High-speed Light-emitting Diode Research Articles

Bandwidth Analysis of High-Speed InGaN Micro-LEDs by an Equivalent Circuit Model

High-speed light-emitting diodes (LEDs) are ideal light sources for low-cost plastic optical fiber communication in data-centers and vehicles. Here, high-speed micro-LEDs based on InGaN quantum dot (QD) active region with different diameters are fabricated and characterized. The -3dB bandwidth around 3.6 GHz and 1.4 GHz are achieved in the blue and green micro-LEDs under current density less than 1 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively, which can work at an environmental temperature of 125 °C. An equivalent circuit model is proposed to analyze the limiting factors of the bandwidth, using which the effects of carrier recombination and RC time constant on modulation bandwidth are separated. It is found that carrier recombination rate is fast enough and the RC constant is still the main limiting factor for the blue micro-LEDs at an injection current density of more than 500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> even when the diameter scales down to 20 μm, while the recombination rate mainly limits the bandwidth of the green micro-LEDs.

Image contrast enhancement and denoising in micro-gap weld seam detection by periodic wide-field illumination

Due to the speckle noise and welding light noise, it's still a challenge for the detection of tight weld joints by wide-field illumination. To circumvent this problem, we proposed a novel weld joint detection system by using periodic wide-field illumination. A specific frequency component can be encoded into the image sequence by modulating the intensity of the illumination. By leveraging high-speed light-emitting diode (LED) driver and high-speed camera, an image sequence can be obtained in milliseconds. Most of the light noises contain no modulation information. By extracting the modulation frequency from the image sequences, we achieved ~35% improvement in signal-to-noise ratio (SNR) of the seam image. In addition, the light noise can be removed effectively in the frequency domain. The comparisons under different welding conditions revealed that the proposed method could accurately locate the seam position in ultra-tight seam and under strong light noise while the method without modulation failed.

An Equivalent Circuit Model of a Commercial LED With an ESD Protection Component for VLC

An equivalent circuit model for a typical commercial light-emitting diode (LED) used in visible light communication (VLC) applications is proposed. The parameters of the elements in the model are extracted from the measured impedance using a vector network analyser. The model shows high accuracy in fitting the measured results and shows the effect of bonding/packaging parasitics and the electrostatic discharge (ESD) component on the LED's impedance and bandwidth. This study aims to raise the designer's awareness when designing and fabricating high-speed LED/ μLED devices to achieve the optimal modulation bandwidth in the emerging VLC applications.

Adaptive Extreme Learning Machine-Based Nonlinearity Mitigation For LED Communications

This work concerns the receiver design for light emitting diode (LED) communications, where the LED nonlinearity can severely degrade the system performance. The LED nonlinearity makes high speed LED communications more challenging when it is time-varying (e.g., due to temperature drifting) and/or combined with time-varying channels (due to the relative movement between the transmitter and the receiver). In this work, we use adaptive neural network techniques to address this issue. We first propose a new adaptive extreme learning machine (AELM) with a variable forgetting factor for adaptive learning in dynamic scenarios. Then, an AELM based (turbo) receiver is designed to handle the time-varying LED nonlinearity and memory effects jointly. It is demonstrated that the proposed AELM based receiver can efficiently mitigate the dynamic nonlinearity and memory effects, and outperform the state-of-the-art adaptive techniques significantly.

Experimental analysis of the injection pressure effect on the near-field structure of liquid fuel sprays

Primary atomization and initial spray development are critical in the operation of high-pressure liquid sprays such as the ones used in internal combustion engines. In the current paper, an analysis of the effect of the fuel injection pressure on the near-nozzle spray morphology is performed. For this purpose, a high-magnification diffused backlight imaging technique is set by means of a high-speed light-emitting diode and a long-distance microscope. The technique allows to visualize the first 2 mm of the spray development with a resolution of approximately 250 pixels per millimeter. The images obtained are processed with two different criteria to define the spray contour: one using an optical thickness threshold and a second one based on the intensity derivative on the radial direction. The analysis is focused on three aspects of the spray morphology: the spray angle, its standard deviation, and the standard deviation of the spray contour itself. The results show that the average spray angle is only slightly affected by the injection pressure, being the discharge density the main boundary condition affecting this parameter. Instead, the two standard deviation parameters show a clear increasing trend with the injection pressure, confirming that higher turbulence at the nozzle outlet induces higher variability in the spray shape. Regarding the two contour detection methodologies, similar values are reached in the average spray angle, while some more impact is seen in the spray dispersion, although the same trends are found.

High-Speed Lens-Free Holographic Sensing of Protein Molecules Using Quantitative Agglutination Assays.

Accurate, cost-effective, easy-to-use, and point-of-care sensors for protein biomarker levels are important for disease diagnostics. A cost-effective and compact readout approach that has been used for several diagnostic applications is lens-free holographic microscopy, which provides an ultralarge field of view and submicron resolution when it is coupled with pixel super-resolution techniques. Despite its potential as a diagnostic technique, lens-free microscopy has not previously been applied to quantitative protein molecule sensing in solution, which can simplify sensing protocols and ultimately enable measurements of binding kinetics in physiological conditions. Here, we sense interferon-γ (an immune system biomarker) and NeutrAvidin molecules in solution by combining lens-free microscopy with a one-step bead-based agglutination assay, enabled by a custom high-speed light-emitting diode (LED) array and automated image processing routines. We call this a quantitative large-area binding (QLAB) sensor. The high-speed light source provides, for the first time, pixel super-resolved imaging of >104 2 μm beads in solution undergoing Brownian motion, without significant motion blur. The automated image processing routines enable the counting of individual beads and clusters, providing a quantitative sensor readout that depends on both bead and analyte concentrations. Fits to the chemical binding theory are provided. For NeutrAvidin, we find a limit of detection (LOD) of <27 ng/mL (450 pM) and a dynamic range of 2-4 orders of magnitude. For mouse interferon-γ, the LOD is <3 ng/mL (200 pM) and the dynamic range is at least 4 orders of magnitude. The QLAB sensor holds promise for point-of-care applications in low-resource communities and where protocol simplicity is important.

Impact of crystal orientation on the modulation bandwidth of InGaN/GaN light-emitting diodes

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities. At 500 A/cm2, the nonpolar LED has a 3 dB bandwidth of ∼1 GHz, while the semipolar and polar LEDs exhibit bandwidths of 260 MHz and 75 MHz, respectively. A lower carrier density for a given current density is extracted from the RF measurements for the nonpolar and semipolar LEDs, consistent with the higher wavefunction overlaps in these orientations. At large current densities, the bandwidth of the polar LED approaches that of the nonpolar and semipolar LEDs due to coulomb screening of the polarization field. The results support using nonpolar and semipolar orientations to achieve high-speed LEDs at low current densities.

Differential carrier lifetime and transport effects in electrically injected III-nitride light-emitting diodes

This work describes a small-signal microwave method for determining the differential carrier lifetime and transport effects in electrically injected InGaN/GaN light-emitting diodes (LEDs). By considering the carrier diffusion, capture, thermionic escape, and recombination, the rate equations are used to derive an equivalent small-signal electrical circuit for the LEDs, from which expressions for the input impedance and modulation response are obtained. The expressions are simultaneously fit to the experimental data for the input impedance and modulation response for nonpolar InGaN/GaN micro-LEDs on free-standing GaN substrates. The fittings are used to extract the transport related circuit parameters and differential carrier lifetimes. The dependence of the parameters on the device diameter and current density is reported. We also derive approximations for the modulation response under low and high injection levels and show that the transport of carriers affects the modulation response of the device, especially at low injection levels. The methods presented are relevant to the design of high-speed LEDs for visible-light communication.

Novel Scanning Technique for LED Dot-Matrix Display to Reduce Flicker

A Conventional progressive scanning method can be used for small and medium size LED (Light Emitting Diode) dot-matrix display board. However, this conventional scanning approach has some drawbacks such as flicker, lower luminous intensity, requirement of high speed LED driver, etc., specifically for large LED dot-matrix display. This paper brings forward the analysis of scanning method clarifying the limitations of conventional scanning approach with several critical design parameters. After that, a new scanning approach, congenial to both small and large has been proposed to get over these limitations. Eventually, simulation of the mathematical models of the two methods are expounded that points to the vital design parameters constraints along with the advantages of new method. The mathematical models and analyses give understanding of system design approach to the LED driver circuit designers to estimate various design parameters.

High-Speed LED Driver for ns-Pulse Switching of High-Current LEDs

This letter analyzes the intrinsic optical switching response of state-of-the-art high power, multiquantum wells, thin-film, and surface-textured infrared light emitting diodes (LEDs). For the switching time response of such devices, the theoretical basics are presented and their predictions are experimentally verified. In the second part of this letter, a novel, fast switching LED driving circuit is presented which enables nanosecond-pulse operation. The circuit reduces significantly the optical rise and fall time of LEDs from 10/15 ns, respectively, down to 2.6 ns at 2-A pulse current without compromising the peak output power.

High-Speed Light-Emitting Diodes Emitting at 500 nm With 463-MHz Modulation Bandwidth

Light-emitting diode (LED) is one of the most important light sources due to its low power consumption and long lifetime. In this letter, we present the high-frequency characteristics of GaN-based green LEDs with different aperture diameters. In order to get higher current density, we use ring-shaped electrode to confine the current injection. Unlike conventional LEDs, we only use its natural feature to get a high modulation bandwidth. The LEDs investigated have a peak emission wavelength of 500 nm. The highest optical 3-dB modulation bandwidth is ~463 MHz at 50 mA for the 500-nm green GaN-based LED with an aperture diameter of 75 μm. It is the highest bandwidth yet reported for the green GaN-based LEDs. The LED also exhibits a relatively high output power of ~1.6 mW at 50 mA as compared with other high-speed LEDs. Such the LEDs can be applied to plastic optical fiber and visible light communication in the future.

A High-Speed LED Driver That Sweeps Out the Remaining Carriers for Visible Light Communications

The modulation speed of light-emitting diodes (LEDs) must be increased to improve the speed, increase the bandwidth, and miniaturize the hardware of visible light communication (VLC) systems. The LED modulation speed is limited by the remaining carriers that remain in the depletion capacitance. In this paper, we evaluate the increase in optical transmission rate for an LED driver for the first time by sweeping out the remaining carriers in a GaN-based LED for VLC system. The driver is fabricated using discrete GaAs FETs and passive elements on a board. An optical transmission experiment is performed. The experimental results demonstrate that the driver increases the maximum error-free bit rate for a NRZ PRBS signal from 69 to 95 Mb/s (38% increase). However, the additional current path for carrier sweep-out increases the power dissipation of the driver. To reduce the power dissipation, we employ a CMOS inverter, in which a PMOS FET sweeps out the carriers. The driver is fabricated using a 0.18-μm CMOS IC process and then implemented in a package. The experimental results demonstrate that this design increased the maximum error-free bit rate from 27.5 to 51.8 Mb/s (88% increase). The maximum bit rate for the packaged driver is 20% higher than that of the driver measured on the wafer.

GaN-Based Miniaturized Cyan Light-Emitting Diodes on a Patterned Sapphire Substrate With Improved Fiber Coupling for Very High-Speed Plastic Optical Fiber Communication

We demonstrate the performance of a novel cyan light-emitting diode (LED) on a patterned sapphire (PS) substrate as a light source for plastic optical fiber (POF) communications with the central wavelength at 500 nm. To further enhance the external quantum efficiency (EQE) and output power of this miniaturized high-speed LED, a LED with a PS substrate is adopted. Furthermore, by greatly reducing the number of active <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{In}_{\rm x}\hbox{Ga}_{1 - {\rm x}}\hbox{N/GaN}$</tex></formula> multiple quantum wells (MQWs) to four and minimizing the device active area, we can achieve a record-high electrical-to-optical (E–O) bandwidth (as high as 400 MHz) among all the reported high-speed visible LEDs under a very small dc bias current (40 mA). The fiber coupling efficiency has been improved in 4 dB using lens with a 500- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu \hbox{m}$</tex></formula> diameter mounted on the LED chip. Thus, the maximum fiber-coupled power was <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex></formula> 2.67 dBm at the bias current of 40 mA. The 1.07-Gb/s data transmissions over a 50-m SI-POF fiber have been successfully demonstrated using this device at the bias current of 40 mA.

Very-High Temperature (200 $^{\circ}$C) and High-Speed Operation of Cascade GaN-Based Green Light- Emitting Diodes With an InGaN Insertion Layer

We demonstrate a novel type of linear cascade green light-emitting diode (LED) arrays as a light source for in-car or harsh environment plastic optical fiber (POF) communications. To further enhance its dynamic and static performance, an InGaN layer is inserted between an n-type GaN cladding layer and InGaN-GaN multiple quantum wells as an efficient current spreading layer. Compared with the control device without that layer, our three-LED cascade array demonstrates a smaller turn-on voltage (9.3 versus 11 V at 20 mA) and a larger output power (25.5 versus 22.5 mW at 180 mA), corresponding to an enhancement of around 31% in wall-plug efficiency. Furthermore, under the constant voltage bias of an in-car battery (12 V), our three-LED array exhibits an electrical-to-optical 3-dB bandwidth (100 versus 40 MHz) performance superior to that of the control device. Even under high-temperature dynamic operation, we observe that the InGaN insertion layer gives strong enhancement of modulation speed with negligible degradation of the output power, unlike the red resonant-cavity LEDs conventionally used for POF. We achieve 200-Mb/s error-free transmission at 200°C which is the highest operation temperature among all the reported high-speed LEDs.

Surface plasmon enhanced spontaneous emission rate of InGaN∕GaN quantum wells probed by time-resolved photoluminescence spectroscopy

We observed a 32-fold increase in the spontaneous emission rate of InGaN/GaN quantum well (QW) at 440 nm by employing surface plasmons (SPs) probed by time-resolved photoluminescence spectroscopy. We explore this remarkable enhancement of the emission rates and intensities resulting from the efficient energy transfer from electron-hole pair recombination in the QW to electron vibrations of SPs at the metal-coated surface of the semiconductor heterostructure. This QW-SP coupling is expected to lead to a new class of super bright and high-speed light-emitting diodes (LEDs) that offer realistic alternatives to conventional fluorescent tubes.

Wideband squeezing in photon number fluctuations from a high-speed light-emitting diode

Wideband, highly noise-suppressed squeezing was observed by using a high-speed, high-quantum-efficiency light-emitting diode. The squeezing bandwidth extended over 200 MHz. We also have investigated the dependence of the squeezing bandwidth on the pump-current at low temperature. The experimental result was compared with the theoretical predictions based on a unified model of the pump and recombination process and was well explained by the model at the thermionic emission limit.

GHz bandwidth GaAs light-emitting diodes

Double-heterostructure GaAs/GaAlAs light-emitting diodes (LEDs) have been fabricated with the emitter regions beryllium doped to 2×1019 and 7×1019 cm−3. The 7×1019 cm−3 doped emitters have an internal quantum efficiency of 10% and an optical modulation bandwidth of 1.7 GHz. The steady-state optical output power versus the input current shows an external efficiency of 2.5 μW/mA. The 2×1019 cm−3 emitters have internal quantum efficiencies as high as 80%, but a reduced cutoff frequency. The external quantum efficiency reaches 10 μW/mA. These high-speed LEDs are produced by reducing the radiative lifetime to 100–250 ps without significantly degrading internal quantum efficiency. The current results on heavily beryllium-doped LEDs exhibit, to the best of our knowledge, the highest external efficiencies to date for such high doping and efficiencies close to that observed for lower-doped LEDs.

Wide-band suppression of photon-number fluctuations in a high-speed light-emitting diode driven by a constant-current source

We demonstrate experimental results on the generation of sub-Poissonian photon fluxes emanating from an AlGaAs light-emitting diode, which manifest a wide-band (0–100 MHz) noise suppression below the standard quantum limit level despite low current density (∼38 A/cm2) operation at room temperature. The experimental noise power spectrum is well fitted in terms of the theoretical curve estimated with the quantum mechanical Langevin equations.

Observation of the collective Coulomb blockade effect in a constant-current-driven high-speed light-emitting diode

We have investigated the intensity squeezing bandwidth of sub-Poissonian light generated by a high-speed light-emitting diode. The squeezing bandwidth of a constant-current-driven light-emitting diode is proportional to a driving current in a lower-current regime and approaches a constant in a higher-current regime. This dependence on driving current is well explained by the contributions of the thermionic emission time and the radiative lifetime to the squeezing bandwidth.

Optical fiber transmission of multi-channel FM audio and video signals

An optical-fiber transmission system with a bandwidth of greater than 100 MHz for transmitting multichannel FM audio and video signals is described. This system for distributing FM audio and video signals on up to five channels to the respective receiver was constructed using a high-speed LED (light-emitting diode) and a wideband amplifier. The differential gain and differential phase were less than 2% and 3 degrees , respectively, and the SNR was 56 dB. The proposed system will be used for high-quality, high reliability distribution of multiple TV channels instead of the coaxial cable distribution systems. >