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Dispositivos Optoelectrnicos

Dispositivos Semicoductores - DIEC/UNS

TiposFotodetectores: Detectan cambios de energa fotnica, transformndolos en energa elctricaCeldas solares:Transforman energa lumnica en elctricaLED / Diodos Laser: Transforman energa elctrica en energa lumnicaOptoacopladores


4.2x1014 Hz6x1014 Hz


Fotodiodos p-nJuntura p-n que permite penetracin de luz en la vecindad de la unin metalrgicaAbsorcin de luz crea pares hueco-electrn


Fotodiodos p-nPortadores generados en la zona de vaciamiento son arrastrados por el E. Tasa de Generacin ptica: GL

Si W menor que la long. Del diodo, IL es independiente de la tensin aplicada y proporcional a la intensidad de luz


Respuesta espectralVariacin de IL con la longitud de onda de luz incidenteFotones generan pares h-e solo si Eph>EG. Para Si, EG = 1.12 eV, y max=1.1m


Respuesta espectralVariacin de IL con la longitud de onda de luz incidentePotencia lumnica es cte. Por ende, hay menos fotones de mayor energa (

Fotodiodos p-nRespuesta FrecuencialVelocidad de respuesta ante cambios de la luz incidenteFotodiodo p-n tiene capacidades limitadasPortadores minoritarios deben difundir hasta la zona de vaciamiento: proceso lentoMxima velocidad: decenas de Mhz


Fotodiodo p-i-nTop semiconductor thin, to minimize absorptioni region is depletedMost of the photocurrent is generated in the depletion region


Fotodiodo p-i-ni region can be tailored for specific wavelengthsFrequency response is much better than Si diodes (most current is generated in the dep. Region)Freq. response about GHz

Aplicaciones en fibra ptica requieren > 1.1m, y menores bandgap (In Ga As)


Fotodiodos de AvalanchaFotodiodos operados cerca del punto de ruptura inversaGanancia en la generacin de portadoresMultiplicacin de avalancha amplifica los portadores provocados por la luz


FotodiodosFactores ImportantesVelocidad de respuestaEficiencia cuntica: relacin entre fotones y pares h-eLinealidad Uniformidad espacialRuido oscuro (dark noise)


Hamamatsu model S2386 silicon photodiode


Toshiba TPS850


Celdas SolaresHonda dream, the winning car in the 1996 World Solar Challenge. The custom made cells for the car are greater than 20% efficient. (Photograph PVSRC)


Celdas SolaresJunturas p-n de gran areaDesign to minimize energy losses

Voc: open circuit voltageIsc: short circuit currentPmax = Im Vm


Celdas solaresMedida de desempeo: eficiencia de conversin

Output from the sunArea beyond G is power lost cause it cannot be absorbed20% lost in Si, 35% in GaAsThe Eph>Eg energy adds kinetic energy (heat): 40% in Si, 30% in GaAs


Celdas Solares


Celdas SolaresTop contact coverage of the cell surface can be minimised (although this may result in increased series resistance). Anti-reflection coatings on the top surface of the cell. Reflection reduced by surface texturing. The solar cell can be made thicker to increase absorptionThe optical path length may be increased by a combination of surface texturing and light trapping.

http://www.udel.edu/igert/pvcdrom/index.html


Celdas SolaresArea: 22cm2 Efficiency: 23.5% Voc: 703 mV Isc: 914mA Jsc: 41.3mA Vmp: 600mV FF: 0.81 Imp: 868 mA


Light Emitting Diode (LED)Definition: a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction

Courtesy of Wikipedia http://en.wikipedia.org/wiki/LED


PrincipleForward bias injects majority carriers in opposite regions where they recombineIn indirect semiconductor (Si) recombination produces heatIn direct semiconductors (GaAs) recombination occurs from band to band and emits photons


PrincipleThree requirements to produce visible LEDDirect semiconductor (Si, Ge, GaP, AlAs,SiC, excluded)Bandgap between 1.77eV and 3.10eV (GaAs bandgap too small)Able to allow pn junction diodes Need for alloys


LEDsExternal quantum efficiency: is due to reflections in the Interface air-semiconductor = Photo power out/electrical power


Light Emitting Diode (LED)

LED v.s. Incandescent (Edisons lightbulb) and Flourescent BulbsMuch longer life span (105 - 106 hrs v.s. 103 / 104 hrs)Suitable for applications that are subject to frequent on-off cyclingEfficiency: better than incandescent but currently worse than flourescent bulbsSource: US Department of Energy http://www.netl.doe.gov/ssl/faqs.htm


LEDs for lighting12 Volt MR16 LED spotlight bulbsEach LED spotlight has 20 ultrabright 15,000mcd LEDs producing a similar amount of light to a 20W halogen bulb1 Watt of power!


LED EfficiencyInternal Quantum Efficiency (int)Definition: ratio of the number of electrons flowing in the external circuit to the number of photons produced within the device Has been improved up to 80%

External Quantum EfficiencyDefinition: The percentage of photons that can be extracted to the ambient.Typically 1% ~ 10%Limiting factor of LED efficiencyImprovement techniques: dome-shaped package, textured surface, photonic crystal, Source: Lecture Note of Optoelectronic Devices (by Sheng-fu Horng, Dept. of Electrical Engrg, NTHU, Hsinchu, Taiwan)


LED


OptoacopladoresAislacin elctrica entre dos circuitos. Comunicacin pticaTpicamente se utilizan haces de luz entre el rojo al infrarrojo


Caractersticas importantes:Tensin de aislacinBuena relacin de transferenciaBaja capacidad de acoplamientoImnunidad a interferencias


ReferenciasRobert F. Pierret, Semiconductor Device Fundamentals, Addison Wesley, 1996. Captulos 6, 9, 14.Stanley G. Burns, Paul R. Bond, Principles of Electronic Circuits, PWS Publishing Company, 1997. Captulo 3.


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