Threshold Current Increases Research Articles

Контроль качества светодиодных COB матриц по пороговому току

A description of a method for measuring the threshold current, the minimum current at which optical emission is occurs, of chips that are part of COB (chip on board) LEDs is given. The method consists in registering with a digital camera the average values of the emission intensities of the chips at three small values of the electric current and determining the threshold currents by solving a system of equations relating the measured intensities to a function approximating the P-I characteristic in the range of low currents. The measurement method was tested on commercially available COB LEDs consisting of ten series-connected chips. It was determined that the values of the threshold current of the chips differ: for the investigated samples, the coefficient of variation ranged from 2.6% to 39.2%. As a result of the tests of COB LEDs under the action of a current of 140 mA at a maximum allowable ambient temperature of 110 °C for 11 days, it was found that changes in the average values of the threshold currents during tests correlate with changes in the current-voltage characteristics of the matrices measured in the voltage range from 17 to 24 V at which leakage current dominates: the threshold current increases for those matrices in which leakage current increases. The correlation coefficient between the relative changes in the leakage current and the average values of the threshold currents is 0.94. It is shown that the correlation coefficient between the magnitude of the decline in the luminous flux of the matrices during testing and the change in the average value of the threshold current is 0.96. The established strong correlations show the possibility of diagnosing the quality by the threshold current of individual chips in the composition of COB LEDs.

Impact of the Figures of Merit (FoMs) Definitions on the Variability in Nanowire TFET: NEGF Simulation Study

In this article, we investigate the effect of variability in p-type nanowire tunnel FET (TFET) using quantum mechanical transport simulations. The simulations have been carried out using the Nano Electronics Simulation Software (NESS) from the University of Glasgow. Random discrete dopants (RDDs) and work-function variations (WFV) have been investigated in the simulations. Our statistical simulations reveal that key figures of merit (FoMs) such as the current variability generally decrease as the gate voltage decreases, the threshold voltage variability increases as the threshold current increases, and the dependences of these FoM variabilities on criteria become stronger with the switch characteristic ameliorated. Furthermore, it is interesting to find that the band offset in heterostructure can more or less alleviate the current variability, especially around the OFF-state.

The effect of humidity on the degradation mechanisms of GaN-based green laser diodes

The degradation behaviors of InAlGaN-based green laser diodes in humid ambients were studied. Nonhermetic laser diodes were aged at three different relative humidity conditions at constant current aging mode. For all nonhermetic devices failure occurred as a result of a large increase in the threshold current and sharp decrease of light output power. The reverse leakage current for the failures did not increase when the threshold current increased and slope efficiency decreased, indicating that the change in the threshold current was a result of a change in internal and/or mirror loss. Besides the threshold current increase, the photoluminescence intensity of the ridge region decreased more than 80% after aging compared with the non-aged companions. The moisture oxidized the p-Ohmic contact region of the semiconductor and caused the increase of the serial resistance. The mirror facet degradation was observed in scanning electron microscope. This led to change in reflectivity and light scattering and eventually to a large shift in the threshold current and slope efficiency of the device.

Thermal instabilities in micropinches under turbulent heating conditions

Anomalous thermal (AT) instabilities occurring in micropinches are analyzed based on the theory of small perturbations. Instabilities of this type develop in a high-temperature plasma under turbulent heating conditions in the presence of an anomalous resistance resulting from the scattering of electrons by lower hybrid drift oscillations. When turbulent heating occurs in the plasma of a micropinch, its resistivity increases with decreasing density; this may lead to the formation of plasma layers normal to the current flow (stratification). For the AT instability, a dispersion relation taking into account the effect of the plasma self-radiation has been derived, and the characteristic instability growth rates and wavelengths have been determined. A comparison of the development pattern has been performed between AT and sausage-type magnetohydrodynamic instabilities. It has been shown that for any pinch material, there is a threshold current below which AT instabilities prevail over magnetohydrodynamic instabilities. For the metals considered (aluminum, titanium, copper, molybdenum, and tungsten), the threshold currents are hundreds of kiloamperes. The threshold current increases with atomic number: for tungsten, it is approximately 3.5 times higher than for aluminum. The conclusions drawn from the analysis based on the linear small perturbation theory are compared with the results of experiments in which the parameters of ‘hot spots’ formed in X pinches were determined.

Experimental investigation of stripe cavity length effect on threshold current density for InP/AlGaInP QD laser diode

InP/AlGaInP QD laser diodes with various cavity lengths grown on GaAs substrate were synthesized by (MOVPE) technique. L-I characteristics at various temperatures (190–390K) were measured for InP/AlGaInP QD laser diodes to investigate the effect of the various stripe cavity length on threshold current density and characteristic temperature. It is found that the threshold current increases with temperature, reaches a peak, then drops as temperature increases, before it increases again at temperatures over 300K due to carrier redistribution in the quantum dot states with temperature. It is found also that the threshold current density decreases with increasing cavity length due to the increasing threshold gain with the cavity length. However, as the cavity length increases beyond certain length, the rate of the decrease of the threshold current with cavity length decreases. One important finding is that the characteristic temperature of threshold current decreases significantly with the cavity length at the operating temperature range from 190K to 360K.

The Nature of Auger Recombination in Type-I Quantum Well Lasers Operating in the Mid-Infrared

Auger recombination is known to be a significant non-radiative process limiting near- and mid-infrared quantum well lasers. The one-dimensional confinement of quantum wells and small band offsets (relative to the bandgap) permits two fundamentally different categories of Auger mechanisms to operate. These mechanisms may be identified as either <i>activated</i> or <i>thresholdless</i> in nature. In this work, we investigate the nature of the dominant Auger mechanism in mid-infrared emitting quantum wells by characterizing a range of type-I InGaAsSb quantum well lasers operating within the 2 - 3 &#x03BC;m wavelength range. The temperature dependence of both the threshold current density and integrated spontaneous emission reveal that the threshold current is dominated by radiative recombination up to a break-point temperature (occurring below 200 K). Beyond the break point temperature, the exponential dependence of the threshold current increases rapidly. The deterioration in the stability of lasing threshold indicates that a thermally activated Auger process is dominant in all devices and is sensitive to the population of heavy-holes in the quantum wells.

Features of glow discharge ignition through a small hole in the hollow cathode of a large volume

The influence of the size of a cathode gap on the initiation of the effect of a hollow cathode in a glow discharge system with an extended hollow cathode in the forevacuum pressure range is shown. It was found that the threshold current for the transition of the discharge to the burning mode with a hollow cathode is determined by the ratio of the longitudinal and transverse dimensions of the cathode slit. With a decrease in the width of the slot, the threshold current increases disproportionately; at the same time, with an increase in the length of the slot, this current sharply decreases.

Quick Fabrication VCSELs for Characterisation of Epitaxial Material

A systematic analysis of the performance of VCSELs, fabricated with a decreasing number of structural elements, is used to assess the complexity of fabrication (and therefore time) required to obtain sufficient information on epitaxial wafer suitability. Initially, sub-mA threshold current VCSEL devices are produced on AlGaAs-based material, designed for 940 nm emission, using processing methods widely employed in industry. From there, stripped-back Quick Fabrication (QF) devices, based on a bridge-mesa design, are fabricated and this negates the need for benzocyclcobutane (BCB) planarisation. Devices are produced with three variations on the QF design, to characterise the impact on laser performance from removing time-consuming process steps, including wet thermal oxidation and mechanical lapping used to reduce substrate thickness. An increase in threshold current of 1.5 mA for oxidised QF devices, relative to the standard VCSELs, and a further increase of 1.9 mA for unoxidised QF devices are observed, which is a result of leakage current. The tuning of the emission wavelength with current increases by ~0.1 nm/mA for a VCSEL with a 16 μm diameter mesa when the substrate is unlapped, which is ascribed to the increased thermal resistance. Generally, relative to the standard VCSELs, the QF methods employed do not significantly impact the threshold lasing wavelength and the differences in mean wavelengths of the device types that are observed are attributed to variation in cavity resonance with spatial position across the wafer, as determined by photovoltage spectroscopy measurements.

Pulsed Diode Laser Minibars for Pumping Space-Borne Solid-State Lasers

Laser diode benches (LDBs) with lasing wavelength of 808 nm were developed, manufactured and endurance-tested for utilization as pump lasers for the Nd:YAG oscillator in the climate satellite MERLIN. The LDBs combine customized TE-polarized GaAs-based 5-mm minibars with fast axis collimation lenses to produce a reliable, space qualified component at optical power of 63 W per minibar. Accelerated life tests of 20 LDBs are presented. Except for a small increase in threshold current, no wear out in operating current or sudden failures were observed, and reliable operation over the full mission time of >4 gigashots was confirmed. The electro-optical characteristics remain in the specified ranges after exposure to the total operational load of the mission.

Experimental Investigation on the Current Carried by a Cathode Spot of Vacuum Arc in Axial Magnetic Fields

The splitting and threshold currents of vacuum arc cathode spots in axial magnetic fields (AMFs) up to 220 mT are investigated experimentally. Measurements are made on electrodes of Cu and CuCr25 with electrode separations of 2 and 6 mm. The electrode diameter is 30 mm. All the experiments are conducted in a demountable vacuum chamber evacuated continuously by a turbo-molecular pump to a pressure of the order of 10−3 Pa. The spots are photographed using a high-speed digital camera with an exposure time of $2~\mu \text{s}$ . The AMF is applied by means of two permanent magnets placed symmetrically inside the chamber. The experimental results indicate that the splitting and threshold current decrease in the presence of AMF and tend to certain constant values when the field gets stronger. The splitting and threshold currents are considerably larger than Cu electrodes than those with CuCr25 electrodes. With increasing electrode separation, the splitting current decreases while the threshold current increases. Possible explanations are proposed to account for the experimental observations.

Особенности инициирования эффекта полого катода в электродной системе тлеющего разряда с протяженной катодной щелью

The influence of the size of the cathode gap on the initiation of the hollow cathode effect in a glow discharge system with an extended rectangular hollow cathode is presented. It is established that the threshold current of the discharge transition to the mode with a hollow cathode is determined by the size of the cathode gap. With a decrease in the width of the gap, the threshold current increases disproportionately, with an increase in the longitudinal size of the gap, this current decreases sharply

Dendritic Degeneration of Human Auditory Nerve Fibers and Its Impact on the Spiking Pattern Under Regular Conditions and During Cochlear Implant Stimulation.

Due to limitations of human in vivo studies, detailed computational models enable understanding the neural signaling in the degenerated auditory system and cochlear implants (CIs). Four human cochleae were used to quantify hearing levels depending on dendritic changes in diameter and myelination thickness from type I of the auditory nerve fibers (ANFs). Type I neurons transmit the auditory information as spiking pattern from the inner hair cells (IHCs) to the cochlear nucleus. The impact of dendrite diameter and degree of myelination on neural signal transmission was simulated for (1) synaptic excitation via IHCs and (2) stimulation from CI electrodes. An accurate three-dimensional human cochlear geometry, along with 30 auditory pathways, mimicked the CI environment. The excitation properties of electrical potential distribution induced by two CI were analyzed. Main findings: (1) The unimodal distribution of control dendrite diameters becomes multimodal for hearing loss cases; a group of thin dendrites with diameters between 0.3 and 1 μm with a peak at 0.5 μm appeared. (2) Postsynaptic currents from IHCs excite such thin dendrites easier and earlier than under control conditions. However, this advantage is lost as their conduction velocity decreases proportionally with the diameter and causes increased spike latency and jitter in soma and axon. Firing probability reduces through the soma passage due to the low intracellular current flow in thin dendrites during spiking. (3) Compared with dendrite diameter, variations in myelin thickness have a small impact on spiking performance. (4) Contrary to synaptic excitation, CIs cause several spike initiation sites in dendrite, soma region, and axon; moreover, fiber excitability reduces with fiber diameter. In a few cases, where weak stimuli elicit spikes of a target neuron (TN) in the axon, dendrite diameter reduction has no effect. However, in many cases, a spike in a TN is first initiated in the dendrite, and consequently, dendrite degeneration demands an increase in threshold currents. (5) Threshold currents of a TN and co-stimulation of degenerated ANFs in other frequency regions depend on the electrode position, including its distance to the outer wall, the cochlear turn, and the three-dimensional pathway of the TN.

Theoretical Study on the Effects of Dislocations in Monolithic III-V Lasers on Silicon

In this work, we present an approach to modelling III-V lasers on silicon based on a travelling-wave rate equation model with sub-micrometer resolution. By allowing spatially resolved inclusion of individual dislocations along the laser cavity, our simulation results offer new insights into the physical mechanisms behind the characteristics of 980 nm In(Ga)As/GaAs quantum well (QW) and 1.3 μm quantum dot (QD) lasers grown on silicon. We identify two effects with particular importance for practical applications from studying the reduction of the local gain in carrier-depleted regions around dislocation locations and the resulting impact on threshold current increase and slope efficiency at high dislocation densities. First, a large minority carrier diffusion length is a key parameter inhibiting laser operation by enabling carrier migration into dislocations over larger areas, and secondly, increased gain in dislocation-free regions compensating for gain dips around dislocations may contribute to gain compression effects observed in directly modulated silicon-based QD lasers. We believe that this work is an important contribution in creating a better understanding of the processes limiting the capabilities of III-V lasers on silicon in order to explore suitable materials and designs for monolithic light sources for silicon photonics.

Theoretical simulation of quantum cascade lasers based on InGaAs/AlInAs and InGaAs/AlAsSb quantum wells

Quantum cascade lasers based on semiconductor InGaAs/AlInAs and InGaAs/AlAsSb quantum wells exhibit electronic intersubband transitions within conduction bands and attract much attention due to their operation at room-temperature short wavelengths. In this work we performed a theoretical simulation of such quantum cascade lasers and studied the influence of their construction and application conditions on optical behaviour. The intersubband absorption processes in these quanta well structures were studied. Electronic properties and the conduction band edge profiles were simulated as well as the probability densities of the Wannier-Stark states were determined. The simulation results showed that with the rise of the temperature the threshold current density increases which also leads to a decrease in the optical gain. An increase of the applied electric field is accompanied by the optical gain rising and decrease of the threshold current density that results in a blue shift of laser frequency.

Effect of Mg doping concentration of electron blocking layer on the performance of GaN-based laser diodes

Performance of InGaN-based laser diodes (LDs) with different Mg concentrations of electron blocking layer (EBL) is investigated by simulation and experimental methods. It is found from the simulation results that the threshold current decreases and slope efficiency increases, when the Mg concentration of EBL increases from 2 × 1018 to 6 × 1019 cm−3; it is attributed to the suppression of the leakage of electrons and the enhancement of the injection of holes due to the variation of potential barrier for them as the increase of Mg concentration of EBL. These simulation results agree well with the experimental ones, when the Mg concentration of EBL is lower than 7.5 × 1018 cm−3. However, it deteriorates when the Mg concentration increases to 1.2 × 1019 cm−3. It may be due to the increase of the absorption loss of LDs.

Effects of quantum well thickness and aluminum content of electron blocking layer on InGaN-based laser diodes

The effects of quantum well (QW) thickness and aluminum content of electron blocking layer (EBL) on device performance of InGaN-based laser diodes (LDs) are numerically investigated with LASTIP. It is found that the device performance of the 3.0-nm-thick QW LD is the best. The threshold current increases and the output power at 120 mA decreases when the QW thickness is too thin or too thick. Actually, the optical and electrical characteristics of InGaN-based LDs demonstrate that the optical confinement factor decreases and optical loss increases when the QW thickness is too thin. The stimulated recombination rate decreases due to the poorer overlap of electron–hole wave functions and the enhanced polarization-induced built-in electric field when the well thickness is too thick. Moreover, the calculation results of LDs with different aluminum compositions of EBL demonstrate that the effectiveness of EBL would be enhanced through increasing aluminum content when the thickness of QWs decreases, because there is a reduction of ground-state energy level and the energy difference between the ground state and the top of the quantum barrier.

Strain-Related Degradation of GaN-Based Blue Laser Diodes

Degradation of GaN-based laser diodes (LDs) is studied using Electroluminescence (EL) and Transmission Electron Microscopy (TEM). The slope efficiency decrease is observed in addition to threshold current increase. Further studies on the optical loss and microstructures of the LDs indicate that the reduction of the slope efficiency is attributed to the increase of optical loss (from 14.2 cm−1 to 24.2 cm−1) and decrease of injection current efficiency induced by the defect generation around the waveguide and cladding layers. Injection current induced blue-shift of the spontaneous EL peak reduced after >3000 hrs operation, which is ascribed to the compensation of Quantum-Confined Stark Effect (QCSE) caused by partial strain relaxation near the active region.

Investigation of Current-Driven Degradation of 1.3 μm Quantum-Dot Lasers Epitaxially Grown on Silicon

This work investigates the degradation processes affecting the long-term reliability of 1.3 μm InAs quantum-dot lasers epitaxially grown on silicon. By submitting laser samples to constant-current stress, we were able to identify the physical mechanisms responsible for the optical degradation. More specifically, the samples (i) exhibited a gradual increase in threshold current, well correlated with (ii) a decrease in sub-threshold emission, and (iii) a decrease in slope efficiency. These variations were found to be compatible with a diffusion process involving the propagation of defects toward the active region of the device and the subsequent decrease in injection efficiency. This hypothesis was also supported by the increase in the defect-related current conduction components exhibited by the electrical characteristics, and highlights the role of defects in the gradual degradation of InAs quantum dot laser diodes. Electroluminescence measurements were used to provide further insight in the degradation process.

Physical Origin of the Optical Degradation of InAs Quantum Dot Lasers

We present an extensive analysis of the physical mechanisms responsible for the degradation of 1.3-μm InAs quantum dot lasers epitaxially grown on Si, for application in silicon photonics. For the first time, we characterize the degradation of the devices by combined electro-optical measurements, electroluminescence spectra, and current-voltage analysis. We demonstrate the following original results: when submitted to a current step-stress experiment: 1) QD lasers show a measurable increase in threshold current, which is correlated to a decrease in slope efficiency; 2) the degradation process is stronger, when devices are stressed at current higher than 200 mA, i.e., in the stress regime, where both ground-state and excited-state emission are present; and 3) in the same range of stress currents, an increase in the defect-related current components is also detected, along with a slight decrease in the series resistance. Based on the experimental evidence collected within this paper, the degradation of QD lasers is ascribed to a recombination-enhanced defect reaction (REDR) process, activated by the escape of electrons out of the quantum dots.

Ultrasound Stimulation Modulates Voltage-Gated Potassium Currents Associated With Action Potential Shape in Hippocampal CA1 Pyramidal Neurons.

Potassium channels (K+) play an important role in the regulation of cellular signaling. Dysfunction of potassium channels is associated with several severe ion channels diseases, such as long QT syndrome, episodic ataxia and epilepsy. Ultrasound stimulation has proven to be an effective non-invasive tool for the modulation of ion channels and neural activity. In this study, we demonstrate that ultrasound stimulation enables to modulate the potassium currents and has an impact on the shape modulation of action potentials (AP) in the hippocampal pyramidal neurons using whole-cell patch-clamp recordings in vitro. The results show that outward potassium currents in neurons increase significantly, approximately 13%, in response to 30 s ultrasound stimulation. Simultaneously, the increasing outward potassium currents directly decrease the resting membrane potential (RMP) from −64.67 ± 1.10 mV to −67.51 ± 1.35 mV. Moreover, the threshold current and AP fall rate increase while the reduction of AP half-width and after-hyperpolarization peak time is detected. During ultrasound stimulation, reduction of the membrane input resistance of pyramidal neurons can be found and shorter membrane time constant is achieved. Additionally, we verify that the regulation of potassium currents and shape of action potential is mainly due to the mechanical effects induced by ultrasound. Therefore, ultrasound stimulation may offer an alternative tool to treat some ion channels diseases related to potassium channels.