Trichromatic white-light sources based on light-emitting diodes ͑LEDs͒ offer a high luminous efficacy of radiation, a broad range of color temperatures and excellent color-rendering properties with color-rendering indices ͑CRIs͒ 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 CRIϾ 80 can be maintained, by adjusting the LED power ratio so that the chromaticity point is conserved.

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.

Anomalously high ideality factors of 2.0-7.0 have been reported in GaN-based visiblespectrum and ultyviolet light-emitting diodes (LEDs) [I]- [3]. These values are much higher than the expected values of 1.0 to 2.0 predicted by the Sah-Noyce-Shockley theory. High ideality factors (n >> 2.0) in GaN-based LEDs were attributed to tunneling [l], [2]. This attribution was motivated by the temperature-independent slopes of (log &versus-V plots. Ideality factors close to 2.0 were attributed to space charge region recombination [3], consistent with the Sah-NoyceShockley theory, due to temperature-dependent slopes of (log I)-versus-V plots. However, a comprehensive theory for the high ideality factors found experimentally in GaN p-n junctions has not been presented. In this article we propose a new explanation for the high ideality factors.Moderately doped unipolar heterojunctions with large band-discontinuities are generally rectifying. In the 111-V nitride material system, rectification is expected in unipolar heterojunctions as well. In fact, the high barriers inherent to the 111-V nitride material system and polarization effects, which can further increase heterojunction barrier heights, make such rectification likely. The two metal-semiconductor junctions of a diode ideally have ohmic characteristics. However, contacts can exhibit non-linear characteristics. This is especially the case for the metal contact to p-type GaN. In the limit of high contact resistance, the metalsemiconductor contact can be considered as a reverse-biased Schottky diode. A GaN-based p-n junction diode can then b e modeled by a series o f d iodes, the actual G aN p-n j unction diode, unipolar heterojnnction diodes, and a Schottky diode at the metavp-type GaN junction. Such a structure and the corresponding equivalent circuit are schematically shown in Fig. 1 . The circuit also shows resistors indicating the leaky nature Melalof the diodes. Assuming that these resistors are large, we can derive the current-voltage remicondudor characteristic of this system of junctions, which vz indicates that the extemally measured ideality factor is the sum of the ideality factors of the be generalized as n = xi ni , where ni represent the ideality factors of the p-n junction, the unipolar heterojunctions, and the metalsemiconductor junctions. This result elucidates Unipolar individual rectifying junctions. This result can fig. 1. Schemalic s t~a u r e of p n junction dlcde and equivalent circuil consisling of three diodes.that ideality factors >> 2.0 can be measured,The experimental study involves diodes fabricated from two different stmctures, a bulk GaN p-n junction structure and a p-n junction structure incorporating a p-type AlGaNiGaN superlattice. The superlattice structure is included to facilitate ohmic contact formation. I-V characteristics of the p-n junctions measured at room temperature are shown in Fig. 2 and Fig. 3. The I-Vcharacteristics display a linear dependence of In I on Y over several orders of magnitude of the injection current. The ide...