Project Lighting - April 2008 - (Page 12)

FeAtURe ensure particle conservation when a net photon recycling recombination rate is fed back into the continuity equations. This enables the important physical effects in white-LED structures to be captured in a fully coupled and self-consistent electro-opto-thermal simulation. simuLation of a 3-D aton whitE LED the bottom electrode covers the entire base. The LED structure contains two InGaN/ GaN QWs designed to have peak spontaneous emissions at around 460 nm. The substrate is silicon carbide (SiC) and the buffer layers are aluminum gallium nitride (AlGaN) Figure 2 shows the current spreading and temperature profiles, both of which are not directly measurable and add key physical insight. Good current spreading is essential to reducing photon absorption by the QWs. On the other hand, current spreading also affects the way that heat production is distributed via the Joule heating and recombination processes, and it becomes a design issue to simultaneously minimize both absorption and temperature rise. The output spectra of both devices are shown in Figure 3, which shows that blue emissions from the InGaN/GaN QWs Contents Viewpoint Selecting HBLED Drivers for Lighting Apps Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs Lighting News FIg 3 the outPut SPectra of Both DeViceS ShoW that Blue emiSSionS from the ingan/gan QWS Were SPectrally conVerteD SucceSSfully. To illustrate the techniques discussed in the previous section, let’s look at the simulation results of a 3-D ATON white LED using the Sentaurus Device simulator from Synopsys. Figure 1 shows two simplified 3-D ATON structures that are encased in luminescent material. Figure 1(a) is about 50% covered with phosphor, while Fig. 1(b) is totally covered. The top electrode is a cross pattern to facilitate current spreading without over-blocking the topsurface light extraction, and were spectrally converted successfully. This particular simulation used a simple phosphor with an absorption peak at 460 nm and emission peak at 560 nm. The measured output power is a product of the extraction efficiency and the total power. LED extraction efficiency is generally low, implying that most of the light is trapped within the device. However, a properly designed ATON structure can exceed 50% extraction efficiency, providing a strong motivation for simulation studies. The trapped light undergoes spectral evolution as it traverses active QWs and luminescent regions multiple times in succession. Home Page Product Training Module Online Ordering Suppliers Catalog 12 | Project lighting | aPr 08 http://digikey.com/ http://ad.doubleclick.net/clk;198717884;26055153;t http://ad.doubleclick.net/clk;198717989;26055167;e http://ad.doubleclick.net/clk;198717988;26055168;e http://ad.doubleclick.net/clk;198717986;26055170;v http://ad.doubleclick.net/clk;198717985;26055171;v

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Project Lighting - April 2008 Contents Viewpoint About Project Lighting Sponsor Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs LED Lighting Needs Smarts to be Competitive Lighting News Enter to Win an iPhone Refernce Materials Lighting Quick Links

Project Lighting - April 2008

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