Hole Drift Velocity Research Articles

Enhanced hole injection in Ga-polar 290 nm AlGaN-based DUV LEDs with a p-n junction hole accelerator

The limited kinetic energy of holes in AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) poses a challenge in their transportation into the active region across the Al-rich electron blocking layer (EBL) and significantly restricts the electrical and optical performance of DUV LEDs. In this work, we propose a hole accelerator structure composing a p-AlxGa1-xN/n-AlxGa1-xN junction to improve the hole injection efficiency and explore the mechanism behind the enhanced performance with the Advanced Physical Models of Semiconductor Devices software (APSYS). The built-in electric field of the p-n junction distributes along the [000-1] direction, which can enhance the hole drift velocity and improve the hole injection into the active region. Moreover, with an optimum Al composition of 50%, [000-1] oriented polarization-induced electric field can be generated at the vicinity of both the p-EBL/accelerator and accelerator/hole supplier interfaces, which further boosts the holes into the active region. Besides, the original steep barrier for holes at the EBL/hole supplier interface can be splited into a two-step barrier which is more favorable for hole transportation. As a result, an enhanced optical power by 49.4% and alleviated efficiency droop by 76.3% can be achieved with the proposed p-n junction-based hole accelerator. The results can pave the way for AlGaN-based DUV LEDs towards high-power and high-efficiency applications.

Experimental Determination of the Charge Carrier Transport Models for Improving the Simulation of the HR GaAs:Cr Detectors' Response.

The response of Timepix3 detectors with 300 µm and 500 µm thick HR GaAs:Cr sensors was studied with particle beams at the Danish Centre for Particle Therapy in Aarhus, Denmark. Therefore, the detectors were irradiated at different angles with protons of 240 MeV. The precise per-pixel time and energy measurements were exploited in order to determine the charge carrier transport properties. Using the tracks left by the penetrating charged particles hitting the sensor at the grazing angle, we were able to determine the charge collection efficiency, the charge carrier drift times across the sensor thickness, the dependency of the electron, and for the first time, the hole drift velocity on the electric field. Moreover, extracting the dependence of the charge cloud size on the interaction depth for different bias voltages, it was possible to determine the dependence of the diffusion coefficient on the applied bias voltage. A good agreement was found with the previously reported values for n-type GaAs. The measurements were conducted for different detector assemblies to estimate the systematic differences between them, and to generalize the results. The experimental findings were implemented into the Allpix Squared simulation framework and validated by a comparison of the measurement and simulation for the 241Am γ-ray source.

Gradient bandgap modification for highly efficient carrier transport in antimony sulfide-selenide tandem solar cells

Antimony chalcogenides emerge as a type of efficient material for solar cells. In particular, antimony sulfide-selenide (SbSSe) has attracted significant interests based on their simple preparation, excellent photoelectric performance, and tunable bandgaps. In this study, by applying energy-band engineering technologies, we achieved carrier transport balance and light absorption balance for SbSSe single- and triple-junction solar cells, respectively. First in a single junction solar cell, the photoelectric conversion efficiency (PCE) of SbSSe solar cells is improved from 13.14% to 16.16% with a front-gradient Se content structure compared to a non-gradient Se content SbSSe solar cell. This improvement is attributed to the additional electric field induced by such a gradient bandgap, promoting the carrier motion. Consequently, the balance of carrier transport is realized by adjusting the drift velocities of holes and electrons simultaneously, thereby surpassing carrier recombination and improving the device parameters of short-circuit current density ( J sc ) and fill factor (FF). In a next step, an SbSSe of advanced gradient bandgap has been applied as the absorber layer of middle-cell in an antimony chalcogenide based triple-junction solar cell. Based on the high J sc and FF advantages of SbSSe sub-cells with front-gradient Se content structure, the uniform absorption of sunlight in each sub-cell and current matching of tandem solar cells could be easily realized. Eventually, the PCE of the triple-junction solar cell exhibits an enhancement from 17.34% to 19.51%. Our results demonstrate that the application of energy-band engineering technology can effectively improve device performance, providing theoretical guidance for the refined design and nanomanufacturing development of antimony chalcogenide solar cells. • Balanced carrier transport can be realized in a front-gradient SbSSe absorber. • Front-gradient SbSSe can form an additional electric field to promote hole transport. • A front-gradient SbSSe solar cell yields a high FF and J sc . • Applying a front-gradient SbSSe middle cell can enhance the triple-junction solar cell performance.

Hot carrier dynamics of photoinjected plasma in indium nitride

A theoretical study using the nonequilibrium statistical operator method is performed to investigate the ultrafast transient transport properties of photoexcited carriers in wurtzite InN subjected to electric fields up to 20 kV/cm. The evolution toward the steady state of the drift velocity of carriers (electrons and holes) and nonequilibrium temperature (carriers and phonons) is determined. The transient regime occurs at the subpicosecond scale with maxima and minima in the hole and electron drift velocities.

Self-heating controlled current–voltage and noise characteristics in graphene

Pulsed current–voltage characteristics have been measured for epitaxial SiC graphene under controlled lattice temperature conditions. The monolayer and AB stacked bilayer graphene is dry transferred on SiO2. The measured characteristics of two-terminal samples with coplanar electrodes demonstrate non-ohmic behaviour up to intermediate-high fields. At high currents thermal effects come into play and soft damage of the samples takes place. Bilayer graphene samples withstand 20 kV cm−1 electric field when few nanosecond voltages pulses are applied. The results are treated in terms of hole drift velocity estimated from the data on current under the assumption of uniform electric field and constant hole density. The highest velocity of ∼3.5–5 × 106 cm s−1 (∼1.5 × 107 cm s−1) is estimated for the bilayer (monolayer) graphene. The velocity results are interpreted with hot phonons. High frequency noise is measured at 10 GHz for the monolayer graphene and the associated shot noise is resolved.

Behavioral Contrast of Electron and Hole Transport in High-Resolution Diamond Detectors: A Biparametric Correlation Study

In this letter we investigate for the first time the behavioral differences of transport properties of electrons and holes observed in high-resolution radiation detectors fabricated on high-purity single crystalline diamond using biparametric (BP) correlation studies. The chemical vapor deposition (CVD) grown 500 μm thick diamonds exhibited an excellent energy resolution of 0.5% for 5486 keV alpha particles for both electron and hole transport in a vertical electrode geometry. Interestingly, the behavior of the electron and hole transport were found to be vastly different at bias voltages less than 100 V. No discernable detector signal was observed due to electron transit below an applied bias of -100 V. The hole transit on the other hand, resulted in resolvable pulse height spectrum (PHS) from bias voltages as low as +10 V. Digital biparametric correlation studies of the rise-times and induced charge from each interaction of the incident alpha particles, revealed that the observed difference is due to the excessive electron trapping below bias voltages of -100 V. At lower biases, the width of the alpha peak in the PHS due to hole movement, was hardly affected by trapping and was observed to be limited due to lower hole drift velocities. The developed method with new insights of correlating the biparametric plots and the detector performance can be extended to vast range of semiconductor detectors.

Carrier Scattering Mechanisms: Identification via the Scaling Properties of the Boltzmann Transport Equation

AbstractA method based on the scaling properties of the Boltzmann transport equation is proposed to identify the dominant scattering mechanisms that affect charge transport in a semiconductor. This method uses drift velocity data of mobile charges at different lattice temperatures and applied electric fields and takes into account the effect of carrier heating. By performing time‐of‐flight measurements on single‐crystalline diamond, hole and electron drift velocities are measured under low‐injection conditions within the temperature range 10–300 K. Evaluation of the data using the proposed method identifies acoustic phonon scattering as the dominant scattering mechanism across the measured temperature range. The exception is electrons at 100–200 K where conduction‐band valley repopulation has a prominent effect. At temperatures below ≈80 K, where valley polarization is observed for electrons, transport dominated by acoustic phonon scattering is observed in different valleys separately. The scaling model is additionally tested on data from highly resistive gallium arsenide samples to demonstrate the versatility of the method. In this case, impurity scattering can be ruled out as the dominant scattering mechanism in the samples for the temperature range 80–120 K.

Fabrication, Characterization and Analysis of Concentrically Strained Silicon Nanowires With Extremely-High Hole Mobility

The findings from the time-of-flight (TOF) experiment for strained silicon nanowires are reported in this article, in combination with numerical simulations demonstrating new physical features involved in the quasi-quantum regime. The drift velocity of valence holes is demonstrated to exceed greatly that of conduction electrons. These experimental data are accurately reproduced by our simulations, revealing a dramatic reduction in the hole's effective mass as well as a significant extension of the mean-free time between two consecutive random scattering events due to combined effects of both quantum confinement and biaxial strain. Many-body theory is further introduced for calculating elastic-scattering rate of impurities with the inclusion of static screening under the random-phase approximation. Dark current, photo-current and photoluminescence spectra are all measured in our experiments, in addition to direct TOF experiment by employing a Ti-sapphire mode-locked femtosecond laser, and their data are fully analyzed and physically explained by using both simulations and many-body theory. Our observation that the Einstein's relation does not hold true for 1-D transport under a strong electric field is supported by a quantum-statistical calculation. Our discovery with a very large hole mobility is expected to play a key role in establishing a much more straight-forward technical path towards high-performance CMOS and radio-frequency amplifiers than current ones.

3-D Monte Carlo device simulator for variability modeling of p-MOSFETs

A device simulator for p-MOSFETs, based on the Monte Carlo method for the solution of the Boltzmann transport equation, was developed, and results and implementation challenges are presented and discussed in detail in this paper. By using a Monte Carlo device simulator (MCDS), it is possible to consider effects that affect state-of-the-art devices that cannot be adequately considered using other methods (drift–diffusion, hydrodynamic, etc.). Novel feature of the simulator is that it treats hole–hole and hole–impurity interactions in real space using particle–particle–particle–mesh coupling method, allowing the simulator to account for random dopant fluctuation and charged traps, responsible for random telegraph noise and bias temperature instability, while having a small computational cost enabling statistical simulations. The MCDS shows excellent agreement between experimental data for the hole drift velocity versus electric field and low-field hole mobility versus doping density.

Experimental Determination of Velocity-Field Characteristic of Holes in GaN

This study presents a photo-assisted method to measure the drift velocity of carriers in semiconductors, and successfully used to determine the drift velocity of holes in GaN. A p-i-n diode with a buried p-type layer was designed and fabricated on a free-standing GaN substrate. By reverse-biasing the p-i-n diode and illuminating the cathode layer using an ultraviolet light simultaneously, photo-generated holes were injected into the depletion region and accelerated by the electric field to reach the saturation velocity. The drift velocity ( $\text{v}_{\text {d}}$ )–electric field (E) characteristic can be obtained from the photocurrent induced by photo-generated holes. The measured hole drift velocity can be written as $v_{\text {d}}=\mu _{\text {LF}}E /[1+(\mu _{\text {LF}}E/v_{{\mathrm {sat}}})^{{\beta }}]^{{1}/\beta }$ , where $\mu _{\text {LF}}\,\,=17$ cm2/Vs is the low-field hole mobility, $v_{\text {sat}}= 6.63\times 10^{{6}}$ cm/s is the saturation velocity, $\beta =1.75$ is the fitting parameter. The method presented in this study is a unique way of determining the saturation drift velocity of holes in GaN.

Anisotropic hole drift velocity in 4H-SiC

A theoretical study on the nonlinear transport of holes in the transient and steady state of p-doped 4H-SiC under the influence of high electric fields is presented. It is based on a nonlinear quantum kinetic theory which provides a clear description of the dissipative phenomena that are evolving in the system. The hole drift velocity and the nonequilibrium temperature are obtained, and their dependence on the electric field strength is derived and analyzed.

Physical Model of the Effects of Drift Velocity on Current Transport in PN Junctions under the Forward Electric Field

Current transport in PN junctions under a forward electric field is physically modeled. The space-charge regions or the depletion regions at thermal equilibrium in both n-type and p-type sides are found to be divided into two regions: one is the depletion region; the other is the accumulation region. The proposed current equation under a forward electric field reveals that a drift velocity caused by the forward voltage is a physical origin of the ideality factor. The voltage and temperature dependent ideality factors determined by the current-voltage curve predicted by this diode current equation agree well with experimental results. Due to the electron drift velocity higher than the hole drift velocity, electron transport dominates the current transport. This current equation of PN junctions will benefit to better physically understand the current transport in semiconductor materials and devices. It is also helpful to improve their performance.

High-Performance Black Phosphorus Field-Effect Transistors with Long-Term Air Stability.

Two-dimensional layered materials (2DLMs) are of considerable interest for high-performance electronic devices for their unique electronic properties and atomically thin geometry. However, the atomically thin geometry makes their electronic properties highly susceptible to the environment changes. In particular, some 2DLMs (e.g., black phosphorus (BP) and SnSe2) are unstable and could rapidly degrade over time when exposed to ambient conditions. Therefore, the development of proper passivation schemes that can preserve the intrinsic properties and enhance their lifetime represents a key challenge for these atomically thin electronic materials. Herein we introduce a simple, nondisruptive, and scalable van der Waals passivation approach by using organic thin films to simultaneously improve the performance and air stability of BP field-effect transistors (FETs). We show that dioctylbenzothienobenzothiophene (C8-BTBT) thin films can be readily deposited on BP via van der Waals epitaxy approach to protect BP against oxidation in ambient conditions over 20 d. Importantly, the noncovalent van der Waals interface between C8-BTBT and BP effectively preserves the intrinsic properties of BP, allowing us to demonstrate high-performance BP FETs with a record-high current density of 920 μA/um, hole drift velocity over 1 × 107 cm/s, and on/off ratio of 1 × 104 to ∼1 × 107 at room temperature. This approach is generally applicable to other unstable two-dimensional materials, defining a unique pathway to modulate their electronic properties and realize high-performance devices through hybrid heterojunctions.

A study of electric transport in n- and p-type modulation-doped GaInNAs/GaAs quantum well structures under a high electric field

We present the results of longitudinal carrier transport under a high electrical field in n- and p-type modulation-doped Ga0.68In0.32NyAs1−y/GaAs (y = 0.009, 0.017) quantum well (QW) structures. Nitrogen composition-dependent drift velocities of electrons are observed to be saturated at and at 77 K for the samples with y = 0.009 and y = 0.017, respectively, while the drift velocities of holes do not saturate but slightly increase at the applied electric field in the range of interest. The hole drift velocity is observed to be higher than the electron drift velocity. The electron mobility exhibits an almost temperature-independent characteristic. On the other hand, the hole mobility exhibits a conventional temperature dependence of modulation-doped QW structures. As the temperature increases, the drift velocity of the electrons exhibits an almost an temperature-insensitive characteristic, but, on the other hand, for holes, drift velocity decreases approximately from 107–106 cm s−1. It is observed that the drift velocities of electrons and holes are N-dependent and suppressed at higher electric fields. Furthermore, experimental results show that there is no evidence of negative differential velocity (NDV) behaviour for both n- and p-type samples. To explore the observed electron and hole drift velocity characteristic at high electric fields, we use a simple theoretical model for carrier transport, which takes into account the effect of non-drifting hot phonons. The mobility mapping technique (comparison method) is used to extract hot hole temperature in order to employ it in the non-drifted phonon distribution and to obtain the drift velocity–electric field curves. Then hot electron temperatures are obtained from the drift velocity–electric field curves as a fit parameter using non-drifted hot phonon dynamics. The analytical model is well-matched to the experimental –E curves, indicating that carrier-hot phonon scattering is the main reason for suppressing the NDV mechanism in GaInNAs/GaAs QW structures with a carrier density higher than 1017 cm−3.

4H-SiC Based Subnanosecond (150 ps) High-Voltage (1600 V) Current Breakers

High-voltage (1600 V) diodes based on epitaxial 4H-SiC p++–p+–n0–n+ structures are tested as fast current breakers included in a special pulsed circuit. The measured current-breakdown time is about 150 ps. This is a record short time for high-voltage (above 1000 V) silicon-carbide diode breakers. The saturated hole-drift velocity in 4H-SiC of p type is experimentally estimated for the first time: v sp = 3 × 106 cm/s.

Высоковольтные (1600 V) размыкатели тока с субнаносекундным (150 ps) быстродействием на основе 4H-SiC

AbstractHigh-voltage (1600 V) diodes based on epitaxial 4 H -SiC p ^++– p ^+– n _0– n ^+ structures are tested as fast current breakers included in a special pulsed circuit. The measured current-breakdown time is about 150 ps. This is a record short time for high-voltage (above 1000 V) silicon-carbide diode breakers. The saturated hole-drift velocity in 4 H -SiC of p type is experimentally estimated for the first time: v _ sp = 3 × 10^6 cm/s.

Analysis of high-field hole transport in germanium and silicon nanowires based on Boltzmann’s transport equation

In this paper, high-field hole transport in germanium nanowires was studied by using Boltzmann's transport equation in an atomistic framework. The scattering mechanisms taken into account are phonon and surface roughness. The hole drift velocities of [110], [111], and [112]-oriented germanium nanowires showed negative-differential characteristics, while that of the [001] nanowire did not. The behavior of hole drift velocity was analyzed based on the highly nonparabolic and orientation-dependent valence band structure. High-field hole transport properties in silicon nanowires were also calculated, and the differences between germanium and silicon nanowires were discussed, focusing mainly on momentum and energy relaxation times. The [110], [111], and [112] silicon nanowires showed faster hole drift velocity at high field than the germanium nanowires with the same orientation. This was attributed to faster energy relaxation in silicon nanowires, which mitigates the negative differential mobility in silicon nanowires compared to germanium nanowires.

Simulating the Role of TCO Materials, their Surface Texturing and Band Gap of Amorphous Silicon Layers on the Efficiency of Amorphous Silicon Thin Film Solar Cells

In this work, through modeling we propose how the choice of the TCO material, its texturing and optimization of band gap of a-Si:H layers help to increase the efficiency of a-Si:H solar cells. While selecting plane and textured indium tin oxide (ITO) and zinc oxide (ZnO) as TCOs, the solar cell parameters and performance are examined as a function of band gap of different a-Si:H layers. The optimum band gap values of 2.1 eV, 1.9 eV and 1.85 eV are obtained for p, i and n-layers of a-Si:H with maximum efficiencies of ~ 15.5 % and 17.7 % using plane ITO and ZnO contacts respectively. Interestingly, the conversion efficiency is further increased to ~ 16.3 % and 18.6 % when textured ITO and textured ZnO are used as TCOs. Moreover the higher efficiencies with ZnO-based contact than ITO-based contact can be explained due to slightly higher drift velocity of holes nearer to the junction and little improved optical properties which may also attributes to the enhanced trapping of the light. These results are very encouraging and may help in developing a-Si:H based solar cell technology for thin films.

Precision measurement of the carrier drift velocities in ⟨100⟩ silicon

Measurements of the drift velocities of electrons and holes as functions of electric field strength and temperature in high-purity n- and p-type silicon with ⟨100⟩ crystal orientation are presented. The measurements cover electric field strengths between 2.4 and 50 kV/cm and temperatures between 233 and 333 K. Two methods have been used for extracting the drift velocities from current transient measurements: a time-of-flight (tof) method and fits of simulated transients to the measured transients, with the parameters describing the field and temperature dependence of the electron and hole mobilities as free parameters. A new mobility parametrization, which also provides a better description of existing data than previous ones, allowed an extension of the classical tof method to the situation of non-uniform field strengths. For the fit method, the use of the convolution theorem of Fourier transforms enabled us to precisely determine the electronics transfer function of the complete set-up, including the sensor properties. The agreement between the tof and the fit method is about 1%, which corresponds to a time-of-flight uncertainty of 30 ps for a pad diode of 200μ m thickness at the highest voltages. Combining our results with published data of low-field mobilities, we derive parameterizations of the drift velocities in high-ohmic ⟨100⟩ silicon for electrons and holes for field strengths between 0 and 50 kV/cm and temperatures between 233 and 333 K.

Measurement of the drift velocities of electrons and holes in high-ohmic [formula omitted] silicon

Measurements of the drift velocities of electrons and holes as functions of electric field and temperature in high-purity n- and p-type silicon with 〈100〉 lattice orientation are presented. The measurements cover electric field values between 2.5 and 50kV/cm and temperatures between 233 and 333K. For both electrons and holes differences of more than 15% are found between our 〈100〉 results and the 〈111〉 drift velocities from literature, which are frequently also used for simulating 〈100〉 sensors. For electrons, the 〈100〉 results agree with previous 〈100〉 measurements; however, for holes differences between 5 and 15% are observed for fields above 10kV/cm. Combining our results with published data of low-field mobilities, we derive parametrizations of the drift velocities in high-ohmic 〈100〉 silicon for electrons and holes for fields up to 50kV/cm, and temperatures between 233 and 333K. In addition, new parametrizations for the drift velocities of electrons and holes are introduced, which provide somewhat better descriptions of existing data for 〈111〉 silicon than the standard parametrization.