Search

Characterization and structural design on monolithic stacked InGaN light-emitting diode (LED) are investigated numerically in an attempt to pursue multicolor light emission and wide spectral width. Crucial physical properties such as the energy band configurations, carrier distributions, and interband transitions are analyzed in detail, which are also utilized as an aid to justify the desired characteristics of the LED structures under study. The compositions of multi-quantum wells, as well as the thicknesses of quantum barriers in each unit stacked LED are appropriately adjusted to simplify the tandem LED structure and optimize the overall emission spectra. Upon optimization, a monolithic tunnel-junction LED with an emission spectral width of approximately 110 nm, full width at half maximum, is demonstrated with only three unit stacked LEDs and two tunnel junctions.

In deep-ultraviolet (DUV) light-emitting diodes (LEDs), it is difficult to obtain both efficient carrier confinement and high light extraction, which are quite sensitive to optical polarization and other physical parameters. In this paper, characteristics of DUV LEDs with various n-AlGaN layers and quantum barriers (QBs), and various widths of quantum wells (QWs) are investigated. Specifically, the capability of carrier confinement and properties of optical polarization are analyzed in detail. The simulation results show that LED structure with Al0.64Ga0.36N QBs, n-Al0.7Ga0.3N layer, and 4-nm-thick QWs, which has a peak emission wavelength of 284.5 nm at 60 mA, exhibits high internal quantum efficiency of 25% and high degree of optical polarization of 0.874.

In this study, the effect of exciton-blocking layer (EBL), employed between the electron-transporting layer (ETL) and the undoped host spacer layer, on the characteristics of fluorescent/phosphorescent multilayer white organic lightemitting diode (OLED) is investigated numerically with the APSYS (Advanced Physical Model of Semiconductor Devices) simulation program. The validation of simulation model is confirmed by the good agreement of photoelectric characteristics between the results obtained numerically and those obtained experimentally. Simulation results suggest that singlet excitons and triplet excitons are generated at both hole-transporting layer (HTL)/emitting layer (EML) and EML/ETL interfaces, where electrons and holes accumulate and recombine, with certain thickness of host spacer layers employed on both sides of EML of white OLED structure. Further study shows that a better choice for the trade-off between color stability and electroluminescence (EL) efficiency can be achieved by properly adjusting the number of EBLs. An optimized performance is achieved if two pairs of EBLs are used.

The design of monolithic multicolor tunnel-junction (TJ) light-emitting diodes (LEDs) are investigated numerically. This paper primarily aims to design and study monolithic TJ LED devices that possess high luminous efficiency and wide color gamut using a simple fabrication process. First, the characteristics of green and blue single LEDs are explored. Nonuniform carrier distribution inside quantum wells (QWs), which is due to the deep QWs and severe polarization effect, is observed for both single LEDs under study. Based on structural analysis of the single LEDs, streamline dual-color and broadband TJ LED structures, in which the blue unit LED has two pairs of 3-nm-thick QWs and the green unit LED has 3-nm-thick single QW, are designed and investigated. Specifically, monolithic dual-color blue/green (459/530 nm) TJ LED is proposed and analyzed using effective n+-GaN/i-In0.2 Ga0.8N/p+-GaN TJ. Furthermore, broadband TJ LED with simulated full width at half maximum of around 100 nm is achieved.

Characteristics of deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated. DUV LEDs possess severe electron current leakage due to insufficient carrier confinement of the active region. Simply increasing the Al composition of quantum barriers (QBs) or electron-blocking layer (EBL) to enlarge the relevant potential barrier height would arise unexpected detrimental effects, such as extra polarization effect or more obstruction for hole injection. In this paper, band-engineered composition-graded QBs and EBL are proposed to resolve this issue. Simulation results show that, with appropriate designs, the leakage current of DUV LEDs could be effectively diminished with just slight side effects.

The polarization effect in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated, which is critical for the development of DUV LEDs, because the basal material, epitaxial structure, and polarization characteristics are very distinct to those of the well-developed (In)GaN-based near-ultraviolet and visible light emitters. In this paper, the influence of the polarization effect in multi-quantum well active region and p-type layers on the characteristics of DUV LEDs with Ga-face or N-face polarization is explored. Simulation results show that the severe band bending of the p-type layers induced by the polarization field markedly affects the optical and electrical performance. A band-engineered DUV LED structure with compositional grading electron-blocking layer and p-interlayer is proposed to enhance the electron confinement and hole injection with the mitigation of polarization effect. The device performance of the proposed LED structure with Ga-face or N-face polarization is comparable with that of the DUV LED without polarization.

Green tunnel-junction (TJ) light-emitting diodes (LEDs) are investigated theoretically in this letter. The influence of Auger recombination on the optical performance of green single LED and TJ LED is explored. Simulation results show that non-uniform carrier distribution and Auger recombination are critical issues influencing the performance of conventional green LED. The TJ LED can be operated at relatively low current density that possesses suppressed Auger recombination and high quantum efficiency when compared with its single LED counterpart at the same level of output power. Simulated wall-plug efficiency of the TJ LEDs with two and three unit LEDs is increased from 5.67% to 10.51% and 12.54%, respectively, in comparison with the single LED when the output power is 21 mW.

Auger recombination of dual-wavelength emission in monolithic InGaN light-emitting diodes (LEDs) is numerically investigated. Simulation results show that effective suppression of Auger recombination plays an important role toward the realization of dual-wavelength emission in InGaN LEDs. With appropriate design, the carriers in active region can be spatially and spectrally balanced and thereby effective dual-wavelength emission can be achieved.

The optical performance and relevant physical properties of near-ultraviolet (NUV) GaN-based light-emitting diodes (LEDs) are investigated. Specifically, the influence of traditional AlGaN bulk electron blocking layer (EBL) and AlGaN/GaN superlattice (SL) EBL with various thicknesses of AlGaN layers on NUV LEDs is explored. It is indicated from the band diagrams, electrostatic field profile, electron reflecting and hole transmitting spectra, and carrier concentrations profile that the use of a thin AlGaN layer of AlGaN/GaN SL EBL is beneficial to the electron confinement and hole injection in the active region, which results in the high internal quantum efficiency and low efficiency droop at high injection current. Moreover, the experimental results show that replacing the traditional AlGaN bulk EBL with the AlGaN/GaN SL EBL can markedly improve the optical performance. When compared with the NUV LED with traditional AlGaN bulk EBL, the output power of the NUV LED with the proposed AlGaN/GaN SL EBL increases from 13.5 to 48.7 mW at 100 mA.

The effect of polarization-matched AlInGaN/AlGaN superlattice (SL) electron blocking layer (EBL) on the physical characteristics of blue InGaN light-emitting diodes (LEDs) is investigated numerically. Simulation results show that the optical performance of the LEDs with polarization-matched SL EBL can be markedly improved due to the effectively suppressed polarization effect, enhanced hole injection efficiency, and reduced electron overflow. Comparing to the LEDs with conventional AlGaN EBL, an improvement of 53% in light output power is achieved for the proposed LED structure.

Carrier transport characteristics of AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) are theoretically investigated. Simulation results reveal that hole transport/injection may be severely obstructed by the large potential barrier at the p- electron-blocking layer/p-GaN interface. Under this circumstance, the slope efficiency degrades and electron leakage increases accordingly. By inserting the AlGaN interlayers to form band-engineered staircase p-region, both the transport/injection of holes and $I$ – $V$ characteristic are improved. Moreover, the LED characteristics become less sensitive to the polarization field, which is beneficial for obtaining high LED performance with the LED of high crystalline quality.

The band-engineered structure design for electron-blocking layer (EBL) and hole-blocking layer (HBL) in Al x Ga1– x N-based ultraviolet light-emitting diodes (UV LEDs) is performed and analyzed theoretically. Simulation results show that the severe polarization effect is efficiently mitigated and the downward-bended band profile of the EBL is improved when the EBL is designed with a graded-composition and multiquantum barrier (GMQB) structure. As a result, the capabilities of both electron confinement and hole injection, and also the light output power are promoted. On the contrary, for the HBL, the design of composition graded and/or multiquantum barrier structures reduces the effective potential barrier height for holes in the valence band and, consequently, causes a considerable hole overflow. The UV LED, thus, exhibits superior optical performance when the LED structure is simultaneously designed with a GMQB EBL and a bulk HBL.

Effects of the number of quantum wells (QWs) and Shockley–Read–Hall (SRH) recombination in deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated theoretically. Simulation results show that, for DUV LEDs with high crystalline quality, light output power increases with an increasing number of QWs. As for the DUV LEDs with poor crystalline quality, light output power may decrease with an increasing number of QWs due to the deteriorated SRH recombination. The injection current density is also an important factor regarding the impact of the number of QWs. When operated at low current density, for the DUV LED with poor crystalline quality, light output power may decrease with an increasing number of QWs.

The illumination efficiency and the relevant physical mechanism of near-ultraviolet (NUV) AlGaN-based light-emitting diodes (LEDs) are investigated numerically. In particular, the interrelationship of quantum well (QW) thickness and degree of polarization, and the relevant influence on the light output power of NUV LEDs are systematically studied. Simulation results indicate that the use of wider QWs with less polarization field is beneficial in suppressing the Auger recombination by reducing the carrier density. However, the structure with wider QWs suffers from severer spatial separation of electron and hole wave functions within the QW, which is more sensitive to the degree of polarization in its optical performance. To resolve this problem, the quaternary Al0.1In0.05Ga0.85N is proposed as the material of quantum barriers in wide QWs, in which the polarization mismatch between the QW and the barrier is reduced and the relevant quantum-confined Stark effect is relieved consequently.

The optical and electrical characteristics of the double-heterostructure (DH) blue InGaN light-emitting diodes (LEDs) with composition-graded interlayers are studied numerically. Specifically, the detailed physical mechanisms and influences of grading layer thickness on the LED performance are explored systematically. Simulation results reveal that besides the advantages of mitigating lattice relaxation in real epitaxy, the employment of thick composition-graded interlayers can benefit from the enhanced spatial separation of electron–hole wavefunctions, suppressed Auger recombination, and reduced polarization effect in DH active region. The illumination efficiency and electrical characteristics are markedly improved when the thick grading layers are employed.

Undergraduate science, technology, engineering, and mathematics (STEM) curriculum emphasize project-based learning (PBL) with peer assessment/on-line peer assessment (PA/OPA). Many studies have stressed that students did not improve over two rounds in PBL with OPA studies and PBL with PA have to adopt team mutual cooperation to reap effective learning. This study proposed an innovative approach that incorporate OPA with TQM as a macro-level instructional tool to guide students in teacher's directing of collaborative project development as well as seeking continuous improvement to elevate Project-Based Learning Performance in a STEM course. The effects of OPA with TQM were examined through an experiment with PBL performances hypotheses. A total of 63 junior students in an university of Taiwan voluntarily participated in this study and a quasi-experimental approach with a two-group design was adopted. The results revealed that the team members using the OPA with TQM approach tended to have higher design skill performance, better cohesive teamwork and creative problem solving attitude. Thus, the proposed approach facilitated team members to collaborate for seeking continuous improvement. However, no significant difference was reported on the influence of enhancing students’ design concept. Implication and suggestions for educators to promote the PBL with OPA and TQM were also provided in the study. Key words: collaborative learning, online peer assessment, project-based learning, STEM course, Total Quality Management.

In this paper, the electrical polarization effects on the optical polarization properties of Al 0.35 Ga 0.65 N multiple quantum well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) by varying the Al content in AlGaN quantum well (QW) barriers are qualitatively analyzed. Numerical simulation results show that the varied potential barrier height and different polarization-induced electric field resulting from varying the Al content in AlGaN QW barriers have great influence on the transverse electric (TE) and transverse magnetic (TM) emission intensities. Both the TE- and TM-polarized spontaneous emissions from the Al 0.35 Ga 0.65 N QWs decrease with the increase of the Al content in AlGaN QW barriers, which is attributed to the poor carrier confinement resulting from the more serious band bending effect and the reduced effective potential barrier height in the conduction band, especially, the degree of optical polarization increases with the increase of the Al content in AlGaN QW barriers due to the rearrangement of the valence subbands near the Γ-point of the Brillouin zone. It is consequently concluded that the TE and TM spontaneous emissions and the degree of optical polarization are closely related to the Al content in AlGaN QW barriers of the Al 0.35 Ga 0.65 N MQW UV LEDs.

The improvement in efficiency of nitride-based light-emitting diodes by the implementation of a vertically stacked tandem structure is investigated. The electrical and optical characteristics of an LED with a tunnel junction inserted between two active regions are modeled, and the wall-plug efficiency gain of the tandem LED is shown to start at 4.2% at low output powers (27.6 mW), with increasing efficiency gains with increased output power due to the alleviation of efficiency droop. The TLED concept further enables optimization of device structure, allowing removal of electron blocking layer, and optimization of number of quantum wells for improvement in efficiency.

Recently, nanoporous (NP) GaN has emerged as a promising photonic material in the III-N family. Due to its attractive properties, such as its large refractive index contrast and perfect lattice matching with GaN, as well as its good electrical conductivity, photonic components and devices involving NP GaN have been successfully demonstrated. However, further development of high-performance NP GaN based electrically injected devices, such as vertical-cavity surface-emitting lasers (VCSELs) and edge emitting lasers, requires efficient heat dissipation. Therefore, in this paper, we study thermal conductivity (TC) of NP GaN, especially when incorporated into a practical distributed Bragg reflector (DBR) in a VCSEL device. Through an effective medium model, we study the theoretical effect of NP GaN morphological properties over its TC. We then experimentally measure the TC of NP GaN, with different porosities and pore wall thicknesses, which shows a high agreement with the theoretical model. We also fabricate actual NP GaN DBRs and study the large tunability and interdependence among their TC (1–24 W/m K), refractive index (0.1–1.0), and electrical conductivity (100–2000 S/m) compared to other conventional DBRs. Finally, we perform a finite-element simulation of the heat dissipation within NP GaN-VCSELs, revealing their superior thermal dissipation compared to dielectric DBR based VCSELs. In this regard, this study lays the foundation for nanoscale thermal engineering of NP GaN optoelectronic and photonic devices and paves the way for their successful commercialization.

A four-junction InGaN-based multijunction solar cell structure is proposed theoretically. The simulation results show that, with the use of appropriately designed compositional grading layers, the performance of InGaN-based multijunction solar cell can be maintained without the cost in performance degradation caused by the polarization-induced electric field and the potential barriers resulting from the heterointerfaces. After the optimization in thicknesses for current matching, a high conversion efficiency of 46.45% can be achieved under 1000-sun AM1.5D illumination, in which the short-circuit current density, open-circuit voltage, and fill factor are 12.2×103 mA/cm2, 4.18 V, and 0.77, respectively. The simulation results suggest that, in addition to the detrimental effects caused by the built-in electric polarization and potential barriers, the issue of crystalline quality is another critical factor influencing the performance of multijunction solar cells.

The use of polarization charges in nitride based tunnel junctions enables a wide range of design approaches to increase the tunneling current to magnitudes usable in high efficiency GaN-based devices, including enhanced multijunction solar cells, optoelectronic and electronic devices. Here, an integrated computational model is used to explore and design the dopant concentration profile and implement the hybrid use of both AlGaN and InGaN layers to systematically optimize the configuration of polarization charges in the structure. The proposed tunnel junction structure, with indium composition and doping density compatible for insertion into a typical Ga-polar InGaN multiple-quantum well light-emitting diode structure, allows a high tunneling efficiency under reverse bias condition, achieving a resistivity of 7.8 × 10-3 Ω·cm2.

The effect of using polarization-reversed AlInGaN-based quantum well active region in ultraviolet light-emitting diode is numerically investigated. By employing ${\rm Al}_{0.54}{\rm In}_{0.26}{\rm Ga}_{0.20}{\rm N}$ and ${\rm Al}_{0.83}{\rm In}_{0.17}{\rm N}$ as barrier and electron blocking layers, which ably reverse the direction of the polarization in ${\rm Al}_{0.005}{\rm In}_{0.02}{\rm Ga}_{0.975}{\rm N}$ quantum well and provide sufficient potential barrier height to confine electrons in the conduction band, simulation results show that the energy band profile of the ${\rm Al}_{0.005}{\rm In}_{0.02}{\rm Ga}_{0.975}{\rm N}$ quantum well is tilted up, resulting in better carrier confinement and more uniform distribution of carriers in the quantum well. Moreover, the overlap between electron and hole wave functions is increased and the Auger recombination is suppressed effectively, which in turn improves the radiative recombination efficiency.

Blue InGaN light-emitting diodes (LEDs) with composition-graded barriers and composition-garded electron-blocking layer (EBL) are studied theoretically. Simulation results show that the employment of composition-graded barriers and compositon-graded EBL simultaneously can benefit from the enhanced hole-injection efficiency and more uniform carrier distribution among the quantum wells. Moreover, the optical and electrical properties of the InGaN blue LEDs are markedly promoted. The graded barriers are investigated systematically in an attempt to obtain the optimal situation.

The characteristics of green InGaN light-emitting diodes (LEDs) are investigated. The results obtained numerically show that the optical performance of green InGaN LEDs is deteriorated severely by Auger recombination, which becomes more severe at high current injection levels. Through employing In0.2Ga0.8N as barrier material, the radiative recombination efficiency is enhanced and the Auger recombination is suppressed efficiently because of the better electron-hole spatial overlap and the more uniform carrier distribution in the active region.

The photovoltaic characteristics of Ga-face GaN/InGaN p-i-n solar cells are investigated numerically. The severe polarization and barrier effects induced by the GaN/InGaN hetero-interfaces are demonstrated to be detrimental for the carrier collection. The conversion efficiency could be degraded to be out of application when the degree of polarization and/or indium composition are high. To efficiently eliminate both critical issues, the solar cell structure with appropriate band engineering is introduced. In the proposed structure, the photovoltaic characteristics not only show high-grade performance but also become insensitive to the degree of polarization, even in the situation of high indium composition.

Optical performance of blue InGaN light-emitting diodes (LEDs) with multiple GaN-InGaN barriers is investigated. The energy band diagrams, light-current performance curves, and internal quantum efficiency are studied numerically. The simulation results show that the InGaN LED has markedly improved the optical performance when the first conventional GaN barrier is replaced by the GaN-InGaN-GaN barrier, in which the indium composition is less than 5%, with other GaN barriers remain unchanged. The improved performance is due to the enhanced injection efficiency of holes and relatively uniform distribution of electrons and holes.

Advantages of near-ultraviolet light-emitting diodes with polarization-matched InGaN/AlGaInN multi-quantum wells (QWs) are investigated numerically. Simulation results show that, the polarization-matched structure possesses improved efficiency droop and higher output power under high current injection due to better overlap of electron-hole wavefunctions and slighter Auger recombination. The optical performance of the polarization-matched structure can benefit from the enhanced capability of carrier confinement with the employment of larger bandgap electron-blocking layer and wider QWs.

The photovoltaic properties of (0001) face GaN/InGaN p-i-n solar cell are studied numerically. The simulation results show that the detrimental effects of hetero-interfaces and polarization charges will seriously degrade the solar cell performance, especially when the indium composition is high. If these effects are not eliminated or diminished, the photovoltaic properties would not be good enough for practical applications even if a high-quality crystal could be obtained. For this purpose, the step-graded interlayers between the GaN-InGaN interfaces are introduced in order to overcome both aforementioned critical effects. The impacts of the thickness and p-type doping concentration of the step-graded interlayers are also investigated in detail.

First-principles calculations are applied to investigate the effect of biaxial strain on the band gap of wurtzite Al x Ga1−x N. The band gap and band gap bowing parameter increase with compressive strain and decrease with tensile strain. The strain-induced changes in the band gap of Al x Ga1−x N are linear in the strain range of about −1% to 1% while the linearity is invalid out of the range. The linear coefficient B(x), characterizing the relationship between the band gap and the biaxial stress, with a quadratic form is obtained. The value of the band gap bowing parameter decreases from 1.0 eV for −2% strain to 0.91 eV for unstrained and to 0.67 eV for 2% strain.

In response to the growing industrial demand for light-emitting diode (LED) design professionals, based on industry-university collaboration in Taiwan, this paper develops a novel instructional approach: a simulation-based learning course with peer assessment to develop students' professional skills in LED design as required by industry as well as to enhance cognition and metacognition in students in higher education. The simulation-based learning course enables students to understand the influences on LED performance of the variation of different parameters and to seek the best design through comparing the effectiveness of different combinations of parameters. The evaluation results of pre- and post-test knowledge maps and a photonics scoreboard indicate that this project-based instruction may help students understand the operating principles of LEDs and enhance their skill in LED design. The Constructivist Project-based Learning Environment Survey is adopted to demonstrate that the proposed project-based learning environment is beneficial in cultivating student inquiry learning, reflective thinking, teamwork, and skills in creative problem solving.

The concept of a step-like quantum well is proposed with the purpose to reduce the influence of electrostatic field resulting from the piezoelectric effect on the optical performance of blue InGaN light-emitting diodes. Particularly, the optical properties of the LED structures with the In0.23Ga0.77N single quantum well, In0.20Ga0.80N/In0.26Ga0.74N step-like quantum well, and In0.26Ga0.74N/In0.20Ga0.80N step-like quantum well are numerically investigated in detail. Simulation results show that the In0.20Ga0.80N/In0.26Ga0.74N step-like-quantum-well LED structure has the best optical performance in virtue of the improvement in spatial overlap of electrons and holes in the quantum well.

Optical properties of the InGaN/(In)GaN light-emitting diodes (LEDs) with varied barrier materials and quantum-well (QW) numbers are studied numerically. The simulation results show that, for the LEDs with GaN barriers, the single quantum-well (SQW) structure has the best optical performance. However, for the LEDs with InGaN barriers, the 5-QW structure has less serious efficiency droop and higher output power at high current than the SQW one, which makes it a better structure for high-power LEDs. The physical mechanisms of the aforementioned phenomena can be well explained by uniformity of carrier distribution, band-filling effect, and overlap between the electron and hole wavefunctions.

The optical characteristics of ultraviolet-violet InGaN laser diodes with different numbers of quantum wells under normal and reversed polarizations are numerically investigated. For the laser structures under normal polarization, the lowest threshold current is obtained when the number of quantum wells is two in the spectral range of 380–408 nm. For the laser structures under reversed polarization, the single quantum-well laser structure possesses the lowest threshold current. The simulation results suggest that the physical origin for these phenomena is caused by the sufficiently suppressed electron and hole leakage currents when the laser diode is under reversed polarization.

P-type doping in the last barrier is proposed to improve the efficiency droop of the blue InGaN light-emitting diodes (LEDs). The light-current curves, energy band diagrams, carrier concentrations, radiative recombination efficiency, and internal quantum efficiency of the blue LEDs under study are investigated. The simulation results show that the efficiency droop is significantly improved when the last undoped GaN barrier in a typical blue LED is replaced by a p-type GaN barrier. The simulation results suggest that the improvement in efficiency droop is mainly due to the decrease of electron current leakage and increase of hole injection efficiency.

The impact of quantum barriers (QBs) and electron-blocking layers (EBL) on the output performance of deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated systematically. Specific LED structures with strain-compensated and p-staircase-interlayer configurations are proposed and explored in detail. Simulation results show that Al compositions of both QBs and EBL are critical to the capability of carrier confinement of the active region. A DUV LED structure with excellent output performance can be obtained provided that the QBs and EBL are properly designed via band engineering. Furthermore, LED structures without EBL are also investigated. If the DUV LED has well-designed deeper quantum wells, an excellent output performance can be maintained, even when the LED structure is without EBL.

The physical mechanisms leading to the effi- ciency droop of InGaN/GaN light-emitting diodes (LEDs) are theoretically investigated. We first discuss the effect of Auger recombination loss on efficiency droop by taking dif- ferent Auger coefficients into account. It is found that the Auger recombination process plays a significant nonradia- tive part for carriers at typical LED operation currents when the Auger coefficient is on the order of 10 −30 cm 6 s −1 . Fur- thermore, the InGaN/GaN multiple-quantum-well (MQW) LEDs with varied indium compositions in InGaN quan- tum wells are studied to analyze the wavelength-dependent efficiency droop. The simulation results show that the wavelength-dependent efficiency droop is caused by sev- eral different effects including non-uniform carrier distrib- ution, electron overflow, built-in electrostatic field induced by spontaneous and piezoelectric polarization, and Auger recombination loss. These internal physical mechanisms are the critical factors resulting in the wavelength-dependent ef- ficiency droop in InGaN/GaN MQW LEDs.