Interband Impact Ionization Research Articles

NUMERICAL SIMULATION OF ELECTRIC CHARACTERISTICS OF DEEP SUBMICRON SILICON-ON-INSULATOR MOS TRANSISTOR

Today submicron silicon-on-insulator (SOI) MOSFET structures are widely used in different electronic components and also can be used as sensing elements in some applications. The development of devices based on the structures with specified characteristics is impossible without computer simulation of their electric properties. The latter is not a trivial task since many complicated physical processes and effects must be taken into account. In current study ensemble Monte Carlo simulation of electron and hole transport in deep submicron n-channel SOI MOSFET with 100 nm channel length is performed. The aim of the study is investigation of the influence of interband impact ionization process on the device characteristics and determination of the transistor operation modes when impact ionization process starts to make an appreciable influence on the device functioning. Determination of the modes is very important for adequate and accurate modeling of different devices on the basis of SOI MOSFET structures. Main focus thereby is maid on the comparison of the use of two models of impact ionization process treatment with respect to their influence on the transistor current-voltage characteristics. The first model is based on the frequently used Keldysh approach and the other one utilizes the results obtained via numerical calculations of silicon band structure. It is shown that the use of Keldysh impact ionization model leads to much faster growth of the drain current and provides earlier avalanche breakdown for the SOI MOSFET. It is concluded that the choice between the two considered impact ionization models may be critical for simulation of the device electric characteristics.

Evaluation of the effective threshold energy of the Interband impact ionization in a deep-submicron silicon n-channel MOS transistor

Using the ensemble Monte Carlo method allowing for the main features of charge-carrier transport in conditions of strong electric fields, a deep-submicron silicon n-channel MOS transistor with a channel length of 50 nm is simulated. In the Keldysh impact ionization model with a soft threshold in a channel of the simulated transistor, the effective threshold energy of this process is calculated.

Refined dielectric breakdown model for crystalline organic insulators: electro-thermal instability coupled to interband impact ionization

A refined, substantially improved dielectric breakdown model is presented and applied to solution grown, single crystalline alkane type polymeric (n-C36H74) insulator, representing the iso-electronic analog to polyethylene. Ultraviolet illumination of attached electrodes allows controlled generation, injection into and transport of free charge carriers through the insulator. At sufficiently high electric fields, carrier transport is mediated by delocalized states in the conduction and valence band, respectively. At low and moderate fields, charge transport is suppressed by carrier trapping effects. Electric field induced inter-band impact ionization and generation of electron-hole pairs has been identified from these experiments as the dominant carrier multiplication and breakdown triggering mechanism. Critical field magnitudes >;1.26 MV/cm have been recorded experimentally for injected electrons and >;0.8 MV/cm for defect electrons, in reasonable agreement with the theoretical model. Application of the energy conservation principle, in accord with the solid state band model, allows determination of critical fields from the insulators electronic band-gap, effective mass and mobility of minority charge carriers. The related electrical breakdown feature and associated rapid dynamic temperature evolution has been explored on basis of the electro-thermal heat balance equation, following previous concepts applied to phase transitions. The non-linear differential equation has been solved numerically, using appropriate thermo-physical materials parameters, while considering the dielectric breakdown phenomenon as a singularity of the solution. Thermal and current run-away is due to strong positive electro-thermal feedback, in connection with an initial transient resistive behavior. Very small thermo-physical parameters are attributed to and explain filamentary charge transport. The temporal evolution of temperature and current in the conducting section or filament during the breakdown event exhibits a time scale up to the microsecond range. Shock wave emission is apparent, since the spatial temperature propagation exceeds the velocity of sound.

Calculation of the electron and hole impact ionization rate for wurtzite AlN and GaN

The interband impact ionization transition rate (IIR) of electrons and holes in wurtzite GaN and AlN is calculated using full local density approximation (LDA) band structures under the scissor approximation. The IIR of AlN is first calculated and the results show a large difference between the IIR of electrons and holes. The IIR of electrons grows from zero to 1014 at energies of ∼6.6–10 eV, while the IIR of holes is zero below 12.2 eV. The difference is interpreted by the energy gap between the sixth and the seventh valence bands. Analysis shows that it is hard for holes in AlN to tunnel through the large energy gap between the sixth and the seventh valence bands; as a result, holes can hardly get enough energy for impact ionization in the electrical field of several MV cm−1. This indicates that AlN is a good material for avalanche photodetectors of controllable gain, low noise and wide bandwidth with a cutoff wavelength of 210 nm. To further facilitate Monte Carlo simulation, the AlN and GaN IIR are fitted to the Keldysh formula.

Terahertz-Induced Impact Ionization Effect in Semiconductor Heterojunctions

We have extended the balance equations to account for conduction–valence interband impact ionization (II) induced by an intense terahertz (THz) electromagnetic irradiation in two-dimensional semiconductors. We have studied the effect of II on electron transport and electron–hole pair generation–recombination rate in THz-driven InAs/AlSb heterojunctions (HJs). As many as needed multiphoton channels are self-consistently taken into account. Usually II acts as a cooling mechanism in semiconductors. In the present THz-radiation-driven case with a multiphoton process, the electron temperature with II, however, is higher than that without this process. We propose to explain the counterintuitive behavior of electron temperature in THz-radiation-driven HJs.

Interband impact ionization and nonlinear absorption of terahertz radiation in semiconductor heterostructures.

We have theoretically investigated nonlinear free-carrier absorption of terahertz (THz) radiation in InAs/AlSb heterojunctions. By considering multiple photon process and conduction-valence interband impact ionization (II), we have determined the field and frequency dependent absorption rate. It is shown that (i). electron-disorder scatterings are important at low to intermediate field, and (ii). most importantly, the high field absorption is dominated by II processes. Our theory can satisfactorily explain a long-standing experimental result on the nonlinear absorption in the THz regime.

Multiphoton-assisted absorption of terahertz radiation in InAs/AlSb heterojunctions

We have calculated the free-carrier absorption percentage of intense terahertz (THz) radiation in InAs/AlSb heterojunctions (HJ's), by considering multiple photon process and conduction-valence interband impact ionization within the extended balance-equation theory. In the calculations we have included the electron-acoustic-phonon scattering, electron-polar-optical-phonon scattering, and elastic scattering both from the remote charged impurities and from the background impurities, and as many as needed hole subbands and multiphoton channels are self-consistently taken into account for getting a required accuracy. It is indicated that the THz radiation with a larger amplitude ${E}_{\mathrm{ac}}$ or a lower frequency ${f}_{\mathrm{ac}}$ has a stronger effect on electric transport characteristics. Good agreement is obtained between the calculated absorption percentages and the available experimental data for InAs/AlSb HJ's at ${f}_{\mathrm{ac}}=0.64\mathrm{THz}.$

Balance-equation approach to impact ionization induced by an intense terahertz radiation: Application to InAs/AlSb heterojunctions

We have extended the balance equations to account for conduction-valence interband impact ionization (II) process induced by an intense terahertz (THz) electromagnetic irradiation in semiconductors, and applied them to study the II effect on electron transport and electron-hole pair generation-recombination rate in THz-driven InAs/AlSb heterojunctions (HJ). As many as needed multiphoton channels are self-consistently taken into account for yielding a given accuracy. The time evolution of transport state including THz-radiation-induced II process are monitored in details by an extensive time-dependent analysis. Two different physical stages, the quasi-steady state and the complete steady-state, are clearly identified from the present calculations. Intersubband electron transfer rate and net electron-hole generation rate are derived as functions of the THz radiation strength E ac for various radiation frequencies from f ac = 0.42 to 6 THz at lattice temperatures T = 6 K. It's indicated that the THz radiation with a larger E ac or a lower f ac, has a stronger effect on electron transport and II process. Qualitative agreement is obtained between the calculated electron-hole generation rate and the available experimental data for InAs/AlSb HJ's at T = 6 K.

Interband impact ionization in THz-driven InAs/AlSb heterostructures

Extended balance equations accounting for the conduction–valence interband impact ionization (II) process in semiconductor heterostructures are presented. The II effect and terahertz (THz) field influence on electron transport in InAs/AlSb heterostructures is studied. It is shown that the II process usually results in a decrease in electron velocity and temperature when compared to the case without the II process. Qualitative agreement is obtained between the calculated electron–hole generation rates and available experimental data. Comparison with published experimental data on THz-driven heterostructures is also made.

Voltage switching and oscillations in a single barrier heterostructure hot-electron diode

S-type negative differential conductivity and relaxation oscillations in a single-barrier n−-GaAs/Al0.34Ga0.66As/n+-GaAs heterostructure hot-electron diode are studied experimentally and by ensemble Monte Carlo simulations at 300 and 77 K lattice temperature. The influence of interband impact ionization on the instability is investigated. The frequency is found to be limited by a slow electron dispersal from the accumulation layer adjacent to the n−-GaAs/Al0.34Ga0.66As interface to less than 10 GHz. The maximum frequency of the oscillations observed experimentally was about 0.5 GHz on account of the parasitic capacitance of the investigated device structure.

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.

Interband impact ionization by terahertz illumination of InAs heterostructures

Experimental studies of InAs heterostructures illuminated by far-infrared (FIR) radiation reveal an abrupt increase in the charge density for FIR intensities above a threshold value that rises with increasing frequency. We attribute this charge density rise to interband impact ionization in a regime in which ωτm∼1, where τm is the momentum relaxation time, and f=ω/2π is the FIR frequency. The dependence of the density rise on the FIR field strength supports this interpretation, and gives threshold fields of 3.7–8.9 kV/cm for the frequency range 0.3–0.66 THz.

Hot carrier transport in multibarrier heterostructure diode

Current switching and instability in a multibarrier heterostructure GaAs/Al0.45Ga0.55As diode is studied by Monte Carlo particle technique. It is found that interband impact ionization plays an important part in the phenomena considered. Current switching and instability originate from electron tunneling across the heterointerface nearest to the cathode with the subsequent impact ionization. Tunneling across the remaining heterointerfaces is of no importance in the diode operation under an instability regime.

Theoretical Prediction of Zinc Blende Phase GaN Avalanche Photodiode Performance Based on Numerically Calculated Electron and Hole Impact Ionization Rate Ratio

AbstractIn this paper, we present the first calculations of the electron and hole initiated interband impact ionization rate in zinc blende phase GaN as a function of the applied electric field strength. The calculations are performed using an ensemble Monte Carlo simulator including the full details of the conduction and valence bands along with a numerically determined, wave-vector dependent interband ionization transition rate determined from an empirical pseudopotential calculation. The first four conduction bands and first three valence bands, which fully comprise the energy range of interest for device simulation, are included in the analysis. It is found that the electron and hole ionization rates are comparable over the full range of applied electric field strengths examined. Based on these calculations an avalanche photodiode, APD, made from bulk zinc blende GaN then would exhibit poor noise and bandwidth performance. It should be noted however, that the accuracy of the band structure employed and the scattering rates is presently unknown since little experimental information is available for comparison. Therefore, due to these uncertainties, it is difficult to unequivocally conclude that the ionization rates are comparable.

Gain properties of doped GaAs/AlGaAs multiple quantum well avalanche photodiode structures

A comprehensive characterization has been made of the static and dynamical response of conventional and multiple quantum well (MQW) avalanche photodiodes (APDs). Comparison of the gain characteristics at low voltages between the MQW and conventional APDs show a direct experimental confirmation of a structure-induced carrier multiplication due to interband impact ionization. Similar studies of the bias dependence of the excess noise characteristics show that the low-voltage gain is primarily due to electron ionization in the MQW-APDs, and to both electron and hole ionization in the conventional APDs. For the doped MQW APDs, the average gain per stage was calculated by comparing gain data with carrier profile measurements, and was found to vary from 1.03 at low bias to 1.09 near avalanche breakdown.

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.

Theoretical Study of Electron Initiated Impact Ionization Rate in Bulk GaN using a Wave Vector Dependent Numerical Transition Rate Formulation

ABSTRACTIn this paper, we present ensemble Monte Carlo based calculations of electron initiated impact ionization in bulk zincblende GaN using a wavevector dependent formulation of the interband impact ionization transition rate. These are the first reported estimates, either theoretical or experimental, of the impact ionization rates in GaN. The transition rate is determined from Fermi’s golden rule for a two-body screened Coulomb interaction using a numerically determined dielectric function as well as by numerically integrating over all of the possible final states. The Monte Carlo simulator includes the full details of the first four conduction bands derived from an empirical pseudopotential calculation as well as all of the relevant phonon scattering mechanisms. It is found that the ionization rate has a relatively "soft" threshold.

Numerical study of the wave-vector dependence of the electron interband impact ionization rate in bulk GaAs

Ensemble Monte Carlo calculations of the electron interband impact ionization rate in bulk GaAs are presented using a wave-vector (k)-dependent formulation of the ionization transition rate. The transition rate is evaluated through use of numerically generated wavefunctions determined via a k⋅p calculation within the first two conduction bands at numerous points within a finely spaced three-dimensional grid in k space. The transition rate is determined to be greatest for states within the second conduction band. It is found that the interband impact ionization transition rate in bulk GaAs is best characterized as having an exceedingly ‘‘soft’’ threshold energy. As a consequence, the dead space, defined as the distance over which the ionization probability for a given carrier is assumed to be zero, is estimated to be much larger than that estimated using a ‘‘harder’’ threshold. These results have importance in the design of multiquantum-well avalanche photodiodes.

Semiclassical study of the wave vector dependence of the interband impact ionization rate in bulk silicon

We present calculations of the interband impact ionization rate calculated using a wave vector dependent (k-dependent) semiclassical formulation of the transition rate. The transition rate is determined using Fermi’s golden rule from a two-body screened Coulomb interaction assuming energy and momentum conservation. The transition rate is calculated for the first two conduction bands of silicon by numerically integrating over the full Brillouin zone. The overlap integrals in the expression for the transition rate are determined numerically using a 15 band k⋅p calculation. It is found that the transition rate depends strongly on the initiating electron wave vector (k vector) and that the transition rate is greatest for electrons originating within the second conduction band than the first conduction band. An ensemble Monte Carlo simulation, which includes the numerically determined ionization transition rate as well as the full details of the first two conduction bands, is used to calculate the total impact ionization rate in bulk silicon. Good agreement with the experimentally determined electron ionization rate data is obtained.