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  Light emitting diode having a p-n junction doped with one or more luminescent activator ionsUSPTO Application #: 20050253162Title: Light emitting diode having a p-n junction doped with one or more luminescent activator ions Abstract: A light emitting diode (LED) includes a p-n junction containing luminescent activator ions. The visible emission from the activator ions preferably complementing the band edge emission of the LED in order to produce an overall white emission from the LED. In a preferred embodiment, the LED has double heterojunction structure having a semiconductor active layer between two confinement layers. The semiconductor active layer includes activator ions preferably selected from among Eu3+, Tb3+, Dy3+, Pr3+, Tm3+, and Mn2+. The electron-hole pairs trapped within the active layer sensitize the activator ions, causing the activator ions to emit light. (end of abstract) Agent: Osram Sylvania Inc - Danvers, MA, US Inventor: Kailash C. Mishra USPTO Applicaton #: 20050253162 - Class: 257102000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Incoherent Light Emitter Structure, With Particular Dopant Material (e.g., Zinc As Dopant In Gaas) The Patent Description & Claims data below is from USPTO Patent Application 20050253162. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. Provisional Application No. 60/601,382, filed Aug. 13, 2004. TECHNICAL FIELD [0002] The present invention is directed to light emitting diodes (LEDs), and more particularly to light emitting diodes that produce a white light emission. BACKGROUND OF THE INVENTION [0003] Solid state lighting uses several approaches to produce white light. The color mixing approach combines the red, green and blue emissions from three monochromatic LEDs to produce white light. Since each monochromatic LED light source can have high internal quantum efficiency, such a device could generate white light at a relatively high lumens per watt. However, the space needed for three LEDs can be burdensome and the packaging to place them together is cumbersome. The wavelength conversion approach uses ultraviolet (UV) emitting LEDs to generate UV light (generally from about 380 nm to about 420 nm) which is then converted to white light using a triblend phosphor system that is excited by the UV light. This is similar to the way white light is produced in known Hg-discharge fluorescent lamps. However, most conventional photoluminescent phosphors are optimized for excitation by the 254 nm radiation emitted by mercury discharges and not the longer wavelength UV radiation of LEDs. Additional work remains to develop a full range of phosphors for use with UV-emitting LEDs. The third approach is a hybrid in which a blue emission is provided by a GaInN LED and part of the blue emission is converted to a complementary emission by a phosphor. White light sources based on this design have been developed using a broad band emitter, in particular, cerium-activated yttrium aluminum garnet, Y.sub.3Al.sub.5O.sub.12:Ce.sup.3+ (YAG:Ce.sup.3+). This design avoids the large Stokes shift associated with a higher energy UV photon at 380 nm being converted to a visible photon. A similar design has been proposed which uses a second semiconducting layer, known as passive layer, that partly converts the emission from InGaN at 450 nm to a red photon near 620 nm with a InGaP alloy. This is essentially a double heterojunction structure with InGaN as the active layer and InGaP as the passive layer; InGaP acts as a phosphor. [0004] Phosphors in lighting devices present various engineering problems, such as lack of stability, degradation in the epoxy dome, coating uniformity, and scattering of visible light, all of which can be avoided if the lighting device does not include phosphors. As used, herein the term phosphor refers to photoluminescent materials, i.e., materials that convert photons of one energy to photons of a different energy. SUMMARY OF THE INVENTION [0005] An object of the present invention is to provide a novel phosphor-less light emitting diode (LED) that avoids the problems of the prior art. [0006] A further object of the present invention is to provide a novel light emitting diode that emits white light and does not include phosphors. [0007] A yet further object of the present invention is to provide a novel light emitting diode having a junction of p-type semiconductor material and an n-type semiconductor material wherein the junction is doped to contain one or more activator ions that produce a visible light emission when the light emitting diode is forward biased. Preferably, the LED includes a layer of a nitride-based semiconductor alloy of AlN, GaN, and InN having a band edge emission in the visible spectrum, and activator ions that have an emission spectra in the visible spectrum that complements the band edge emission so as to produce a white light emission from the LED. [0008] Another object of the present invention is to provide a novel LED that includes a double heterojunction having a semiconductor active layer between two confinement layers, wherein the semiconductor active layer contains one or more activator ions. Preferably, the semiconductor active layer includes a film of AlGaInN having activator ions selected from among Eu.sup.3+, Tb.sup.3+, Dy.sup.3+, Pr.sup.3+, Tm.sup.3+, and Mn.sup.2+. More preferably, the AlGaInN film has a band edge emission in the visible/UV spectrum and the activator ions have an emission spectra in the visible spectrum that complements/converts the band edge emission of the AlGaInN film so as to produce a white light emission from the LED. [0009] A still further object of the present invention is to provide a novel composition that includes AlGaInN having therein ions selected from the group consisting of Eu.sup.3+, Tb.sup.3+, Dy.sup.3+, Pr.sup.3+, Tm.sup.3+, and Mn.sup.2+. [0010] These and other objects and advantages of the invention will be apparent to those of skill in the art of the present invention after consideration of the following drawings and description of preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS [0011] FIG. 1 is a pictorial representation of a p-n junction of an LED according to this invention. [0012] FIG. 2 is a pictorial representation of a cross section of a preferred embodiment of the present invention. [0013] FIG. 3 is a stylized depiction of the movement of electron-hole pairs in the present invention. DETAILED DESCRIPTION OF THE INVENTION [0014] An embodiment of the present invention is a light emitting diode that contains a junction of a p-type semiconductor material and a n-type semiconductor material wherein the junction has been doped with one or more luminescent activator ions that are stimulated to emit a visible light emission by a non-radiative energy transfer from the electron-hole pairs. The p-n junction of an LED according to this invention is shown in FIG. 1. A layer of n-type semiconductor material 12 is joined to a layer of p-type semiconductor material 14. The p-n junction 10 is doped with activator ions 17. Either or both semiconductor materials may contain the activator ions as shown in FIG. 1. The activator ions may also be present only in the junction or throughout the bulk material. [0015] Preferably, the LED includes one or more layers of a semiconductor material selected from GaN, AlN, InN, or an alloy thereof, and suitably chosen activator ions to produce white light. In a preferred embodiment, the activator ions include one or more types of ions whose emission spectra, when superposed on the band edge (intrinsic) emission of the semiconductor material will lead to white light emission with high efficacy and color rendering index. As will be explained below, the activator ions in the p-n junction are excited by a non-radiative energy transfer from the electron-hole (e-h) pairs. In a more preferred embodiment, the layer is an active region of a double heterojunction structure in a LED that produces white light, such as shown in FIG. 2. The active region is between two carrier confinement (cladding) layers. [0016] Non-radiative energy transfer from the e-h pairs to the activator ions is engineered by matching the band edge emission of the semiconductor material with the excitation peaks of the activator ions. The band edge emission of the semiconductor materials depends on their composition, and the excitation peaks of the activator ions depend on the crystal fields at the activator sites. The latter may be obtained from observing the excitation spectra of activator ions in semiconductor materials, in particular wide-gap, nitride-based semiconductor materials. A semiconductor composition can then be developed so that its band edge emission overlaps with the appropriate excitation peaks near the band edge. [0017] In the case of the embodiment shown in FIG. 2, the active layer sandwiched between the two confinement layers converts the electron and holes injected into this region to white light. The underlying electronic processes are shown in FIG. 3. The filled circles represent electrons and the open circles represent holes. E.sub.c denotes the conduction band, E.sub.f the Fermi levels of the materials and E.sub.v the conduction band. Preferably, part of the emission of the stacked layers shown in FIG. 2 is the characteristic band edge emission (h.nu..sub.1) derived from a radiative re-combination of an e-h pair as shown by arrow 1 in FIG. 3. The other part of the emission is generated by activator ions sensitized by a non-radiative energy transfer from e-h pair recombination as shown by arrow 2. The transferred energy excites the activator to a first excited state E.sub.1 (arrow 4) which then may decay non-radiatively to a second excited state E.sub.1'. The intermediate excited state E.sub.1' then further decays to the ground state of the activator, Eg, by a radiative transition (arrow 3) and emits a visible photon, h.nu..sub.2. [0018] In another embodiment, the LED is engineered such that the band edge emission occurs in the blue region of the visible spectrum (preferably about 420 nm to about 490 nm) and the activators ions are chosen so that their visible emissions combine with the blue band edge emission to produce an overall white emission from the LED. Alternatively, the LED is engineered such that the band edge emission occurs at an ultraviolet wavelength and the activator ions are selected to produce a combined white emission. In this case, different activator ions may produce a red emission, a green emission, and a blue emission respectively. While some of the emitted UV radiation from the band edge emission may be subsequently converted to visible light by the addition of a phosphor layer, it is preferred that the materials be engineered such that the energy from the e-h pair re-combinations is more effectively transferred to the activator ions and the number of radiative re-combinations is minimized. Continue reading... Full patent description for Light emitting diode having a p-n junction doped with one or more luminescent activator ions Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Light emitting diode having a p-n junction doped with one or more luminescent activator ions patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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