High-field Drift Velocity Research Articles

Transport and noise of hot electrons in GaAs using a semianalytical model of two-phonon polar optical phonon scattering

Recent ab-initio studies of electron transport in GaAs have reported that electron-phonon (e-ph) interactions beyond the lowest order play a fundamental role in charge transport and noise phenomena. Inclusion of the next-leading-order process in which an electron scatters with two phonons was found to yield good agreement for the high-field drift velocity, but the characteristic non-monotonic trend of the power spectral density of current fluctuations (PSD) with electric field was not predicted. The high computational cost of the ab-initio approach necessitated various approximations to the two-phonon scattering term, which were suggested as possible origins of the discrepancy. Here, we report a semi-analytical transport model of two-phonon electron scattering via the Fr\"ohlich mechanism, allowing a number of the approximations in the ab-initio treatment to be lifted while retaining the accuracy to within a few percent. We compare the calculated and experimental transport and noise properties as well as scattering rates measured by photoluminescence experiments. We find quantitative agreement within 15% for the drift velocity and 25% for the $\Gamma$ valley scattering rates, and agreement with the $\Gamma-L$ intervalley scattering rates within a factor of two. Considering these results and prior studies of current noise in GaAs, we conclude that the most probable origin of the non-monotonic PSD trend versus electric field is the formation of space charge domains rather than intervalley scattering as has been assumed.

Carrier mobility and high-field velocity in 2D transition metal dichalcogenides: degeneracy and screening

The effect of degeneracy and the impact of free-carrier screening on a low-field mobility and a high-field drift velocity in MoS2 and WS2 are explored using an in-house ensemble Monte Carlo simulator. Electron low field mobility increases to for MoS2 and to for WS2 when temperature decreases to and carrier concentration is around . In the case of holes, best mobility values were and , reached at similar temperature and carrier concentration conditions while at room temperature these fall to and for MoS2 and WS2, respectively. The carrier screening effect plays a major role at low fields, and low and intermediate temperatures, where a combination of large occupancy of primary valleys and carrier–phonon interactions dominated by relatively low energy exchange processes results in an enhanced screening of intrinsic scattering. For electrons, degeneracy yields to transport in secondary valleys, which plays an important role in the decrease of the low field mobility at high concentrations and/or at room temperature. The high-field drift velocity is not much affected by carrier screening because of an increased carrier scattering with surface optical polar phonons, favouring larger phonon wavevector interactions with small dielectric function values.

GaN Schottky diode on sapphire substrate for THz frequency multiplier applications

Gallium Nitride, because of its high breakdown field, and peak and saturated electron drift velocity, high chemical stability and high thermal conductivity, is a very promising material as candidate for Schottky varactor diodes for next-generation high-power frequency multipliers. Similarly to the huge interest in the field of HEMT for high frequency and power application, GaN Schottky diodes is of a high interest for the next technological step for frequency multipliers for THz generations, replacing GaAs Schottky diodes. Due to a lack of research focused on GaN Schottky diodes for frequency multiplication, the design and fabrication of GaN frequency multipliers remains a technological challenge. In this paper, we present GaN Schottky diodes fabricated on a heteroepitaxial GaN on sapphire. The barrier height obtained is around 0.53 ​eV, and it increases up to 0.73 ​eV after Schottky contact annealing; and the ideality factor is equal to 1.1 before annealing and 1.2 after annealing. The resulting series resistance is very low, 7Ω and 1Ω, before and after annealing, respectively.

Implantation-free edge termination structures in vertical GaN power diodes

Gallium nitride (GaN) is a promising candidate for power applications, due to its superior electrical properties such as high critical breakdown field and saturation drift velocity. The recent emergence of GaN substrates with low defect densities has made it feasible to develop efficient vertical GaN power diodes. However, the breakdown voltage of these diodes is far from the theoretical value, due to the lack of suitable edge termination techniques in GaN such as junction termination extension (JTE). This is attributed to the processing challenges in fabricating GaN power diodes where it is difficult to form p-type regions through implantation. This work addresses this challenge and proposes the design, optimization, and a comprehensive design methodology for implantation-free, edge termination techniques such as field plate (FP), single and multi-etch JTE, and a hybrid FP-JTE structure, suitable for vertical GaN diodes. The proposed approaches demonstrate that close to theoretical breakdown voltages (>80%) are achievable, with greater area efficiency, simple processing, and high tolerance in process variation.

Counter-Doped Multizone Junction Termination Extension Structures in Vertical GaN Diodes

GaN is an attractive wide bandgap semiconductor for power applications, owing to its superior electrical properties, such as high critical electric field and saturation drift velocity. Recent advancements in developing native GaN substrates has drawn attention toward exploring vertical GaN power diodes with high breakdown voltages (VBR). In practice, effective edge terminations techniques, such as junction termination extension (JTE) structures, play a crucial role in realizing high-voltage devices. Though certain challenges in fabricating GaN diodes, such as difficulty in forming p-type region, makes it difficult to realize edge termination, hence impeding the development and adoption of such devices. This paper aims to address these challenges by presenting the design and methodology of forming multi-zone, counter-doped JTE structures in vertical GaN diodes, which attains close to theoretical breakdown voltage for a wide range of tolerance in implant dose variation. Extensive device simulations using experimental data and including the effects of surface charges and implant profiles, are performed to present realistic results. The results suggest that >80% of ideal VBR is achievable for a wide range of doping concentration ( $2.4\times 10^{17}$ cm $^{-3}$ ) with a maximum VBR reaching 96% of the ideal value. This paper serves as the first step toward leveraging the current challenges in the fabrication of GaN diodes, by proposing optimum design techniques for realizing vertical GaN diodes with high breakdown voltages.

Impact of the hot phonon effect on electronic transport in monolayer silicene

The hot phonon effect is a non-equilibrium phenomenon that has an important role in high-field electronic transport in two-dimensional electron gases and in 2D materials like graphene. In this work, the influence of this effect on the electronic transport properties of silicene is investigated by means on an ensemble Monte Carlo simulator that accounts self-consistently for the out-of-equilibrium phonon population and the scattering events under high electric field conditions. In this way, the observed reduction in the carrier drift velocity is analyzed in terms of the microscopic scattering events, wavevector distributions, net phonon emission and phonon temperatures. A comparison with the case of graphene is also carried out, showing that despite the similarities concerning the linear bandstructure near the Dirac points, the hot phonon effect is less dominant in silicene due to the differences in the phonon branches and electron–phonon couplings. Even so, neglecting the role of an out-of-equilibrium phonon population may overestimate the high-field drift velocity in silicene up to 25% at moderate carrier concentrations.

Assessing Lock-On Physics in Semi-Insulating GaAs and InP Photoconductive Switches Triggered by Subbandgap Excitation

The time-dependent photocurrent response in semi-insulating GaAs and InP was studied based on 1-D, time-dependent simulations with a focus on the Lock-On phenomenon. The results underscore the role of trap-to-band impact ionization from deep traps in rapid charge creation and its subsequent propagation much like a streamer. The numerical results compare well with the actual data. The main findings are that deeper traps nearer the valence band at higher densities, materials with larger high-field drift velocity, and cathode-side illumination would all aid in attaining Lock-On. These could be useful guidelines for producing Lock-On in new materials such as GaN for high-power applications.

High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS2.

Two-dimensional semiconductors such as monolayer MoS2 are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, vsat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS2. We find that in typical device geometries (e.g. on SiO2 substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron vsat = (3.4 ± 0.4) × 106 cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS2 could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management.

Theoretical analysis of high-field transport in graphene on a substrate

We investigate transport in graphene supported on various dielectrics (SiO2, BN, Al2O3, and HfO2) through a hydrodynamic model which includes self-heating and thermal coupling to the substrate, scattering with ionized impurities, graphene phonons, and dynamically screened interfacial plasmon-phonon (IPP) modes. We uncover that while low-field transport is largely determined by impurity scattering, high-field transport is defined by scattering with dielectric-induced IPP modes, and a smaller contribution of graphene intrinsic phonons. We also find that lattice heating can lead to negative differential drift velocity (with respect to the electric field), which can be controlled by changing the underlying dielectric thermal properties or thickness. Graphene on BN exhibits the largest high-field drift velocity, while graphene on HfO2 has the lowest one due to strong influence of IPP modes.

Electron transport in modulation-doped InAlAs/InGaAs/InAlAs and AlGaAs/InGaAs/AlGaAs heterostructures

The possibilities of enhancing the electron mobility and high-field drift velocity in channels of modulation-doped InAlAs/InGaAs and AlGaAs/InGaAs quantum wells by tuning interaction of electrons with interface phonons are tested and reviewed. A large increase in the mobility is achieved in a novel metamorphic In0.7Al0.3As/In0.8Ga0.2As structure with the high InAs content in the InAlAs barrier layer as well as in the AlxGa1-xAs/In0.2Ga0.8As structure with the low AlAs content in the AlGaAs barrier layer. An enhancement in the electron mobility by inserting thin InAs layers into the In0.52Al0.48As/In0.53Ga0.47As quantum well is obtained. The electron drift velocity saturates at high electric fields of 1.5–5 kV/cm and achieves a maximal value of 2.5 . 107 cm/s in the InGaAs quantum well with the thin InAs and GaAs inserts.

Hot electron transport in heterostructures

The enhancement of electron mobility and high-field drift velocity in a HEMT (high electron mobility transistor)-type channel of AlGaAs/InGaAs/AlGaAs double barrier heterostructures by reducing the electron–interface phonon interaction is obtained. The significant change in the electron concentration in the channel of the modulation-doped AlGaAs/InGaAs/AlGaAs heterostructures at low electric fields is experimentally discovered.

High-field electron transport of wide quantum wells under in-plane magnetic fields

High-field transport properties of a bilayer two-dimensional electron gas in a wide quantum well (QW) subjected to an in-plane magnetic field have been investigated by ensemble Monte Carlo simulations. The electron energy spectrum was calculated self-consistently. Due to the effect of Hartree potential, a coupled two-dimensional electron gas appears in the wide QW. The in-plane magnetic field induces significant modifications in bonding and antibonding subbands of the QW. The high-field transport behavior is significantly different from the low-field case. With increasing the magnetic field, the high-field drift velocity increases to its maximum value and then decreases monotonically. The magnetic-field-dependent behavior of drift velocity is qualitatively explained as the competition between the magnetic-field-induced depopulation of the antibonding subband and the electron effective mass enhancement.

The high-field drift velocity in degenerately-doped silicon nanowires

The charge transport in nanowires suitable for high-speed applications depends on charge carrier's mobility and saturation velocity in the conducting channel. It is shown that the high mobility does not always lead to higher carrier drift velocity. The ultimate drift velocity (the intrinsic velocity) due to the high-electric-field streaming is based on the asymmetrical distribution function that converts randomness in zero-field to a streamlined one in a very high electric field. The saturation velocity limited to the intrinsic velocity is an appropriate thermal velocity for a non-degenerately doped nanowire, increasing with the temperature, but independent of carrier concentration. However, this saturation velocity is the appropriate Fermi velocity for a degenerately doped silicon nanowire, increasing with carrier concentration but independent of temperature. The results obtained are applied to the modelling of a silicon nanowire transistor.

Inhibition of negative differential resistance in modulation-dopedn-typeGaxIn1−xNyAs1−y∕GaAsquantum wells

We present the results of hot-electron momentum relaxation studies for longitudinal transport in modulation doped ${\mathrm{Ga}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{N}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}∕\mathrm{Ga}\mathrm{As}$ quantum wells. Experimental results show that the high field drift velocity saturates at a value close to $1\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{s}$ with no evidence for negative differential resistance or instabilities. Experimental results are compared with a simple theoretical model for transport taking into account the effect of nonequilibrium phonon production. Model calculations indicate that enhanced momentum scattering for electrons with nondrifting hot phonons may be the cause for the reduction in drift velocity.

Enhancement of electron drift velocity in a quantum well by confinement of polar optical phonons

AbstractNew approaches to decrease the electron–polar optical (PO) phonon scattering rate in double heterostructures by using phonon barriers (reflective to PO phonons and transparent for electrons) are proposed. Twofold decrease of the electron–PO phonon scattering rate in the AlGaAs/GaAs/AlGaAs quantum well by inserting the InAs phonon barriers is achieved. The enhancement of high electric field drift velocity in the AlGaAs/GaAs/AlGaAs heterostructure with the inserted one monolayer InAs phonon barriers compared with that heterostructure without barriers is observed experimentally. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of hot phonons on the drift velocity in GaN/AlGaN two dimensional electron gas

We present the effect of hot-phonon production on the high-field drift velocity in the steady state in GaN/AlGaN heterostructures . The results are compared with a model taking into account the effect of hot phonon production on the high-field transport of electrons. Experimental results show that the drift velocity saturates at around v d ≈3.8×10 5 cm / s at an electric field of F≈8.9×10 2 V / cm at 3.8 K . The theoretical calculations show that the enhancement of the momentum relaxation rate due to the production of non-drifting hot phonons reduces the drift velocity at high field. The reduction in the drift velocity increases with increasing population of non-drifting non-equilibrium phonons (LO phonons).

The effect of hot phonons on the drift velocity in GaN/AlGaN two dimensional electron gas

We present the effect of hot-phonon production on the high-field drift velocity in the steady state in GaN/AlGaN heterostructures. The results are compared with a model taking into account the effect of hot phonon production on the high-field transport of electrons. Experimental results show that the drift velocity saturates at around v d ≈3.8×10 5 cm/ s at an electric field of F≈8.9×10 2 V/ cm at 3.8 K . The theoretical calculations show that the enhancement of the momentum relaxation rate due to the production of non-drifting hot phonons reduces the drift velocity at high field. The reduction in the drift velocity increases with increasing population of non-drifting non-equilibrium phonons (LO phonons).

Effect of non-drifting hot phonons on high-field drift velocity in GaN/AlGaN

We report on the studies of steady-state high-field drift velocity in GaN/AlGaN HEMT structures. The results are compared with a model taking into account the effect of hot phonon production on the high-field transport of electrons. In the theoretical model Cerenkov effects and phonon drift are assumed to be negligible. Experimental results show that the drift velocity saturates at vd = 1.0 × 107 cm s−1 at electric fields in excess of F ≈ 7.5 kV cm−1 at TL = 77 K. Theoretical calculations indicate that the enhanced scattering rate due to the production of non-drifting hot phonons reduces the drift velocity. The reduction in the drift velocity increases with increasing population of non-drifting non-equilibrium phonons.

Improved Measurements of High-Field Drift Velocity in Silicon Carbide

Improved Measurements of High-Field Drift Velocity in Silicon Carbide

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.