Lecture_15

Lecture_15 - Light Emitting Diodes (LED) Summary...

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1 • Heterostructure diodes • Heterojunction Band Lineup • Single and Double Heterojunctions Light Emitting Diodes (LED) Summary •LED s • Applications • Recombination Mechanisms • Direct vs Indirect Bandgap • CIE Eye Chart and Visible and White Light LEDs • Output Coupling, Packaging and Efficiency Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) • Modulation Speed Light-emitting diodes (LEDs) use pn junctions to inject electrons and holes into same region of semiconductor so that they may recombine and emit light by spontaneous emission Term light-emitting diode (LED) is reserved for spontaneous 5. Light-emitting diodes (LEDs) emission device only (i.e., not lasers) Applications include • Displays • Indicator and traffic lights • High-brightness lamps • Opto-isolators (optocouplers) (e.g., for voltage isolated triggering of triacs and thyristors) Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) • Infrared wireless communication (e.g., remote controls (TV), free-space data links (Palm Pilot, Laptop IR port) LEDs are the highest volume optoelectronic component, due to large area stadium and building displays. Worldwide annual market now greater than 1 billion LEDs/yr.
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2 Large Area, Full Color Displays Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Times Square, NYC LED Traffic Lights Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010)
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3 AlN 6.0 40 (.21 μ m) Technologically Accessible LED Materials AlSb α SiC CdTe InN GaN ZnS MgSe CdS CdSe ZnTe ZnSe GaAs GaP AlAs InP Si (.41 μ m) (.62 μ m) 4.0 3.0 2.0 1.0 dgap energy (eV) (1.24 μ m) Visible Spectrum (.31 μ m) Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) InAs Ge BULK SLE (Ge) InSb Lattice Constant Å) 3.0 3.2 3.4 5.4 5.6 5.8 6.0 6.2 6.4 Ban Response time of LED vs incandescent Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010)
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4 Direct Bandgap Recombination Direct bandgap recombination is by far the most efficient process. Recombination is spontaneous. Emis sio n h ν The electrons and holes can be generated by electrical injection for LEDs (electroluminescence), photo excitation (photoluminescence) or e- beam (cathodoluminescence). The latter 2 are extensively used Prof. J. S. Harris 7 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Relaxation in conduction & valence bands occurs in ~10 -12 sec compared to e-h recombination lifetime, ~10 -8 sec, hence emission is at the bandgap energy and has a thermal distribution for materials characterization. Early LEDs (GaP) used indirect transitions because wide direct bandgap materials were not available. Homojunction vs. Heterojunction LEDs Conventional P/N Junction Heterojunction LED Carriers spread out over ~ ± L D beyond junction Prof. J. S. Harris 8 EE243. Semiconductor Optoelectronic Devices (Winter 2010) D with decreasing density Carriers confined to region W << L D , local concentrations increase for same current, recombination rate R = Bnp
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5 I-V and L-I Characteristics Prof. J. S. Harris 9 EE243. Semiconductor Optoelectronic Devices (Winter 2010)
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This note was uploaded on 06/05/2010 for the course EE 243 taught by Professor Harris,j during the Winter '10 term at Stanford.

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Lecture_15 - Light Emitting Diodes (LED) Summary...

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