Effect of carrier transit time on the bandwidth of traveling-wave photodetector

We present an impulse response of traveling-wave photodetector considering the photo-excited carrier transit time as well as the velocity mismatch between lightwave and microwave. The transit time effect on the bandwidth is found to be more significant than the velocity mismatch effect, if the thickness of the intrinsic absorption region is over 0.2μm. The effect of the intrinsic absorption region thickness is also examined. The optimal thickness of the intrinsic absorption region is determined considering the transit time, the optical confinement factor, and the velocity mismatch.     

Experimental analysis of resonant tunneling transit time using high-frequency characteristics of resonant tunneling transistors

We systematically estimate the resonant tunneling transit time from the high-frequency characteristics of resonant tunneling transistors. It is found that the transit time across an InGaAs/InAlAs resonant tunneling structure is more than one order of magnitude shorter than that for GaAs/AlAs with the same barrier layer thickness. In addition, the obtained times agree reasonably well with calculated phase time. It is probably the elastic resonance width and not the inelastic scattering effect that mainly determines the resonant tunneling transit time.     

Experimental and theoretical study of the carrier capture time

We present an experimental and theoretical study of the carrier capture time into a semiconductor quantum well. We observe for the first time the predicted oscillations of the phonon emission induced capture time experimentally and found good agreement with theory. Calculations show that not only does the LO phonon emission induced capture time (ph capture) oscillate as a function of well width, but also the carrier-carrier scattering induced capture time (c-c capture) oscillates by more than an order of magnitude as a function of the active layer design. Recently, it has been shown that the carrier capture time is directly related to the modulation bandwidth in a quantum well laser. As a result, it might be possible to tailor the modulation bandwidth by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.     

Experimental evidence of ion heating by transit time magnetic pumping

Transit time magnetic pumping has been used in a Tohamak to heat the ions from 200 to 260 eV. Electron density and impurity concentration were not affected during the process. The temperature increase is consistent with theory.     

An alternative method for measurement of charge carrier mobility in semiconductors using photocurrent transient response

We report here a simple alternative method for measuring charge carrier drift mobilities in semiconductor devices. A typical falling photocurrent transient formula for switch-off-state was adjusted to obtain simultaneously electron and hole mobilities. For both undoped ZnO film and InAlAs/InGaAs quantum well structure, electron mobilities extracted from our model were compared with those obtained from maximum-entropy mobility-spectrum analysis method (ME-MSA). Our results demonstrated that electron mobility obtained from our photocurrent response model could serve as substitutes for a representative mobility obtained from ME-MSA.     

Calibration of pulse transit time through a cable for EAS experiments

In ground-based extensive air shower experiments, the direction and energy are reconstructed by measuring the relative arrival time of secondary particles, and the energy they deposit. The measurement precision of the arrival time is crucial for determination of the angular resolution. For this purpose, we need to obtain a precise relative time offset for each detector and to apply the calibration process. The time offset is associated with the photomultiplier tube, cable, relevant electronic circuits, etc. In view of the transit time through long cables being heavily dependent on the ambient temperature, a real-time calibration method for the cable transit time is investigated in this paper. Even with a poor-resolution time-to-digital converter, this method can achieve high precision. This has been successfully demonstrated with the Front-End-Electronic board used in the Daya Bay neutrino experiment.     

Group velocity matters for accurate prediction of phonon-limited carrier mobility

First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors. However, in addition to the high computational cost, it is still a challenge to obtain accurate mobility for carriers with a complex band structure, e.g., hole mobility in common semiconductors. Here, we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations. This treatment greatly reduces the computational cost in two ways: relieves the requirement of an extremely dense κ mesh to obtain a smooth change in group velocity, and reduces the 5-dimensional integral to 3-dimensional integral. Taking Si and SiC as two examples, we find that this simplified approach reproduces the full first-principles calculation for mobility. If we use experimental effective masses to evaluate the group velocity, we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range. These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.     

On triggering role of carrier mobility for Laughlin state organization

Recent experiments with suspended graphene have corroborated an important role of carrier mobility in the competition between Laughlin state and insulating state, presumably of Wigner-type electron crystal. Moreover, the fractional quantum Hall effect (FQHE) in graphene has been observed at low carrier densities when the interaction was reduced due to carrier dilution. This suggests that not solely interaction and the flat band with quenched kinetic energy may be important for formation of FQHE. Here, some exclusive for 2D topological arguments are supposed to explain the triggering role of carrier mobility in formation of the collective FQHE state, when conditions of sufficient flattening of a band and interaction presence are fulfilled.     

The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.     

Experimental neutron flux measurements with a diamond detector at the QUINTA setup

The operational capability of a diamond detector used to measure the neutron spectrum by the response function on the QUINTA setup [1] installed at the proton beam of the phasotron [2] (Laboratory of Nuclear Problems, Joint Institute for Nuclear Research) was demonstrated in the energy interval of 2.1–20 MeV. The neutron-flux count rate was measured. The energy of neutrons was estimated at 7.4–25.7 MeV based on the diamond-detector response spectrum. The dependence of the diamond-detector response spectra on the angle between the proton beam and the line going through the detector and the center of the QUINTA setup was investigated. The angular anisotropy of the neutron flux was demonstrated. Measurements at different distances from the detector to the QUINTA setup were performed.     

Stationary setup for identifying explosives using the tagged neutron method

The results of using a stationary setup for detecting and identifying explosives using the tagged neutron method (or associated particle imaging (API) method) have been presented. The source of 14.1-MeV tagged neutron beams is an ING-27 portable neutron generator made by VNIIA (Moscow). It contains an embedded 64-pixel silicon α detector with double-sided strip readout developed by the Joint Institute for Nuclear Research (JINR). The setup was tested for the identification of more than 30 explosives.     

Highly sensitive laser-based sensor for nanoparticles in air using a dual-ring-mirror setup

One of the most frequently applied techniques to detect nanoparticles in air is analyzing laser light scattering. This technique is very flexible while offering high accuracy and reliability. Yet its functionality highly depends on the sensitivity of the measurement system components. Especially for miniaturized sensor devices with limited space, additional techniques are needed to preserve high intensity of scattered light. In our work we demonstrate a technique using two spherical ring mirrors to identify nanoparticles with diameters below 100 nm in a forward-scattering setup. We succeeded measuring polystyrene particles with diameters of 92 nm with a signal-to-noise-ratio of more than 10.     

Channel temperature determination of a multifinger AlGaN/GaN high electron mobility transistor using a micro-Raman technique

Self-heating in a multifinger AlGaN/GaN high electron mobility transistor (HEMT) is investigated by micro-Raman spectroscopy. The device temperature is probed on the die as a function of applied bias. The operating temperature of the AlGaN/GaN HEMT is estimated from the calibration curve of a passively heated AlGaN/GaN structure. A linear increase of junction temperature is observed when direct current dissipated power is increased. When the power dissipation is 12.75 W at a drain voltage of 15 V, a peak temperature of 69.1°C is observed at the gate edge on the drain side of the central finger. The position of the highest temperature corresponds to the high-field region at the gate edge.     

SiGe HBT集电结耗尽层渡越时间模型

集电结耗尽层的渡越时间是影响晶体管交流放大系数和频率特性的重要参数。本文分三种情况求解了SiGe HBT集电结耗尽层宽度。建立了不同集电极电流密度下的集电结耗尽层渡越时间模型，该模型考虑了基区扩展效应。利用MATLAB对该模型进行了模拟，定量地研究了集电结反偏电压、集电区掺杂磷或砷的浓度、集电区宽度对集电结耗尽层渡越时间的影响。模拟结果表明：随着集电结反偏电压、集电区掺杂浓度以及集电区宽度的增大，集电结耗尽层渡越时间增大；集电结耗尽层渡越时间对薄基区的SiGe HBT频率特性影响显著，不能忽略。     

Experimental determining the alternative coherent-mode representation of a planar source

The coherent-mode representation (CMR) of an optical random source is a very powerful tool in contemporary optics. However, the practical value of the CMR is essentially restricted because of the complexity of solving the Fredholm integral equation with the source cross-spectral density as a kernel. Moreover, in practice, the analytical expression for the cross-spectral density of the source, as a rule, is unknown, a fact that makes this solution impossible in general. Here we propose a technique for the determination of the source CMR that does not involve solving the Fredholm integral equation but is based on usual radiometric measurements. We illustrate the proposed technique with the results of mathematical simulation.     

Experimental setup for quantum cryptography by means of single polarized photons

Pioneering experiments on single-photon quantum cryptography that are performed with a tailored setup are reported. The key is transferred by pulsed semiconductor lasers, which encode the polarization state of the photons in two mutually nonorthogonal bases. Photon detectors are based on C30902S silicon avalanche photodiodes. For a laser pulse repetition rate of 100 kHz and a mean number of photons per pulse of about 0.2, the key generation rate reaches ≈4 kbit/s. The error rate in the key does not exceed 1%.     

Digital holography of self-luminous objects by using a Mach-Zehnder setup

We describe a method where the wavefront emitted by a self-luminous object is superimposed to its filtered counterpart by using a Mach-Zehnder interferometer. The amplitude and phase of the resulting interference pattern is used for digital three-dimensional imaging. Experimental results are presented.