The market entrance of thinfilm based, substrate-less LEDs has stimulated the field of high-brightness LEDs. One of the most prominent advantages of thinfilm LEDs is the possibility to achieve a high light extraction efficiency independently of the chip area. This feature is particularly suitable for large-area, high-flux devices. In this paper, we report on high-power LEDs with a chip-area of 1 mm² for red and infrared emission. Mounted in packages with improved heat sinking and operated at a continuous-wave (cw) current of 800mA, the devices achieve an output power of 440 mW both for red (λ = 615 nm) and infrared (λ = 850 nm) wavelengths. Together with Osram's ThinGaN chips, a family of devices is available with very similar emission characteristics, performance and geometry, which allow the assembly of powerful light engines for a number of advanced applications.

High-power Light Emitting Diodes (LEDs) were introduced in the market in 1998 by Lumileds. High-power LEDs have unique properties, very different from conventional light sources, creating never before possible solutions available to lighting designers. In this paper we will give an update on these devices and discuss the main benefits of LEDs compared to more traditional light sources. We will show that the benefits of high-power LEDs also relate to the organization of light they emit. Comparisons of more and less organized light sources as well as applications for highpower LEDs will be discussed.

A commercial optical wireless LAN system has a base station attached to a ceiling, and a number of LEDs therein broadcast optical beams to movable stations below. The movable stations always communicate with the base station when they are in a service area. The LEDs are arranged so that their combined optical beams effectively irradiate the service area. However, studies concerning such designs have not been reported and thus LEDs in base stations have been arranged empirically. The minimum optical intensity in the service area is a key parameter in designing systems because it determines the optical power margin of the system and should be increased as much as possible. We examined the optimum arrangement of LEDs with which the minimum optical intensity in the service area is maximized, assuming the total number and power of LEDs are fixed. Referring to the commercial systems, we assumed LEDs are aligned on coaxial circles in a rotationally symmetric scheme and thus a disk-shaped service area is implemented. We assumed LEDs have the same beam profile, but each group aligned on a different circle has its own number of elements and inclination angle with respect to the vertical axis. We compared numerical results with our experimental results. This study will contribute to designing the base stations of optical wireless LANs.

The market for high power LED solutions is expanding rapidly with the significant efficiency increase of LED chips. This revolutionizes the optoelectronics market, enabling engineers to use LEDs for general lighting applications as well as medical, specialty lighting and automotive solutions, where previously less efficient technologies had to be used. In order to reap the benefits of high power LEDs, however, special requirements concerning thermal management and optomechanical layout have to be met. We will show concepts of thermal management and technological solutions for the choice of different materials and assembly. Furthermore, we will discuss the gain and challenges of chip-on-board solutions using intelligent optical design in specific applications.

Light emitting diodes (LED) are becoming more and more significant in interior and exterior automotive lighting. The long service life, energy and space savings, shock and vibration resistance and new styling potential are the main advantages of using LEDs in automotive applications. Today, most central high mounted stop lamps use LEDs. In rear combination lamps the number of LEDs in amber and red is increasing rapidly. This year, a first rear combination lamp using LEDs for all functionalities including the back-up lamp function was realized. In addition, first signal functions in headlamps using white High Power LEDs were launched onto the market. The long service life characteristic makes LEDs especially predestined for the DRL function combined with the position/parking light. Exterior automotive applications, including requirements and performance will be discussed and an outlook will be given on future scenarios.

The use of coloured LEDs as light sources in digital projectors depends on an optimal combination of optical, electrical and thermal parameters to meet the performance and cost targets needed to enable these products to compete in the marketplace. This paper describes the system design methodology for a digital micromirror display (DMD) based optical engine using LEDs as the light source, starting at the basic physical and geometrical parameters of the DMD and other optical elements through characterization of the LEDs to optimizing the system performance by determining optimal driving conditions. The main challenge in using LEDs is the luminous flux density, which is just at the threshold of acceptance in projection systems and thus only a fully optimized optical system with a uniformly bright set of LEDs can be used. As a result of this work we have developed two applications: a compact pocket projector and a rear projection television.

The distance between the LED and the surface of the target organ is about 4-5 cm, and we think this will become the "ultimate super-localized LED lighting". In an experiment with swine, we placed a LED module at the tip of the retractor. When compared to endoscopic lighting, this method illuminated the entire thoracic cavity more brightly. Since the light is emitted from the cylinder-shaped camera component, the light is unidirectional, and the shadows from the surgical instruments are moved to the side of the incision. Retractor LED lights provided enough light in the thoracic cavity. we believe that "medical white LEDs" can contribute in clinical settings as a light source for performing safe operations with bright surgical fields in the near future. Also, we use our LEDs for new real business. In the summer of 2004, LED lighting was world first used in the 1200 year-old Gion Festival for the first time in history as "a lighting device that does not destroy cultural assets by light heat". And the next is "Lighting at the "Diva status at diva gate" and the "Thousand Armed Avalokiteshwara in innermost sanctuary in the main hall" at Kiyomizudera in Kyoto". It was a great success, and we were invited back in the spring of 2005 and for future applications. We think this is the first real application of LEDs as an outdoor lighting device. The number of people who visit Kiyomizudera is 4000,000 annually, and LEDs were adopted to illuminate the diva gate.

Trichromatic LED backlights render higher color gamut and panel transmittance to the liquid crystal displays (LCDs) than yellow phosphor-converted white LED backlights can possibly do at their best. In realization, however, several technical challenges arise, such as color mixing, minimizing the total number of chips, and maintaining the color balance. We designed and demonstrated a backlight unit for 2.2 inch TFT LCD using two RGB 3-chip LEDs to assess the feasibility and the technical hurdles to overcome. The average brightness of the backlight is 2509cd/m² at the input power of 200mW. The power efficiency is lower than but comparable to commercially available white LED backlights. The color gamut of the LC panel is increased from 53% to 78% when its conventional white LED backlight is replaced by the trichromatic LED backlight. Panel transmittance is expected to be enhanced as well by about 8%. The ambient temperature change was found to be the most significant cause of the color shift of the trichromatic LED backlight. The forward bias voltage can be used in the feedback, since it changes linearly with temperature.

A novel anode structure comprising a nanocomposite and metal, has enabled highly efficient and stable superluminescent organic light-emitting diodes (SOLED). For C545T singlet green emitter, SOLED can reach 33 cd/A at 1000 cd/m², doubled the efficiency as comparing to conventional devices with ITO/CuPc structure. More importantly, the SOLED can still hold 24 cd/A at 50000 cd/m², indicating a highly efficient hole injection capability at ultra-high brightness. In addition, the simulated electronluminescent spectra with angle dependence, agree with experimental results. It is expected that SOLED might find wide applications, not only in display but in general lighting or ultra-high brightness application, by replacing the problematic ITO anode.

This article discusses possible solutions to limitations in light extraction efficiency of light-emitting diodes (LEDs) using new types of triple-layer omni-directional reflectors (ODRs). The ODRs have lower mirror losses than metal reflectors and distributed Bragg reflectors (DBRs). High-reflectivity ODRs have been incorporated into AlGaInP LEDs and GaInN LEDs. It is shown that the ODR significantly increases light extraction from ODR-LEDs as compared to reference LEDs employing a DBR or metal reflector. Other examples of innovative concepts to be presented include novel materials with unprecedented low-refractive index, which further enhance the optical properties of ODRs.

A 1550 nm polarization-insensitive superluminescent diode (SLED) has been demonstrated with >100 mW continuous wave (CW) output power. More than 40 nm FWHM bandwidth of a Gaussian-shape spectrum, and less than 0.7 dB peak-to-peak ripple were also achieved with a polarization dependence of the output power spectrum that is less than 1 dB over 1530-1610 nm.

Vertical GaN based Light Emitting Diodes on metal alloy substrate were realized and characterized for solid state lighting application. An efficiency of more than 70 lumens/watt was achieved. In addition, these LEDs exhibit many advantages over those on sapphire under extreme operation conditions for general lighting application.

We demonstrate a compact system, incorporating a 32-element linear array of ultraviolet (290 nm and 340 nm) light-emitting diodes (LEDs) and a multi-anode photomultiplier tube, to the in-flight fluorescence detection of aerosolized particles, here containing the biological molecules tryptophan and NADH. This system illustrates substantial advances in the growth and fabrication of new semiconductor UV light emitting devices and an evolution in packaging details for LEDs tailored to the bio-aerosol warning problem.

High-brightness light-emitting diodes (LED) based on AlGaInP combines the possibility to achieve high efficiency with the flexibility of tuning the emission wavelength over a large range of the visible spectrum. For optimizing the device characteristics an accurate determination of the electronic properties, like e. g. the voltage drop across the semiconductor layer sequence, is desirable. We demonstrate the potential of Kelvin Force Microscopy for quantitative investigations of the voltage drop across the heterostructure layers of an operating AlGaInP LED. The surface potential was measured for external biases between -2.0 V and +1.86 V. By subtracting the zero bias result the voltage drop could be extracted quantitatively. In the low voltage regime, most of the voltage drops in the active layer. Above +1.5 V an additional voltage drop occurs on the p-side of the device, i. e. outside the active layer sequence, which reduces the efficiency of the LED. By comparing experimental data with simulations we will discuss possible mechanisms of these findings.

Experimental results on a new type of light-emitting device, the light-emitting triode (LET), are presented. The LET is a three-terminal p-n junction device that accelerates carriers in the lateral direction, i.e. parallel to the p-n junction plane, by means of an electric field between two anodes. The lateral field provides additional energy to carriers thereby allowing them to overcome barriers and increasing the carrier injection efficiency into the active region. LETs were fabricated using a ultraviolet LED structure that has an AlGaN/GaN superlattice in the p-type confinement region for high-conductivity 2 dimensional hole gas. LET mesa structures were obtained by standard photolithographic patterning followed by chemically-assisted ion-beam etching using Cl₂ and Ar to expose the n-type cladding layer. The n-type contact was fabricated by electron-beam evaporation of Ti/Al/Ni/Au. Ni/Au (50/50 Å) metallization was deposited for both anodes, Anode 1 and Anode 2, and subsequently annealed at 500 ^oC in an O₂ ambient. It is shown that both the current between Anode 1 and the cathode, and the light-output power increase with increasing negative bias to the Anode 2. This is consistent with the expectation that a negative bias to the second anode allows carriers to acquire a high kinetic energy thereby enabling them to overcome the barrier for holes, resulting in high injection efficiency into the active region that lies beyond the barrier.

Indium nitride (InN), because of its recently discovered narrow band-gap and superior electron transport properties, has emerged as a potentially important semiconductor for use in near-infrared (NIR) optoelectronics, solar cells, and high-speed electronics. The current barrier for extensive fundamental studies and widespread applications of InN is mostly related to the growth difficulty of high-quality InN heteroepitaxial films. We have recently demonstrated that high-quality InN/AlN heterostructures can be formed on Si(111) due to the existence of "magic" ratios between the lattice constants of comprising material pairs: 2:1 (Si:Si₃N₄), 5:4 (AlN/Si), and 8:9 (InN:AlN). This new route of lattice matching allows the formation of commensurate interface with a common two-dimensional superlattice. For InN growth on AlN with nitrogen polarity, we found that the pseudomorphic to commensurate lattice transition occurs within the first monolayer of growth, resulting in an abrupt heterojunction at the atomic scale. At room temperature, the as-grown InN films on Si exhibit strong NIR photoluminescence with the peak energy at ~0.65 eV (wavelength at ~1.9 μm). Combined with the optical absorption measurements performed by transmission and spectroscopic ellipsometry, we confirmed that InN is a direct narrow band-gap semiconductor. Therefore, InN is a very ideal material for applications in NIR optoelectronics and solar cells, if other technological barriers (e.g., p-type doping) can also be overcome. In addition to the measurements of fundamental optical properties, a large valence band offset (3.10 eV) of type-I band alignment was also determined by photoelectron spectroscopy for the InN/AlN 8:9 commensurate heterojunction. The large band offsets and the strong polarization effects make the InN/AlN heterostructures very promising for applications in heterojunction field-effect transistors.

The ultraviolet AlGaInN light-emitting diode under study is grown on a c-face sapphire substrate by low-pressure horizontal-flow metalorganic chemical vapor deposition (MOCVD). With increasing input current from 10 to 100 mA, the main peak of the emission wavelength shifts from 368 to 372 nm. The room-temperature output power is 0.8 mW at 20 mA. Under continuous-wave operation, an output power of 4 mW is achieved at a driving current of 125 mA. The simulation program, advanced physical model of semiconductor devices (APSYS), is used to fit in our experimental results in order to obtain an optimized structure. The device performance affected by piezoelectric and thermal effects is studied via drift-diffusion model for carrier transport, optical gain and loss. The optical performance of the ultraviolet light-emitting diodes with different numbers of quantum wells at various temperatures is numerically investigated. Preliminary simulated results indicate that when the number of quantum wells is 5 to 7, better output performance is obtained. To raise the internal efficiency and radiative recombination rate, a current blocking layer SiO₂ is used to guide and confine current flows through active region.

We have theoretically investigated the optimized quantum well structure for the ultra-deep ultraviolet (UV) AlGaN light emitting diodes (LEDs) with the consideration of band structure deformation caused by polarization effect. In this paper, we further employ an asymmetric active region to reduce the polarization field in the well-barrier interface and modify the band structure to enhance the power efficiency of the AlGaN LED. By increasing the thickness of p-side barrier from 5 nm to 15 nm, the deformation slope of energy band in the well region is reduced due to the reduction of polarization field, which is caused by the large polarization charges in the interface of p-side barrier and carrier blocking layer. Accordingly, the hole concentration is increased and the carrier distributions are more uniform caused by the less-tilted energy band in the well. Therefore, a higher recombination rate and a higher output power can be obtained. Moreover, the power efficiency of AlGaN LED is barely related to the n-side barrier thickness due to the less polarization field. However, a thinner n-side barrier is preferred to enhance the current spreading. Therefore, an asymmetric QW with a thinner n-side barrier and a thicker p-side barrier is a better choice to enhance the power efficiency for the deep UV AlGaN LED.

Deep UV LEDs emitting at on 280 nm with powers as high as 1 mW at 20 mA have been reported recently. These devices have mesa size of 100 μm x 100 μm to avoid current crowding due to the high Al-composition of the AlxGa_1-xN buffer layers. Small mesa size results in pump current density of 200 A/cm² for a device current of 20 mA. Small area of p-contact also leads to higher operating voltage and higher thermal impedance for the flip-chip packaged device. These factors limit the device lifetime for 50 % power reduction to only a few hundred hours. From temperature and bias dependent power degradation measurements we found the output power to decay with two characteristic time constants indicating two degradation mechanisms. The faster time constant is bias dependent and temperature independent. The slower time constant varies exponentially with junction temperature having the activation energy of 0.27 eV at 200 A/cm² pump current density. For the devices with high thermal impedance this degradation mechanism controls the long term power degradation. To increase the device area for better reliability we used the interconnected micro-pixel device design with 10x10 array of 22 μm in diameter pixels. This design allowed for the four-fold increase of the junction area and thereby led to improved reliability performance with the operation life-time for 50 % power reduction of about 1000 hours. In this paper we will present the details of the reliability measurements and use the experimental results to determine possible degradation mechanisms.

The simple and very popular linear position sensor comprises LED (or laser diode) as light source, rectangular photodiode and cover fixed on mechanical moving part which position is measured. Position of the cover defines the light energy that reaches the photodiode surface. The energy is transformed in sensor output voltage proportional to cover displacement by condition that photodiode illumination is absolutely uniform. In practice the linear sensor is volume limited therefore the sensor illumination uniformity and sensor's scale factor accordingly are not constant values. Non-compensated variable sensor scale factor causes control system destabilization where sensor provides cover linear position measurement. This paper proposes the mathematical model which involves approximated characteristics of the LED irradiance intensity. We apply this approximation for calculating voltage versus displacement characteristic of linear sensor consisting of two overlapped photodiodes. Using this characteristic we provided its linearization and stability of control closed position system, where non-linear sensor was main destabilized factor.

Organic light-emitting diodes (OLEDs) have been extensively developed in the past few years. The OLED displays have advantages over other displays, such as CRT, LCD, and PDP in thickness, weight, brightness, response time, viewing angle, contrast, driving power, flexibility, and capability of self-emission. In this work, the optical and electronic properties of multilayer OLED devices are numerically studied with an APSYS (Advanced Physical Model of Semiconductor Devices) simulation program. Specifically, the emission and absorption spectra of the Alq₃, DCM, PBD, and SA light-emitting layers, and energy band diagrams, electron-hole recombination rates, and current-voltage characteristics of the simulated OLED devices, typically with a multilayer structure of metal/Alq₃/EML/TPD/ITO constructed by Lim et al., are investigated and compared to the experimental results. The physical models utilized in this work are similar to those presented by Ruhstaller et al. and Hoffmann et al. The simulated results indicate that the emission spectra of the Alq₃, DCM, PBD, and SA light-emitting layers obtained in this study are in good agreement with those obtained experimentally by Zugang et al. Optimization of the optical and electronic performance of the multilayer OLED devices are attempted. In order to further promote the research results, the whole numerical simulation process for optimizing the design of OLED devices has been applied to a project-based course of OLED device design to enhance the students' skills in photonics device design at the Graduate Institute of Photonics of National Changhua University of Education in Taiwan. In the meantime, the effectiveness of the course has been proved by various assessments. The application of the results is a useful point of reference for the research on photonics device design and engineering education. Therefore, it proffers a synthetic effect between innovation and practical application.

The effects of linear birefringence (LB) upon Bulk Glass Optical Current Transformers (BGOCTs) with return-back optical path designs, such as the Orthoconjugate Reflection (OCR)-typed, the Direct Reflection (DR)-typed and the Roof-prism Reflection (RPR)-typed BGOCT design, are theoretically analyzed and compared with that of the BGOCT with a single-loop optical path in this paper. The results show that the return-back dual-loop current sensing designs with conventional signal processing scheme of "-/+" cannot eliminate the harmful effects of the LB thoroughly, if suitable signal processing schemes which can separate the LB from Faraday effect are not used.