Voltage Of Light-emitting Diodes Research Articles

Surfactants effect on the electrochemical properties of FeSe2 electrode for supercapacitor with first principles insights into quantum capacitance

Nanostructured material like FeSe2 has been synthesized through a simple and cost-effective hydrothermal method. The effect of cationic, non-ionic, and anionic surfactants has been studied on the structural, morphological, and electrochemical properties of FeSe2 electrode material. The FeSe2 electrode material prepared with the assistance of CTAB surfactant displayed a superior specific capacitance value of 401 F g −1 at a scan rate of 10 mV s −1 and 337 F g −1 at a current density of 3.3 A g −1, respectively. The maximum energy density of the symmetric supercapacitor cell fabricated with the same material is evaluated to be 47 Wh kg−1 at a power density of 1667 W kg−1. The addition of the surfactant cetyltrimethylammonium bromide (CTAB) during the synthesis process is found to have a significant impact on the charge storage capability of the material, which is attributed to an increase in active surface area, porosity, reduced crystallite size, and reduced bulk resistance, etc. The real-time use of CTAB-based FeSe2 symmetric cells has been verified via illuminating different voltage light-emitting diodes. Further, the electronic properties and the quantum capacitance of FeSe2 material synthesized without the assistance of any surfactant have been analyzed theoretically using first-principles density functional theory calculations. Acquired results unveil the potential growth of FeSe2 electrode material in energy storage applications.

Portable Instrument for Hemoglobin Determination Using Room-Temperature Phosphorescent Carbon Dots.

A portable reconfigurable platform for hemoglobin determination based on inner filter quenching of room-temperature phosphorescent carbon dots (CDs) in the presence of H2O2 is described. The electronic setup consists of a light-emitting diode (LED) as the carbon dot optical exciter and a photodiode as a light-to-current converter integrated in the same instrument. The reconfigurable feature provides adaptability to use the platform as an analytical probe for CDs coming from different batches with some variations in luminescence characteristics. The variables of the reaction were optimized, such as pH, concentration of reagents, and response time; as well as the variables of the portable device, such as LED voltage, photodiode sensitivity, and adjustment of the measuring range by a reconfigurable electronic system. The portable device allowed the determination of hemoglobin with good sensitivity, with a detection limit of 6.2 nM and range up to 125 nM.

A High Accuracy Weak Voltage LED Analog Dimming Method

A weak voltage analog dimming method for high precision light emitting diode (LED) current control with N-Bits analog static offset calibration (N-Bits ASOC), dynamic offset cancellation (DOC) and K-times dimming duty to voltage (KD2V) conversion is presented. The N-Bits ASOC circuit decreases the analog dimming error to V OSMAX /2 N - 1 by pre-calibrating the error amplifier offset. Moreover, through a smart combination of KD2V circuits and DOC amplifier, the KD2V and DOC operational amplifier magnify the dimming reference and LED current sense voltage K-times, respectively, which reduces the residual dimming error K-times without any modification to the LED current sampling resistor. Based on the 0.18μm 5V/40V Bipolar-CMOS-DMOS (BCD) process, experimental results for a cell phone backlighting driver show that the analog dimming ILED error reduces to 30μA at maximum 200mV LED current feedback voltage. This deviation mostly keeps constant through a dimming duty range of 0.5% to 100% and VIN range of 2.7V to 5.5V. Furthermore, a 200-sample measurement data suggests that this method is effective even when dimming duty is smaller than 0.5% for mass production.

A 93.3% Peak-Efficiency Self-Resonant Hybrid-Switched-Capacitor LED Driver in 0.18-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m CMOS Technology

This paper presents an integrated light-emitting diode (LED) driver based on a self-resonant hybrid-switched-capacitor converter (H-SCC) operating in the megahertz range. An integrated zero-current detection (ZCD) circuit is designed to enable self-resonant operation and zero-current switching. A self-resonant timer is proposed to set the switching frequency to resonance automatically, accommodating for variations in the LED voltage, output current, inductor value, and/or parasitic components, and improving the converter efficiency at light loads without the need for an accurate clock with variable frequency. A ZCD threshold control is also proposed to enable continuous conduction mode and improve efficiency at large currents. The design of high-speed integrated current sensors to measure the inductor current in the H-SCC is also presented. Capacitors, power switches, ZCD, current monitors, and the control circuitry of the LED driver are integrated on-chip in a low-cost, 5-V, 0.18- $\mu \text{m}$ bulk CMOS technology. The proposed driver was measured using inductor values between 36 and 470 nH. It achieves a peak efficiency of 93.3% and an efficiency of 83.1% at the nominal current. The LED driver is able to control a 700-mA LED down to less than 10% of its nominal current. The effective chip area is 7.5 mm2, and the maximum power density is 373 mW/mm2. To our knowledge, this LED driver can achieve efficiencies comparable to prior art LED drivers using a 6.6 $\times $ smaller inductor.

Research on the relationship between ideality factor and number of units of GaN-based high voltage light-emitting diode

Ideality factor can reflect the current, the carrier leakage, and the phenomenon of non-radiative recombination in light-emitting diode (LED). For the problem of ideality factor from current report about GaN-based LED, the value of ideality factor n is calculated by using the I-V curve fitting of high voltage LED. And the relationship between the ideality factor and the number of units is series in 12, 19, 51 and 80 V GaN-based high-voltage LED are discussed. In addition, the relationship between ideality factor and spectral half width (FWHM) is analyzed, and the impact of current crowding effect on the ideality factor is also studied. Results show that the ideality factor n increases nearly linearly with the number of units in series, indicating that the ideality factor n of high voltage LED is composed of its series units. It is a valuable result for understanding the ideality factor of GaN-based high voltage LED.

Saga of efficiency degradation at high injection in InGaN light emitting diodes

Saga of efficiency degradation at high injection in InGaN light emitting diodes

Design and Implementation of a Dimmable LED Driver with Low-Frequency PWM Control

This paper proposes a high-efficiency dimmable LED driver for light emitting diodes (LED). The developed LED driver consists of a full-bridge resonant converter and six buck converters. The function of the full-bridge resonant converter is to obtain a smooth dc-link voltage for the buck converters by phase-shift modulation (PSM) while that of the six buck converters is to drive six LED modules, respectively. The gate voltage of the active switch of each buck converter is a combination of high-frequency and low-frequency pulses. The duty ratio of the high-frequency pulse controls the LED voltage and thereby, controls the amplitude of LED current. LEDs are dimmed by low-frequency pulse-width modulation (PWM) to vary the average current flowing through LED. Circuit equations are derived and circuit parameters are designed. High circuit efficiency is ensured by operating the active switches at zero-voltage switching-on to reduce the switching loss. Finally, a prototype circuit was built to verify the accuracy and feasibility of the proposed LED driver.

Tunable electroluminescence from low‐threshold voltage LED structure based on electrodeposited Zn1−xCdxO‐nanorods/p‐GaN heterojunction

AbstractViolet light‐emitting diode (LED) structures based on Cd‐alloyed zinc oxide (Zn1−xCdxO) nanorods (NRs)/p‐GaN heterojunction have been fabricated by epitaxial electrodeposition at low temperatures in an aqueous soft bath followed by a mild thermal annealing. The ultraviolet (UV) room‐temperature emission peak at around 397 nm with a full width at half‐maximum (FWHM) of 10 nm observed from pure ZnO‐NRs/p‐GaN at room temperature was shifted to 417 nm with FWHM of 14 nm by employing a Zn0.92Cd0.08O‐NRs/p‐GaN heterojunction. The emission threshold voltage was low at about 5.0 V and the electroluminescence (EL) intensity rapidly increased with the applied forward‐bias voltage. The emission wavelength increased with the Cd content in the alloy. The EL physics mechanism in LED structures of the heterojunctions is discussed.

ZnO nanorod based low turn-on voltage LEDs with wide electroluminescence spectra

Light emitting diodes (LEDs) based on arrays of n-type ZnO Nanorods were fabricated on p-GaN films using a hydrothermal method. The LEDs emit mainly in blue and UV range of the light. Their current–Voltage (I–V) characteristics typically show a low leakage current (7.2μA) and a high rectification ratio (355). Devices operate at a low turn-on voltage of ∼4.5V. Photoluminescence (PL) and electroluminescence (EL) measurements suggest low density of ZnO defects; however, in some aspects density of interfacial defects still might be considerable in the studied devices. The PL emission is deconvoluted to three peaks that are located at wavelengths of 361, 381, and 397nm, while the wide EL spectra are deconvoluted to five peaks appearing at 368, 385, 427, 474, and 515nm. Near-band-edge (NBE) emission of p-GaN and n-ZnO was observed in both the PL and EL spectra. Deconvoluted EL spectra consist of a very wide green band with the peak at 515nm and extending up to 650nm (red), and a rarely reported EL emission at 474nm. Origin of these emissions is discussed, herein. The electrical characteristics together with EL characteristics indicate potential to develop and study p-GaN/n-ZnO nanorod LEDs for white emitting applications.

Light-emitting diode quality investigation via low-frequency noise characteristics

There are presented a comprehensive investigation of noise characteristics (the fluctuations spectra of both the emitted light power and the voltage of light-emitting diodes (LEDs) at constant direct current (d.c.), and their simultaneous cross-correlation factor) for nitride-based green (InGaN) and phosphide-based red (AlInGaP) LEDs. The noise characteristics of LEDs before aging and during their aging have been carried out. It is shown, that large level of electrical fluctuations of the red LEDs did not show on high defectiveness of the active layer. These electrical fluctuations are related with the charge carrier recombination and capture processes in defects outside the active region, and they only weakly influence the emitted light power fluctuations. On the contrary, the large cross-correlation factor for the green LEDs shows that low-frequency electrical and optical noises are mainly due to defects in the active layer and its interfaces. It is shown, that during 1800 h aging the red LEDs characteristics are more stabile: their light output power has negligible changes, while analogical characteristics for the green LEDs gradually decreases over 30%.

Effect of chip geometry on breakdown voltage of GaInN light-emitting diodes

Reverse leakage current characteristics of GaInN/GaN multiple quantum well light-emitting diodes (LEDs) with various chip geometries are examined. The effect of chip geometry on the reverse leakage current is negligible at a low voltage, but becomes apparent at a high voltage. The reverse breakdown voltage of LEDs decreases as the angle of vertex in the chip geometry decreases presumably because of a highly localised electric field strength near the vertex. This suggests that a chip geometry with a rounded vertex is suitable for reliable high-power LEDs.

Highly Reliable GaN-Based Light-Emitting Diodes Formed by p–<tex>$hboxIn_0.1hboxGa_0.9hboxN$</tex>–ITO Structure

Indium-tin-oxide (ITO) is deposited as a transparent current spreading layer of GaN-based light-emitting diodes (LEDs). To reduce the interfacial Schottky barrier height, a thin p-In/sub 0.1/Ga/sub 0.9/N layer is grown as an intermediate between ITO and p-GaN. The contact resistivity around 2.6/spl times/10/sup -2/ /spl Omega//spl middot/cm/sup 2/ results in a moderately high forward voltage LED of 3.43 V operated at 20 mA. However, the external quantum efficiency and power efficiency are enhanced by 46% and 36%, respectively, in comparison with the conventional Ni-Au contact LEDs. In the life test, the power degradation of the p-In/sub 0.1/Ga/sub 0.9/N-ITO contact samples also exhibits a lower value than that of the conventional ones.

Direct current stabilization of scintillation counters used for uranium prospecting

A simple system for stabilizing a scintillation counter is described which uses a dc light source (a green light emitting diode) to illuminate the photo-cathode of the photomultiplier used to detect γ-induced light pulses from the scintillator. Basically, the photomultiplier anode current due to the light emitting diode light is held constant by an automatic control loop acting on the eht voltage to keep the gain of the photomultiplier tube constant. However, because the temperature coefficient of the scintillator does not in general match that of the light emitting diode, further compensation is required to achieve constant γ pulse gain. This is provided by adding to the control circuit a current derived from the light emitting diode voltage which is an excellent measure of temperature; the use of this technique results in gain constancy to within ±1% in the 10–50°C ambient temperature range. Noise and countrate limitations are discussed and it is concluded that the system is generally applicable to uranium prospecting equipment.