A simplified impact ionization model based on the average energy ofhot-electron subpopulation

A previously developed nonlocal impact ionization model based on the average energy of hot-electron subpopulation has been further simplified. The system of hydrodynamic transport equations consisting of three equations for these high-energy electrons has been reduced to a single equation. The simulation results for n⁺-n-n⁺ structures are in good agreement with Monte Carlo (MC) calculations. The model is easily applicable to two-dimensional (2-D) problems by exploiting the current flow line approach     

A non-local impact ionization/lattice temperature model for VLSIdouble-gate ultrathin SOI NMOS devices

This paper presents an analytical drain current model for VLSI double-gate ultrathin SOI NMOS devices considering both the nonlocal impact ionization effect and the lattice temperature effect. Supported by the experimental data, the analytical model predicts that the double-gate SOI NMOS device has a more obvious nonlocal impact ionization effect and a higher lattice thermal effect as compared to the single-gate device     

Resonance impact ionization in superlattices

We propose an enhancement of the electron or hole impact ionization coefficients (α or β) by introducing resonant impact ionization states into the (conduction or valence) band by using suitable lattice matched multilayer heterojunctions (superlattices). Model calculations for the AlAs:GaAs superlattice indicate resonance enhancements can occur over a wide range of energy gaps (1.54–1.9 eV). The gap can be varied by choosing the appropriate ratio of the alternating layer thickness. This effect should be useful for improving the signal/noise ratio of avalanche photodiodes significantly.     

Soft handoff algorithm with variable thresholds in CDMA cellularsystems

Traffic nonuniformity degrades the performance of CDMA cellular systems. The authors present a new algorithm called the variable threshold soft handoff (VTSH) which reduces the performance degradation due to traffic nonuniformity in CDMA cellular systems. Unlike the conventional fixed handoff thresholds, the proposed algorithm allows the handoff thresholds (T ADD and T DROP) to vary dynamically according to the traffic density of each cell. This algorithm has been implemented by means of computer simulation and the results show that VTSH improves the overall CDMA system performance in terms of outage probability. The VTSH algorithm can easily be applied to CDMA cellular systems without any modification     

Internal ionization energy in II-VI compounds

The dependence of sensitivity of surface-barrier p-Cu_1.8S/n-II-VI heterostructures on the energy of exciting photons or of accelerated monoenergetic electrons was studied. The method of the determination of the mean energy ? of internal ionization for the direct-gap II-VI compounds is suggested, and the ? values are found experimentally. The relationship between ? and the band gap of a semiconductor is found to be expressed as ?=2.5E _g .     

A general control-volume formulation for modeling impact ionization in semiconductor transport

A method for incorporating impact ionization into a general control-volume formulation of semiconductor transport is described. The methods for electric-field and current-density vector evaluation and generated charge partitioning within a two-dimensional triangular element are given. The techniques employed allow device breakdown to be accurately determined independent of mesh orientation to current flow direction. The avalanche breakdown of a 1-µm n-MOSFET illustrates the approach. Regions where the drain current is a multivalued function of drain voltage are directly and self-consistently calculated for this device.     

Temperature dependence of impact ionization in GaAs

The temperature dependence of electron and hole impact ionization in gallium arsenide (GaAs) has been determined from photomultiplication measurements at temperatures between 20 K and 500 K. It is found that impact ionization is suppressed by increasing temperature because of the increase in phonon scattering. Temperature variations in avalanche multiplication are shown to decrease with decreasing avalanching region width, and the effect is interpreted in terms of the reduced phonon scattering in the correspondingly reduced ionization path length. Effective electron and hole ionization coefficients are derived and are shown to predict accurately multiplication characteristics and breakdown voltage as a function of temperature in p/sup +/in/sup +/ diodes with i-regions as thin as 0.5 /spl mu/m.     

Transient behavior of impact ionization in silicon

A transient solution of the ionization coefficient for electrons (α) in silicon has been obtained for various electric field strengths. The results show that α takes a longer time to come to its steady-state value for lower electric field strengths. The time the transient lasts is of the order of 10^-13s.     

Experimental determination of impact ionization coefficients in

The room-temperature electron and hold impact ionization coefficients, α and β, respectively, have been determined for     

A thermal activation view of low voltage impact ionization inMOSFETs

The authors present a thermal activation perspective for direct assessment of the low voltage impact ionization in deep-submicrometer MOSFETs. A comparison of the experimentally determined activation energy and a simple theoretical model is used to demonstrate the underlying mechanism responsible for impact ionization at low drain bias. The study indicates that the main driving force of impact ionization changes from the electric field to the lattice temperature with power-supply scaling below 1.2 V. This transition of driving force results in a linear relationship between log(I_SUB/I_D) and V_D at sub-bandgap drain bias, as predicted by the proposed thermally-assisted impact ionization model     

A lucky drift model,including a soft threshold energy,fitted to experimental measurements of ionization coefficients

We have fitted the lucky drift model of impact ionization due to Burt to experimental data for GaAs, InP, Si and In_0.53Ga_0.47As. The agreement is surprisingly good considering the simplicity of that model. The lucky drift model has been further extended to include the effects of a soft threshold energy where carriers do not necessarily ionize immediately on achieving the threshold energy. This new lucky drift model is found to fit the same experimental data better than the model due to Burt in all cases but one.     

Tunneling and impact ionization in thin-film ZnS:Mn-based electroluminescent structures

The errors in determining the depth of the surface-state levels of the insulator-phosphor cathode interface, the width of the potential barrier, and the probability of electron tunneling from the surface states are analyzed using the numerical simulation of experimental time dependences of the current flowing across the phosphor layer. The time dependences of the above parameters for the complete cycle of operation of electroluminescent emitters are obtained, and the effect of the frequency and amplitude of excitation voltage pulses as well as illumination of emitters in the blue spectral region during the interval between the pulses on these dependences is revealed. The time dependences of the multiplication factor and the number of ionization events per electron in the impact ionization region are determined. The largest value of the impact ionization coefficient is estimated to be ≥2.6 × 10⁴ cm^?1.     

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.     

Investigation of impact ionization in thin GaAs diodes

The electron and hole multiplication coefficients, M_e and M_h, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, α and β, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 μm down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M_e/M_h ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w⩽0.1 μm) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (⩾1×10³ kV/cm)     

Experimental evaluation of impact ionization coefficients in GaN

The impact ionization coefficient of electrons (α_n) in GaN is determined as a function of the electric field strength from gate-current analysis in the prebreakdown regime of AlGaN/GaN heterojunction field effect transistors (HJFETs). The experimentally obtained α_n, where the assumed effective length of the high electric field is precisely defined since its upper bound is closely limited due to the small gate-drain separation, agrees with that extrapolated from Monte Carlo simulation. It is experimentally confirmed that the breakdown field of GaN is higher than that of GaAs by a factor of about eight     

Monte Carlo simulation of impact ionization in SiGe HBTs

Measurements and Monte Carlo simulations of impact ionization in the base-collector region of SiGe HBTs are presented. A device with low germanium concentration (graded from 0 to 12%) is considered and no differences are found between the experimental multiplication factor in that device and the corresponding silicon control. Because impact ionization (II) occurs inside the bulk-Si collector, phonon and II scattering rates for bulk silicon can be used in the Monte Carlo simulation, avoiding the need to model the strained SiGe layers. Full-Band Monte Carlo simulations are shown to reproduce the multiplication factors measured in SiGe devices featuring different collector profiles     

Characteristics of tunneling and impact ionization in ZnS:Mn-based thin-film electroluminescent structures

A method for measuring the characteristics of tunneling and impact ionization in thin-film electroluminescent emitters is suggested. This method makes it possible to find time dependences of the space-charge layer thickness near the anode and the length of the impact ionization region, to determine more exactly the time dependence of the field in the potential barrier at the cathode interface, the maximum depth of the surface states from which electron tunneling occurs, the minimum thickness of the barrier, and the electron tunneling probability, as well as the impact ionization rate for the deep centers related to structural defects of the phosphor layer.