More than a century after the introduction of incandescent lighting and half a century after the introduction of fluorescent lighting, solid-state light sources are revolutionizing an increasing number of applications. Whereas the efficiency of conventional incandescent and fluorescent lights is limited by fundamental factors that cannot be overcome, the efficiency of solid-state sources is limited only by human creativity and imagination. The high efficiency of solid-state sources already provides energy savings and environmental benefits in a number of applications. However, solid-state sources also offer controllability of their spectral power distribution, spatial distribution, color temperature, temporal modulation, and polarization properties. Such ‘‘smart’’ light sources can adjust to specific environments and requirements, a property that could result in tremendous benefits in lighting, automobiles, transportation, communication, imaging, agriculture, and medicine.

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Solid-State Light Sources Getting Smart

Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

A lighting device comprising first and second groups of solid state light emitters, which emit light having peak wavelength in ranges of from 430 nm to 480 nm, and first and second groups of lumiphors which emit light having dominant wavelength in the range of from 555 nm to 585 nm. In some embodiments, if current is supplied to a power line, a combination of (1) light exiting the lighting device which was emitted by the first group of emitters, and (2) light exiting the lighting device which was emitted by the first group of lumiphors would have a correlated color temperature which differs by at least 50 K from a correlated color temperature which would be emitted by a combination of (3) light exiting the lighting device which was emitted by the second group of emitters, and (4) light exiting the lighting device which was emitted by the second group of lumiphors.

Lighting device and lighting method

Visible Light Communication (VLC) is an emerging field in Optical Wireless Communication (OWC) which utilizes the superior modulation bandwidth of Light Emitting Diodes (LEDs) to transmit data. In modern day communication systems, the most popular frequency band is Radio Frequency (RF) mainly due to little interference and good coverage. However, the rapidly dwindling RF spectrum along with increasing wireless network traffic has substantiated the need for greater bandwidth and spectral relief. By combining illumination and communication, VLC provides ubiquitous communication while addressing the shortfalls and limitations of RF communication. This paper provides a comprehensive survey on VLC with an emphasis on challenges faced in indoor applications over the period 1979–2014. VLC is compared with infrared (IR) and RF systems and the necessity for using this beneficial technology in communication systems is justified. The advantages of LEDs compared to traditional lighting technologies are discussed and comparison is done between different types of LEDs currently available. Modulation schemes and dimming techniques for indoor VLC are discussed in detail. Methods needed to improve VLC system performance such as filtering, equalization, compensation, and beamforming are also presented. The recent progress made by various research groups in this field is discussed along with the possible applications of this technology. Finally, the limitations of VLC as well as the probable future directions are presented.

LED Based Indoor Visible Light Communications: State of the Art

Light emitting diodes reliability review

Diode ideality factors much higher than the expected values of 1.0 to 2.0 have been reported in GaN-based p-n junctions. It is shown that moderately doped unipolar heterojunctions as well as metal-semiconductor junctions, in particular the metal contact to p-type GaN, can increase the ideality factor to values greater than 2.0. A relation is derived for the effective ideality factor by taking into account all junctions of the diode structure. Diodes fabricated from a bulk GaN p-n junction and a p-n junction structure with a p-type AlGaN/GaN superlattice display ideality factors of 6.9 and 4.0, respectively. These results are consistent with the theoretical model and the fact that p-type AlGaN/GaN superlattices facilitate the formation of low-resistance ohmic contacts.

Experimental analysis and theoretical model for anomalously high ideality factors (n≫2.0) in AlGaN/GaN p-n junction diodes

Trichromatic white-light sources based on light-emitting diodes (LEDs) offer a high luminous efficacy of radiation, a broad range of color temperatures and excellent color-rendering properties with color-rendering indices (CRIs) exceeding 85. An analysis of the luminous efficacy and CRI of a trichromatic light source is performed for a very broad range of wavelength combinations. The peak emission wavelength, spectral width, and the output power of LEDs strongly depend on temperature and the dependencies for red, green, and blue LEDs are established. A detailed analysis of the temperature dependence of trichromatic white LED sources reveals that the luminous efficacy decreases, the color temperature increases, the CRI decreases and the chromaticity point shifts towards the blue as the junction temperature increases. A high CRI>80 can be maintained, by adjusting the LED power ratio so that the chromaticity point is conserved.

Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes

The carrier transport and recombination dynamics of monolithic InGaN/GaN light-emitting p-n junction structures with two active regions are investigated. Room-temperature and low-temperature photoluminescence and room-temperature electroluminescence measurements show two emission bands originating from the two active regions. In electroluminescence, the intensity ratio of the two emission bands is independent of injection current. In contrast, the intensity ratio depends strongly on the excitation intensity in photoluminescence measurements. The dependency of the emission on excitation is discussed and attributed to carrier transport between the two active regions and to the different carrier injection dynamics in photoluminescence and electroluminescence. The luminous efficacy of a Gaussian dichromatic white-light source is calculated assuming a line broadening ranging from 2kT to 10kT. Luminous efficacies ranging from 380 to 440 lm/W are obtained for broadened dichromatic sources.

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Carrier dynamics in nitride-based light-emitting p-n junction diodes with two active regions emitting at different wavelengths

Synchrotron radiation photoemission spectroscopy reveals enhanced oxygen incorporation in AlxGa1−xN as the Al mole fraction increases. It is shown that the increased oxygen donor incorporation can result in a conductivity-type change from p-type to n-type in Mg-doped AlxGa1−xN. Consistent with the conductivity-type change, epitaxial Al0.20Ga0.80N films exhibit n-type conductivity despite heavy Mg doping. The p-type conductivity of bulk AlxGa1−xN with a high Al mole fraction can be improved by employing AlxGa1−xN/AlyGa1−yN superlattices (SLs). At 300 K, Mg-doped Al0.17Ga0.83N/Al0.36Ga0.64N SLs (average Al mole fraction of 23%) exhibit strong p-type conductivity with a specific resistance of 4.6 Ω cm, a hole mobility of 18.8 cm2/Vs, and an acceptor activation energy of 195 meV.

P-type conductivity in bulk AlxGa1-xN and AlxGa1-xN/AlyGa1-yN superlattices with average Al mole fraction > 20%

A technology for low-resistance ohmic contacts to III nitrides is presented. The contacts employ polarization-induced electric fields in strained cap layers grown on lattice-mismatched III-nitride buffer layers. With appropriate choice of the cap layer, the electric field in the cap layer reduces the thickness of the tunnel barrier at the metal contact/semiconductor interface. Design rules for polarization-enhanced contacts are presented giving guidance for composition and thickness of the cap layer for different III-nitride buffer layers. Experimental results for ohmic contacts with p-type InGaN and GaN cap layers are markedly different from samples without a polarized cap layer thus confirming the effectiveness of polarization-enhanced ohmic contacts.

Y. Li

Papers

Experimental analysis and theoretical model for anomalously high ideality factors (n≫2.0) in AlGaN/GaN p-n junction diodes

Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes

Carrier dynamics in nitride-based light-emitting p-n junction diodes with two active regions emitting at different wavelengths

P-type conductivity in bulk AlxGa1-xN and AlxGa1-xN/AlyGa1-yN superlattices with average Al mole fraction > 20%

Ohmic contact technology in III nitrides using polarization effects of cap layers