Inorganic semiconductor light‐emitting diodes (LEDs) have found widespread use in small‐area mobile displays, large‐area displays, signaling, signage, and general lighting. The entire visible spectrum can be covered by light‐emitting semiconductors: AlGaInP and AlGaInN compound semiconductors are capable of emission in the red‐to‐yellow and violet‐to‐green wavelength range, respectively. For white light sources based on LEDs, the most common approach is the combination of a blue LED chip with a yellow phosphor. White LEDs are currently used to replace incandescent and fluorescent sources. In the present review, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current flow patterns. LED structures providing high internal quantum efficiency, namely heterostructures and multiple quantum well structures, will be discussed. Techniques enhancing the external quantum efficiency will be reviewed, including chip shaping and surface roughening. Different approaches to white LEDs will be presented and figures‐of‐merit such as the color rendering index and luminous efficacy will be explained. Besides visible LEDs, the technical challenges in deep ultraviolet LEDs will be introduced. Finally, the packaging of low power and high power LED chips will be discussed.

About twenty years ago, in the autumn of 1996, the first white light‐emitting diodes (LEDs) were offered for sale. These then‐new devices ushered in a new era in lighting by displacing lower‐efficiency conventional light sources including Edison's venerable incandescent lamp as well as the Hg‐discharge‐based fluorescent lamp. We review the history of the conception, improvement, and commercialization of the white LED. Early models of white LEDs already exceeded the efficiency of low‐wattage incandescent lamps, and extraordinary progress has been made during the last 20 years. The review also includes a discussion of advances in blue LED chips, device architecture, light extraction, and phosphors. Finally, we offer a brief outlook on opportunities provided by smart LED technology.

Efficiency droop, i.e. the loss of efficiency at high operating current, afflicts nitride-based light-emitting diodes (LEDs). The droop phenomenon is currently the subject of intense research, as it retards the advancement of solid-state lighting which is just starting to supplant fluorescent as well as incandescent lighting. Although the technical community does not yet have consented to a single cause of droop, this article provides a summary of the present state of droop research, reviews currently discussed droop mechanisms, and presents a recently developed theoretical model for the efficiency droop. In the theoretical model, carrier leakage out of the active region caused by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, is discussed in detail. The model is in agreement with the droop's key behaviors not only for GaInN LEDs but also for AlGaInP LEDs.

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GaInN LEDs with a six‐layer graded‐ refractive‐index antireflection coating made entirely of indium tin oxide (ITO) are demonstrated to have 24.3 % higher light output than LEDs with dense ITO coating. The increased light‐output of the LEDs with graded‐refractive‐index antireflection coating is attributed to the virtual elimination of Fresnel reflection and surface roughening of low‐refractive index ITO.

We observed a significant enhancement in light output from GaN-based light-emitting diodes (LEDs) in which two-dimensional photonic crystal (PC) patterns were integrated. Two-dimensional square-lattice air-hole array patterns with a period that varied from 300 to 700 nm were generated by laser holography. Unlike the commonly utilized electron-beam lithographic technique, the holographic method can make patterns over a large area with high throughput. The resultant PC-LED devices with a pattern period of ∼500nm had more than double the output power, as measured from the top of the device. The experimental observations are qualitatively consistent with the results of three-dimensional finite-difference-time-domain simulation.

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BackgroundInterest in smartphone health apps has been increasing recently. However, we have little understanding of the cognitive and motivational factors that influence the extent of health-app use.ObjectiveThis study aimed to examine the effects of four cognitive factors—health consciousness, health information orientation, eHealth literacy, and health-app use efficacy—on the extent of health-app use. It also explored the influence of two different use patterns—information and information-behavior use of health apps—with regard to the relationships among the main study variables.MethodsWe collected and analyzed 765 surveys in South Korea. According to the results, there was a negligible gender difference: males (50.6%, 387/765) and females (49.4%, 378/765). All participants were adults whose ages ranged from 19 to 59. In order to test the proposed hypotheses, we used a path analysis as a specific form of structural equation modeling.ResultsThrough a path analysis, we discovered that individuals’ health consciousness had a direct effect on their use of health apps. However, unlike the initial expectations, the effects of health information orientation and eHealth literacy on health-app use were mediated by health-app use efficacy.ConclusionsThe results from the path analysis addressed a significant direct effect of health consciousness as well as strong mediating effects of health-app use efficacy. These findings contribute to widening our comprehension of the new, digital dimensions of health management, particularly those revolving around mobile technology.