Carrier Drift Velocity Research Articles

Carrier drift velocity in semiconducting strings

The carrier drift velocity in a 1D semiconducting sample attached to electrodes is derived from the Luttinger model, where the electric field generated by image charges within the electrodes is explicitly taken into account. If the length of the electrode is small enough compared to the sample length, the drift velocity v d can be estimated as v d≃(4 e 2/ h)(1/ ε s), where e, h, and ε s are the elementary charge, the Planck constant, and the dielectric constant of the semiconductor, respectively. Compared to the experimental values of v d, the obtained relationship for v d is found to be still valid for a 3D sample in the large limit of the external field.

Onhigh-field departures from Ohm's local law

This paper analyses the theoretical ingredients of Ohm's locallaw in solids, that is, the linear response of the carrierdrift velocity to the applied electric field. The aim of thepresentation is to facilitate the understanding of departuresfrom that law at high electric fields.

Two-level model for the generation and relaxation of hot electrons near the breakdown of the quantum Hall effect

The time evolution of excited charge carriers near the breakdown of the quantum Hall effect is studied within a simple two-Landau-level model. The model incorporates impurity-assisted inter-Landau-level tunneling as the main mechanism, however, with a characteristic length that is much smaller than the scale set by the width of a sample with homogeneous current distribution. Using the carrier drift velocity to convert time into position dependence, we find that the model can reproduce recent experimental results astonishingly well.

Current transients in extrinsic photoconductors: comprehensive analytical description of initial response

We revisit the theory of the transients in low-temperature extrinsic photoconductors, with emphasis on the initial fast component response. Application of a photon-generation step launches a traveling zone boundary between part of the device where, in the short term, a quasi-equilibrium has been reached and a part where the carrier concentration is still growing. As the boundary passes, the exponentially slowing growth of carriers is abruptly terminated. The time to reach this quasi-equilibrium depends on the sample length and the carrier drift velocity rather than on the carrier lifetime. We note restricting errors in earlier analyses and produce an improved formula for the subsequent slow fraction of the total current change by including carrier diffusion. Comparison of numerical and analytical results suggests that measurement of the fast or slow fraction of the response can provide a highly accurate measure of the mobility-lifetime (μτ) product.

Studies of single-event transient current induced in GaAs and Si diodes by energetic heavy ions

To investigate the tolerance of single-event upset (SEU) for GaAs devices, we have measured the transient currents induced in GaAs diodes by energetic heavy particles such as 15 MeV oxygen ions. The measurements were made by combining a focused ion microbeam and a wide-bandwidth current-pulse measurement system. The resulting transients were found to be markedly different to those measured in Si diodes. This result can be mostly attributed to a difference in the carrier drift velocities between GaAs and Si. The amount of collected charge in GaAs diodes was also found to be less than that in Si diodes with similar device properties. This fact strongly suggests that GaAs devices have a higher SEU tolerance than Si devices.

A new method of carrier trapping time measurement

A new method of measuring carrier trapping time by a simple analysis of the current pulse shape is proposed and demonstrated for irradiated silicon detectors. This method which we call Exponentiated Charge Crossing (ECC) requires no knowledge of either the electric field profile in the detector or of the relation between the carrier drift velocity and the electric field. It is general enough to be valid not only for solid-state particle detectors but also for other devices such as some gaseous and liquid detectors. The results obtained by the proposed method are consistent with those obtained by an earlier method.

The role of induced contrast in images obtained using the environmental scanning electron microscope

Generation of contrast in images obtained using the environmental scanning electron microscope (ESEM) is explained by interpretation of images acquired using the gaseous secondary electron detector (GSED), ion current, and the Everhart-Thornley detector. We present a previously unreported contrast component in GSED and ion current images attributed to signal induction by changes in the concentration of positive ions in the ESEM chamber during image acquisition. Changes in positive ion concentration are caused by changes in electron emission from the sample during image acquisition and by a discrepancy between the drift velocities of negative and positive charge carriers in the imaging gas. The proposed signal generation mechanism is used to explain contrast reversal in images produced using the GSED and ion current signals and accounts for discrepancies in contrast observed, under some conditions, in these types of images. Combined with existing models of signal generation in the ESEM, the proposed model provides a basis for correct interpretation of ESEM images.

The influence of anisotropic electron drift velocity on the signal shapes of closed-end HPGe detectors

This study is concerned with the anisotropy of the electron drift velocity in germanium crystals at high electric fields and low temperature, and its influence on the charge collection process in n-type, high-purity germanium (HPGe) detectors of closed-end, coaxial geometry. The electron trajectories inside HPGe detectors are simulated using a phenomenological model to calculate the dependence of the drift velocity on the angle between the electric field and the crystal orientation. The resulting induced currents and pulse shapes for a given detector geometry and preamplifier bandwidth are compared to experiment. Experimentally, the dependence of the pulse shapes on the conductivity anisotropy in closed-end HPGe detectors was observed. The experimental data on pulse shapes were obtained by sampling preamplifier signals of an encapsulated, hexaconical EUROBALL detector, which was irradiated by collimated 22 Na and 241 Am sources. The crystal orientation was measured by neutron reflection. Qualitative agreement between the simulated and experimental pulse shapes was found. A variation in the charge collection time of up to 50 ns was observed for different drift directions of the carriers relative to the crystal orientation. Furthermore, a deflection of the trajectories from a straight radial drift direction of about 20° was predicted by the simulations. These two main effects of charge carrier drift velocity anisotropy in coaxial Ge detectors are expected to play an important role in the development of γ-ray tracking detectors.

Critical voltage growth rate when initiating the ultrafast impact ionization front in a diode structure

The threshold of the voltage-rise rate was estimated; this threshold should be achieved in a reversely biased diode structure to excite an impact ionization front propagating with a velocity higher than saturated carrier drift velocities.

Structured Ultrafast Carrier Drift Velocity in Photoexcited Zincblende GaN

Structured Ultrafast Carrier Drift Velocity in Photoexcited Zincblende GaN

Simulation of terahertz Doppler wavelength shifting of infrared optical pulses in an active semiconductor layer

In this paper, a time-domain model of wavelength shifting in a semiconductor layer with a constant stimulated gain level and moving Bragg grating of permittivity is used to investigate Doppler conversion of an infrared (f/sub 0/=200 THz) ultrashort (0.4-ps width) optical pulse. Simulations of the electromagnetic-field evolution show that the high drift velocity of carriers in InGaAsP can produce at least 1-THz conversion span. The optical power of the converted pulse can be as much as 20% of the power in the initial pulse. The backscattered pulses and pulses transmitted through the semiconductor layer depend dramatically on the permittivity modulation depth and length of the layer. It has been demonstrated that the length of the semiconductor layer can be optimized to produce strong converted pulses of short duration.

Materials theory based modeling of wide band gap semiconductors: from basic properties to devices

In this paper we present a general methodology, materials theory based modeling, for predicting device performance in technologically immature materials that can proceed relatively independently of experiment. The models incorporated within this general approach extend from a fundamental physics based, microscopic analysis to macroscopic, engineering based device models. Using this scheme, we have investigated the transport and breakdown properties of several emerging wide band gap semiconductor materials, i.e. GaN, InN, 3C-SiC, and 4H-SiC. The carrier drift velocities, mobilities, and impact ionization coefficients for these materials can be predicted using the materials theory based modeling method. Using these results, device level simulations can then be made. Here we report Monte Carlo and selfconsistent charge control modeling of GaN based devices. Comparison to experimental measurements is made when possible. Good agreement between the selfconsistent charge control model calculations and experiment is obtained. Some of the issues pertinent to heterostructure bipolar transistor modeling of GaN are discussed.

New Materials-Theory-Based Model for Output Characteristics of AlGaN/GaN Heterostructure Field Effect Transistors

A new model is used to examine the DC output characteristics of AlGaN/GaN heterostructure field effect transistors. The model is based on the charge-control/gradual-channel approximation and takes into account the non-linear current vs. voltage characteristics of the ungated AlGaN/GaN heterostructure channel regions. The model also includes thermal effects associated with device self-heating. For the power dissipation levels considered for many applications, the thermal degradation of the carrier drift velocity is shown to cause a negative output conductance in saturation. The temperature is incorporated self-consistently into the model through the field and temperature dependent mobility obtained from Monte Carlo transport simulations for electron transport in GaN. Calculated results presented for the DC output characteristics of several AlGaN/GaN field effect transistors show a strong dependence on the thermal properties of the substrate material. The substrate materials considered in this work are sapphire, SiC, AlN, and GaN.

A study of electron beam-induced conductivity in resists.

The charging of polymeric resist materials during electron beam irradiation leads to significant problems during imaging and lithography processes. Charging occurs because of charge deposition in the polymer and charge generation/trapping due to formation of electron-hole pairs in the dielectric. The presence of such charge also results in the phenomena of electron beam-induced conductivity (EBIC). Electron beam-induced conductivity data have been obtained for three commercial e-beam resists under a variety of dose rate and temperature conditions. From the observed values of induced conductivity under varying conditions significant information about the generation of electron-hole pair and the transport of charge in the resist can be obtained. Three electron beam resists, EBR900, ZEP7000, and PBS are examined by an external bias method. The difference in resist chemistry is considered to play the role in the initial state EBIC behaviors among three resists even though the way that it affects the behaviors is not clear. A comparison of the power consumption comparison is proposed as a measure to give a preliminary estimate of the carrier concentration and carrier drift velocity differences among the resists. A simple single trap model with constant activation energy is proposed and provides good agreement with experiment.

Double-Doppler wavelength conversion of infrared optical pulses by moving grating of refractive index in semiconductors

A time-domain model of an excited semiconductor medium as a moving Bragg grating was used to investigate a double-Doppler wavelength conversion of an infrared (λ0 = 1.5 μm) 0.8 ps width optical pulse. In order to calculate ultrafast electromagnetic transients, a time-domain Volterra integro-differential equation was used for simulations. It is shown here that the high drift velocity of carriers in an InAs semiconductor produces a 3.5 nm up and down (9.7 THz) conversion span. The converted pulse width and spectral density strongly depend on the contrast of permittivity modulation. The depth of modulation has to be at least 5–7. A −3 dB conversion efficiency can be gained with a depth of modulation more than 7 and a length of semiconductor layer greater than 20 μm.

New Materials-Theory-Based Model for Output Characteristics of AlGaN/GaN Heterostructure Field Effect Transistors

AbstractA new model is used to examine the DC output characteristics of AlGaN/GaN heterostructure field effect transistors. The model is based on the charge-control/gradualchannel approximation and takes into account the non-linear current vs. voltage characteristics of the ungated AlGaN/GaN heterostructure channel regions. The model also includes thermal effects associated with device self-heating. For the power dissipation levels considered for many applications, the thermal degradation of the carrier drift velocity is shown to cause a negative output conductance in saturation. The temperature is incorporated self-consistently into the model through the field and temperature dependent mobility obtained from Monte Carlo transport simulations for electron transport in GaN. Calculated results presented for the DC output characteristics of several AlGaN/GaN field effect transistors show a strong dependence on the thermal properties of the substrate material. The substrate materials considered in this work are sapphire, SiC, AlN, and GaN.

Coupled electron and nonequilibrium optical phonon transport in a GaAs quantum well

The self-consistent Monte Carlo technique has been used to solve coupled nonlinear kinetic equations for electrons and optical phonons confined in a GaAs quantum well. We have studied the influence of nonequilibrium phonons on quasi-two-dimensional electron transport for a lattice temperature of 30 K and for a wide range of applied electric fields. A substantial difference in generation and decay times as well as the confinement inside the GaAs/AlAs heterostructure-bounded active region lead to a significant growth of nonequilibrium optical-phonon population generated by a heated electron gas. We have found that when the phonon generation (as well as phonon reabsorption by the quasi-two-dimensional carriers) becomes significant, there are substantial effects on transport in the quantum well. We show that for low electron concentrations, the hot optical-phonon distribution reflects the main features of the carrier distribution; indeed, it preserves an average quasi-momentum in the forward (opposite to electric field) direction. However, hot-phonon feedback to the electron system is found to be not essential in this case. For high electron concentrations, enhanced nonequilibrium optical-phonon reabsorption results in phonon distribution which spreads significantly in the quasi-momentum space and essentially loses the characteristic of the forward-peaked anisotropy. The interactions with the confined electron subsystem typically result in an isotropic phonon distribution. In this case, nonequilibrium optical phonons lead to an increase in the mean electron energy and a reduction in the carrier drift velocity.

Autocorrelation of mode-locked laser pulses based on the synchronous drift of photogenerated carriers

We show that the autocorrelation function of mode-locked laser pulses can be measured by interfering a pulse with its time-delayed replica in a conventional photoconductor to generate a pulsating grating that can be represented as a superposition of moving gratings. The resonance current peak resulting from matching the carrier drift velocity to the velocity of one of the grating components is proportional to the autocorrelation function. Unlike conventional autocorrelators, the proposed method uses no nonlinear effects and thus can be utilized for measuring weak pulses.

Superlattice space-charge-wave analyses in a modified drift-diffusion model and in a simplified balance-equation formulation

Unstable space-charge-waves in superlattice miniband transport are investigated using a modified drift-diffusion model and using a simplified one-dimensional hydrodynamic balance-equation formulation with a relaxation-time approximation. We point out that the earlier widely accepted notion, that in the small wavevector limit the space-chargewave (SCW) propagates at a phase velocity equal to the carrier drift velocity and with an amplitude-growth rate equal to the negative value of the mobility frequency, -ωc, was crucially based on the implicit assumption that the system momentum relaxation time τm is extremely small. Taking account of a finite momentum relaxation time, we find that even the drift-diffusion model would yield results significantly different from the above predictions: the phase velocity of a long-wavelength traveling SCW can be much slower than the carrier drift velocity and its amplitude-growth rate much smaller than -ωc. A hydrodynamic balance-equation formulation, which properly treats energy dissipation and further reduces the amplitude-growth rate, provides a convenient tool for improved qualitative analyses in SCW-related problem in semiconductor superlattices.

High-field transport of inversion-layer electrons and holes including velocity overshoot

In this paper, we experimentally address the effect of a wide range of parameters on the high-field transport of inversion-layer electrons and holes. The studied parameters include substrate doping level, surface micro-roughness, vertical field strength, nitridation of the gate oxide, and device channel length. We employ special test structures built on Silicon-On-Insulator (SOI) and bulk wafers to accurately measure the high-field drift velocity of inversion-layer carriers. Our findings point to electron velocity overshoot at room temperature, dependence of electron and hole saturation velocities on nitridation of the gate oxide, dependence of the high-field drift velocity on the effective vertical field, and relative insensitivity of electron and hole mobility and saturation velocity to moderate surface roughness.