency is caused by the decrease of impurity assimilation rate with the increase of total injected impurity quantity. The decrease of impurity assimilation rate may be caused by the saturation of impurity density growth rate during the short dissipation phase. A simple estimate also shows that the increase of internal inductance leads to the slowing down of the growth of runaway current dissipation rate during the runaway current decay phase. The results may have important implication for ITER disruption mitigation.

Nano-second timescale high-field phase transition in hydrogenated amorphous silicon

In this work, we report the phase transition behavior of hydrogenated amorphous silicon on the application of nanosecond timescale high-field pulse electrical stress. The transition of amorphous silicon to nanocrystalline silicon, confirmed through Raman spectroscopy, is marked by an abrupt change in the pulse I-V characteristics. The mechanism of the phase transition at high electric field involving the avalanche generation of charge carriers and optical phonon generation is discussed. The role of defect states in optical phonon localization and eventual phase transition is explored. The phase transition in the case of devices with a drain-gate underlap is also studied. The role of self-heating in accelerating the phase transition has also been explored. The impact of channel dimensions on the onset of the phase transition is also discussed. Characterization of the resultant nc-Si is done through deconvolution of the Raman spectra, and the quality of nc-Si created is found comparable to earlier studies.

Insights into the off-state breakdown mechanisms in power GaN HEMTs

We analyze the off-state, three-terminal, lateral breakdown in AlGaN/GaN HEMTs for power switching applications by comparing two-dimensional numerical device simulations with experimental data from device structures with different gate-to-drain spacing and with either undoped or Carbon-doped GaN buffer layer. Our simulations reproduce the different breakdown-voltage dependence on the gate-drain-spacing exhibited by the two types of device and attribute the breakdown to: i) a combination of gate electron injection and source-drain punch-through in the undoped HEMTs; and ii) avalanche generation triggered by gate electron injection in the C-doped HEMTs.

High-Temperature Impact-Ionization Model for 4H-SiC

Silicon carbide (4H-SiC) devices experiencing avalanche conditions can reach temperatures above 1500 K. Simulation of impact ionization in devices should, therefore, include models valid up to such high temperatures. However, calibrations of impact ionization coefficients are available only up to 580 K, and simulations of switching show deviations from measurements at higher temperatures. In this paper, a more accurate model based on the underlying physics of high temperature and anisotropic avalanche generation is proposed. This model enables calibrations performed at room temperature along the ${c}$ -axis to be more precisely extrapolated to any temperature of interest in 2-D device simulations. The model provides an excellent fit to the measurements of breakdown voltages during unclamped inductive switching tests of power MOSFETs, enabling a more accurate prediction of ruggedness. The more comprehensive physics included in the model makes it also applicable to other wide bandgap semiconductors such as GaN, diamond, and Ga2O3, which also exhibit a high critical electric field.

A coarse-graining framework for spiking neuronal networks: from strongly-coupled conductance-based integrate-and-fire neurons to augmented systems of ODEs.

Homogeneously structured, fluctuation-driven networks of spiking neurons can exhibit a wide variety of dynamical behaviors, ranging from homogeneity to synchrony. We extend our partitioned-ensemble average (PEA) formalism proposed in Zhang et al. (Journal of Computational Neuroscience, 37(1), 81-104, 2014a) to systematically coarse grain the heterogeneous dynamics of strongly coupled, conductance-based integrate-and-fire neuronal networks. The population dynamics models derived here successfully capture the so-called multiple-firing events (MFEs), which emerge naturally in fluctuation-driven networks of strongly coupled neurons. Although these MFEs likely play a crucial role in the generation of the neuronal avalanches observed in vitro and in vivo, the mechanisms underlying these MFEs cannot easily be understood using standard population dynamic models. Using our PEA formalism, we systematically generate a sequence of model reductions, going from Master equations, to Fokker-Planck equations, and finally, to an augmented system of ordinary differential equations. Furthermore, we show that these reductions can faithfully describe the heterogeneous dynamic regimes underlying the generation of MFEs in strongly coupled conductance-based integrate-and-fire neuronal networks.

Part I: Physical Insight Into Carbon-Doping-Induced Delayed Avalanche Action in GaN Buffer in AlGaN/GaN HEMTs

Physics behind the improvement in breakdown voltage of AlGaN/GaN HEMTs with carbon-doping of GaN buffer is discussed. Modeling of carbon as acceptor traps and self-compensating acceptor/donor traps is discussed with respect to their impact on avalanche breakdown. Impact of carbon behaving as a donor as well as acceptor traps on electric field relaxation and avalanche generation is discussed in detail to establish the true nature of carbon in GaN that delays the avalanche action. This understanding of the behavior of carbon-doping in GaN buffer is then utilized to discuss design parameters related to carbon doped buffer. Design parameters such as undoped channel thickness and relative trap concentration induced by carbon-doping are discussed with respect to the performance metrics of breakdown voltage, leakage current, sheet charge density, and dynamic ON-resistance.

Role of Kinetic Instability in Runaway-Electron Avalanches and Elevated Critical Electric Fields.

The effects of kinetic whistler wave instabilities on the runaway-electron (RE) avalanche is investigated. With parameters from experiments at the DIII-D National Fusion Facility, we show that RE scattering from excited whistler waves can explain several poorly understood experimental results. We find an enhancement of the RE avalanche for low density and high electric field, but for high density and low electric field the scattering can suppress the avalanche and raise the threshold electric field, bringing the present model much closer to observations. The excitation of kinetic instabilities and the scattering of resonant electrons are calculated self-consistently using a quasilinear model and local approximation. We also explain the observed fast growth of electron cyclotron emission signals and excitation of very low-frequency whistler modes observed in the quiescent RE experiments at DIII-D tokamak. Simulations using ITER parameters show that by controlling the background thermal plasma density and temperature, the plasma waves can also be excited spontaneously in tokamak disruptions and the avalanche generation of runaway electrons may be suppressed.

Effect of Negative Gate Bias on Single Pulse Avalanche Ruggedness of 1.2 kV Silicon Carbide MOSFETs

When power MOSFETs experience a voltage spike initiating avalanche generation, a large amount of power is dissipated at the device junction. This leads to self-heating and lowers the threshold voltage. Some sources indicate that unintended opening of the channel creates a positive feedback, thereby increasing heat generation and leading to thermal runaway. Therefore, keeping MOSFETs off by applying a negative gate bias should improve avalanche ruggedness. In this report, this claim is investigated by comparing single pulse avalanche ruggedness of commercial 1.2 kV, 80 mΩ planar and trench MOSFETs at -10 V and 0 V off-state gate bias. Both planar and trench devices show a small increase in their breakdown voltage with negative gate bias. However, there is no significant difference in avalanche withstanding energy. Even in investigated trench gate devices where the gate oxide is susceptible to interface as well as oxide defects, keeping the gate voltage at VGS = -10 V did not result in improvements in ruggedness.

Limiting Effects on the Design of Vertical Superjunction Collectors in SiGe HBTs

The implementation of a “superjunction” collector design in a silicon–germanium heterojunction bipolar transistor technology is explored for enhancing breakdown performance. The superjunction collector is formed via the placement of a series of alternating the p/xn-doped layers in the collector-base space charge region and is used to reduce avalanche generation leading to breakdown. An overview of the physics underlying superjunction collector operation is presented, together with TCAD simulations, and a parameterization methodology is developed to explore the limits of the superjunction collector performance. Measured data demonstrate the limitations explored in simulation.