Hole-initiated Impact Ionization Research Articles

Carrier type and density dependence of impact ionization characteristics in WSe2.

The impact ionization process offers advantages in achieving low-power and high-speed switching in transistors and also provides high internal gain for photodetectors. We investigate the density dependence of both electron- and hole-initiated impact ionization in WSe2. We observe a record-low critical electric field for impact ionization and a large multiplication factor in WSe2 when the impact ionization is initiated by holes, particularly near a charge-neutral point. As the carrier density increases, the impact ionization decreases, which is attributed to the increase in Fermi energy and the weakening of the carrier-carrier interaction by increasing the screening effect. To understand the role of screening effects on the impact ionization, we examine the carrier scatterings of several scattering sources and the rate of change of the channel current with respect to the drain source voltage, ΔIDS/ΔVDS. We obtain the optimal conditions for impact ionization and apply these findings to fabricate avalanche photodetectors (APDs). This study proposes that the performance of low-power transistors or APDs, utilizing impact ionization, can be enhanced under optimized conditions by examining and controlling specific external parameters.

Terrestrial Neutron-Induced Failures in Silicon Carbide Power MOSFETs and Diodes

Investigations of terrestrial neutron radiation-induced failures in silicon carbide power MOSFETs and diodes indicate that the failures are related to a hole-initiated impact ionization process followed by a thermal transient resulting in the loss of device voltage blocking ability due to the damage of lattice along a filament within the device volume. Irrespective of device type, MOSFET or diode, the start of these failures exhibits the same characteristics, and can be mitigated by decrease in field in device OFF state. These failures with origins in impact ionization and fast thermal transients are fundamentally different from those of bipolar burn-out events, observed in silicon power devices. In addition, once a failure event starts, the failure ends with all terminals shorting.

Multiplication gain and excess noise factor in thin SiC avalanche photodiodes

This work simulated the avalanche characteristics of 4H- and 6H-SiC avalanche photodiodes (APDs) at 0.1 µm, 0.2 µm and 0.3 µm avalanche widths. A Monte Carlo model with random ionization path length techniques is developed to simulate mean multiplication gain and excess noise factor in thin SiC APDs. Mean multiplication gain, breakdown voltage and excess noise factor are simulated based on the electric field dependent impact ionization coefficients with the inclusion of dead space effect. Our results show that hole-initiated impact ionization gives high multiplication gain with low excess noise factor in both devices. We observed that dead space effect is more pronounce in thin structure since it covers a significant portion of the avalanche region. In thick device structure, a high breakdown voltage is observed. A comparison between these two polytypes shows that 4H-SiC provides high multiplication gain with low excess noise factor than 6H-SiC.

A study of hot-hole injection during programming drain disturb in flash memories

Program disturbs in NOR-type Flash arrays significantly degrade the tunnel oxide by hot-hole injection (HHI) induced by band-to-band tunneling at the drain overlap. This paper provides a comprehensive experimental and modeling analysis of HHI in Flash memories under program-disturb conditions. Carrier-separation measurements on arrays of Flash memories with contacted floating-gate (FG) allows for a direct investigation of hole-initiated impact ionization and HHI. A Monte Carlo (MC) model is used to simulate carrier multiplication and injection into the FG. After validating the MC model against experimental data for both secondary electron generation and HHI, the model is used to provide further insight into the hole-injection mechanism.

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.

Estimates of Impact Ionization Coefficients in Superlattice-Based Mid-Wavelength Infrared Avalanche Photodiodes

ABSTRACTWe describe band engineering strategies to either enhance or suppress electron-initiated impact ionization relative to hole-initiated impact ionization in type II superlattice mid-wavelength infrared avalanche photodiodes. The strategy to enhance electron-initiated impact ionization involves placing a high density of states at approximately one energy gap above the bottom of the conduction band and simultaneously removing valence band states from the vicinity of one energy gap below the top of the valence band. This gives the electrons a low threshold energy and the holes a high one. The opposite strategy enhances hole-initiated impact ionization. Estimates of the electron (α) and hole (β) impact ionization coefficients predict that α/β>>1 in the first type of superlattice and α/β<<1 in the second type.

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 Schrödinger 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 multiband Schrödinger equation model is used for holes in the c-axis direction, a significant change in the electron energy distribution is found, since the hole impact-ionization rate is very much increased with this model. The secondary electrons increase the average energy of the electron distribution leading to a significant increase in the electron-initiated impact-ionization coefficients. Our simulation results clearly show that both electrons and holes have to be considered in order to understand electron-initiated impact ionization in 4H-SiC.

Band structure engineering of superlattice-based short-, mid-, and long-wavelength infrared avalanche photodiodes for improved impact ionization rates

We study the effects of the electronic band structure on the hole- and electron-initiated impact ionization in Sb-based superlattice avalanche photodiodes. Earlier calculations have revealed that bulk alloy AlGaSb avalanche photodiodes with alloy composition near the resonance between the energy gap and the spin-orbit splitting have an enhanced hole to electron impact ionization coefficient ratio for a low electric field but not for a high electric field. The absence of an enhancement under high fields is due to carrier heating spreading the hole distribution in the splitoff band. A strategy to extend this type of enhancement to high fields in a superlattice involves band engineering the superlattice to place flat bands at approximately one energy gap below the top of the valence band. This prevents holes from spreading in energy and hence gives rise to strong hole-initiated impact ionization. Quantitative results are presented for short-infrared AlAs/InGaAs/AlAs/InGaSb, midinfrared InAs/InGaSb/AlSb, and long-infrared InAs/InGaSb/AlSb superlattices.

Full-Band Monte Carlo Simulation of Electron Transport in 3C-SiC

A full band Monte Carlo study of the electron transport in 3C-SiC is presented based on ab initio band structure calculation using the Local Density Approximation (LDA) to the Density Functional Theory (DFT). The scattering rates and impact ionization transition rates have been calculated numerically from the band structure using both energy dispersion and wave functions. Coupling constants for the phonon interactions have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The saturation velocity was found to be approximately 2.2*107 cm/s with a visible negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2 to 10 times stronger than reported values for the hole initiated impact ionization

Dead space effect in space-charge region of collector of AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.

Modelling of the hole-initiated impact ionization current in the framework of hydrodynamic equations

Several research papers have shown the feasibility of the hydrodynamic transport model to investigate impact ionization in semiconductor devices by means of mean-energy-dependent generation rates. However, the analysis has been usually carried out for the case of the electron-initiated impact ionization process and less attention has been paid to the modelling of the generation rate due to impact ionization events initiated by holes. This paper therefore presents an original model for the hole-initiated impact ionization in silicon and validates it by comparing simulation results with substrate currents taken from p-channel transistors manufactured in a 0.35 μm CMOS technology having three different channel lengths. The experimental data are successfully reproduced over a wide range of applied voltages using only one fitting parameter. Since the impact ionization of holes triggers the mechanism responsible for the back-bias enhanced gate current in deep submicron nMOS devices, the model can be exploited in the development of non-volatile memories programmed by secondary electron injection.

Avalanche multiplication and ionization coefficient in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

The hole-initiated impact ionization multiplication factor Mp−1 and the ionization coefficient αp in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors (HBTs) are presented. A large discrepancy is observed at low electric field when the measured data from the p-n-p HBTs are compared with those given from avalanche photodiode. The results show that the conventional impact ionization models, based on local electric field, substantially overestimate the hole impact ionization multiplication factor Mp−1. We believe that the hole ionization coefficient in p-n-p HBTs where significant dead space effects occur in the collector space charge region.

Modeling of band-to-band tunneling transitions during drift in Monte Carlo transport simulations

The conventional method of semiconductor charge carrier transport investigations using full band ensemble Monte Carlo simulations is extended to allow for tunneling between bands during accelerated drift of the carriers. The essentially classical picture of transport, as simulated, is preserved by implementing a stochastic selection of the band index of the initial state of each scattering process associated with phonons, with impurities, or with impact ionization. Relative probabilities for the band assignment are calculated from the overlap integrals of the cell-periodic parts of Bloch wave functions belonging to different bands, for k-vectors along the carrier k-space trajectory between successive scattering events. As an example, the method is applied to Monte Carlo transport simulations for holes in 4H SiC in a homogeneous applied electric field. Tunneling between valence bands during the drift phases is shown to have a significant impact on the carrier energy distributions when large electric fields are applied, and on physical parameters that directly depend on the carrier energy, such as the hole initiated impact ionization coefficient.

Monte Carlo calculation of hole initiated impact ionization in 4H phase SiC

In this article, we present a comprehensive, full band theoretical study of the high field, hole transport properties of the 4H phase of silicon carbide (4H-SiC). The calculations are performed using a full band ensemble Monte Carlo simulation that includes numerically tabulated impact ionization rates, and phonon and ionized impurity scattering rates. In addition, the simulation includes a mechanism, interband tunneling, by which the holes can move between bands in the proximity of band intersection points. It is found that there exists a significant anisotropy in the calculated steady-state hole drift velocity for fields applied parallel and perpendicular to the c-axis direction. Good agreement with experimental measurements of the hole initiated impact ionization coefficient for fields applied along the c axis is obtained, provided that interband tunneling in the proximity of band intersections is included in the model. If interband tunneling is not included, the calculated ionization coefficients are orders of magnitude lower than the experimental measurements.

Hole initiated impact ionization in wide band gap semiconductors

Band-to-band impact ionization by hot electrons and holes is an important process in high-field transport in semiconductors, leading to carrier multiplication and avalanche breakdown. Here we perform first principles calculations for the respective microscopic scattering rates of both electrons and holes in various wide band gap semiconductors. The impact ionization rates themselves are calculated directly from the electronic band structure derived from empirical pseudopotential calculations for cubic GaN, ZnS, and SrS. In comparison with the electron rates, a cutoff in the hole rate is found due to the relatively narrow valence bandwidths in these wide band gap semiconductors, which correspondingly reduces hole initiated carrier multiplication.

Theory of hole initiated impact ionization in bulk zincblende and wurtzite GaN

In this article, the first calculations of hole initiated interband impact ionization in bulk zincblende and wurtzite phase GaN are presented. The calculations are made using an ensemble Monte Carlo simulation including the full details of all of the relevant valence bands, derived from an empirical pseudopotential approach, for each crystal type. The model also includes numerically generated hole initiated impact ionization transition rates, calculated based on the pseudopotential band structure. The calculations predict that both the average hole energies and ionization coefficients are substantially higher in the zincblende phase than in the wurtzite phase. This difference is attributed to the higher valence band effective masses and equivalently higher effective density of states found in the wurtzite polytype. Furthermore, the hole ionization coefficient is found to be comparable to the previously calculated electron ionization coefficient in zincblende GaN at an applied electric field strength of 3 MV/cm. In the wurtzite phase, the electron and hole impact ionization coefficients are predicted to be similar at high electric fields, but at lower fields, the hole ionization rate appears to be greater.

Comparison of Electron and Hole Initiated Impact Ionization in Zincblende and Wurtzite Phase Gallium Nitride

ABSTRACTIn this paper, we present the first calculations of the electron and hole impact ionizatioi coefficients for both wurtzite and zincblende phase GaN as a function of the applied electrii field. The calculations are made using an ensemble Monte Carlo simulator including the ful details of the conduction and valence bands derived from an empirical pseudopotentia calculation. The interband impact ionization transition rates for both carrier species an determined by direct numerical integration including a wavevector dependent dielectric function It is found that the electron and hole ionization coefficients are comparable in zincblende Ga> at an applied field of ∼ 3 MV/cm, yet vary to a slight degree at both higher and lower applied field strengths. In the wurtzite phase, the electron and hole coefficients are comparable at hig] fields but diverge at lower applied fields. The most striking result is that the ionization rates an predicted to be substantially different for both carrier species between the two phases. It i predicted that the ionization rates for both carrier species in the zincblende phase are significanti; higher than in the wurtzite phase over the full range of applied fields examined.

Theoretical study of hole initiated impact ionization in bulk silicon and GaAs using a wave-vector-dependent numerical transition rate formulation within an ensemble Monte Carlo calculation

In this paper, calculations of the hole initiated interband impact ionization rate in bulk silicon and GaAs are presented based on an ensemble Monte Carlo simulation with the inclusion of a wave-vector-dependent numerical transition rate formulation. The ionization transition rate is determined for each of the three valence bands, heavy, light, and split-off, using Fermi’s golden rule with a two-body, screened Coulomb interaction. The dielectric function used within the calculation is assumed to be wave-vector-dependent. Calculations of the field-dependent impact ionization rate as well as the quantum yield are presented. It is found from both the quantum yield results and examination of the hole distribution function that the effective threshold energy for hole initiated impact ionization is relatively soft, similar to that predicted for the corresponding electron initiated ionization rate threshold in both GaAs and silicon. It is further found that light-hole initiated ionization events occur more frequently than either heavy or split-off initiated ionization events in bulk silicon over the applied electric field strengths examined here, 250–500 kV/cm. Conversely, in GaAs, the vast majority of hole initiated ionization events originate from holes within the split-off band.

Hole impact ionization rates in InP and In0.53Ga0.47As

Threshold energies and rates for hole-initiated impact ionization are calculated as functions of energy and direction in k-space. The calculations are based on anisotropic band structure and wavefunctions derived from four-band k.p theory with remote bands taken into account by perturbation theory. It is found that in InP the transitions initiated by carriers in the split-off band are dominant, while for In0.53Ga0.47As transitions initiated by light and heavy holes are also important. Simple algebraic expressions depending on energy and one direction parameter have been developed to approximate proximate the computed transition rates so that these may be used in further calculations or simulations of transport involving hot carriers.

Ga<inf>1-x</inf>Al<inf>x</inf>Sb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga 1-x Al x Sb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10-4A/cm2have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found for x = 0.065 (300 K) where the ratio \beta/\alpha exceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy E g , the threshold energy for hole initiated impact ionization reaches the smallest possible value ( E_{i} = E_{g} ) and the ionization process occurs with zero momentum. This leads to a strong increase of β at \Delta/E_{g} = 1 . The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.