OptoElectronic DevicesMade By: Gaurav Kothari Mentor: Prof. A Das Gupta Indian Institute Of Technology Kanpur Outline • Radiometry And Photometry • Radiation Source: Controllable Uncontrollable • Photovoltaic Devices: Photodiodes & Its Characteristics Photoresistors , its Specifications & its Applications Phototransistors & its Uses Solid State Lighting Solar Cells: Photovoltaic Cells ,Modules & Systems Operations Photocurrent & Quantum efficiency Structure & Its Functioning Solar Cell Compared to Battery Optoelectronics Devices 2 Outline contd… • Electroluminescence based device: LEDs ,its Operations & Applications Solid State Lighting Laser: • • • • Absorption & Emission Elements Of Laser Optics Feedback & Tuning Method In A Laser Laser Diode Optical Sensors Optocouplers & Its Applications Summary References Optoelectronics Devices 3 Optoelectronics Devices 4 • Un-Controllable : Little Control over performance.Radiation Sources Controllable Un-Controllable • Controllable : Can exercise full control by selecting operational parameters. Ex. Optoelectronics Devices 5 . ExCRTs & Electroluminescent Displays. Incandescent bulbs & Fluorescent Panels.LEDs. or silicon dioxide for protection & serve as anti reflection coating. • Active area coated with either silicon nitride.Photodiode Construction • p-type layer is formed by thermal diffusion or ion implantation of doping material (usually Boron). • Metal contacts are applied to the front & back surface. • Thickness of coating optimised for particular irradiation wavelengths Optoelectronics Devices 6 . • Front surface acts as anode while back surface as cathode. Photovoltaic Devices … Optoelectronics Devices 7 . Photovoltaic Devices … Optoelectronics Devices 8 . Photovoltaic Devices … Optoelectronics Devices 9 . Photovoltaic Devices … Optoelectronics Devices 10 . Photovoltaic Effect • Photons In Electrons Out Optoelectronics Devices 11 . Solar Cells Structure & Its Functioning • • Convert Light into Electrical Energy basically just P-N junction photodiodes with a very large light-sensitive area. which causes the cell to convert light directly into electrical energy.type semiconductor). top junction layer (made of N. core of the device. occurs in the three energyconversion layers. this is the absorber layer (the P-N junction). Optoelectronics Devices 12 . back junction layer (made of P-type semiconductor). • The photovoltaic effect. so it is always made of metal. Optoelectronics Devices 13 . but are generally not transparent to light.to minimise light reflection from the top surface. layer on the face of the cell where light enters is generally present in some grid pattern. back layer must be a very good electrical conductor.Structure & Its Functioning contd… • There are two additional layers : these are the electrical contact layers. ARC . The cell is covered with a thin layer of dielectric material . Grids are good electrical conductors.the anti-reflection coating. Photovoltaic array integrated with components for charge regulation and storage. Optoelectronics Devices 14 . Module are connected in series in strings & then in parallel into an array.Modules & Systems a) b) c) d) Photovoltaic cell showing surface contact patterns.Photovoltaic Cells . Cells are usually connected in series to give a standard dc voltage of 12V. So . Since power can be computed via this equation: • P=I*V • Then with one term at zero these conditions (V = Voc / I = 0. V = 0 / I = Isc ) also represent zero power. the "maximum power point") Optoelectronics Devices 15 . Operation • Solar cells are characterized by a maximum Open Circuit Voltage (Voc) at zero output current and a Short Circuit Current (Isc) at zero output voltage.a combination of less than maximum current and voltage can be found that maximizes the power produced (called. Jm. of photons with energy in the range E and E+dE q= electronic charge FF= JmVm/JscVoc Where FF= Fill Factor is the ratio of a solar (photovoltaic) cell's actual power to its power if both current and voltage were at their maxima. Vmare current density & voltage at maximum power limit. bs=no.Photocurrent & Quantum Efficiency • • Jsc=q ∫bs(E)QE(E) dE where Jsc =photocurrent density QE=probability that incident photon of energy E will deliver one electron to an external circuit. • η =JscVocFF/Ps Where η=power efficiency Ps=incident light power density Optoelectronics Devices 16 . Solar Cell compared to Battery Battery Solar Cell • • • Delivers constant current for any given illumination level. Essentially a current source. • • Delivers constant emf at different levels of current drain Essentially a voltage source. Voltage largely determined by the resistance of loads. Optoelectronics Devices 17 . • Telecommunication systems: radio transceivers on mountain tops. Domestic Supply. seismic recording. • Electric Power Generation In Space: the space array also have a high power-to-weight ratio. in combination with a dependable battery. etc. • Professional Applications: • Ocean Navigation Aids: many lighthouses and most buoys are now powered by solar cells. • Remote monitoring and control: scientific research stations.Lighting etc.Applications Of Solar Cell • Rural Electrification: Water Pumping. is provided reliably by a small PV module. weather stations. or telephone boxes can be solar powered. Optoelectronics Devices 18 . use very little power which. • Solar cells are very reliable and require little maintenance. Optoelectronics Devices 19 .Advantages Of Solar Cells • There are no fuel costs or fuel supply problems. • The equipment can usually operate unattended. • How does that work? Optoelectronics Devices 20 .Electroluminescence • Electroluminescence is the conversion of electrical energy into light. Direct And Indirect Band gap Semiconductor • In a direct bandgap semiconductors • • • • • minimum energy of the conduction band lies directly above the maximum energy of the valence band in momentum space. • • • the momentum of the conduction band minimum and valence band maximum are not the same. These recombinations will often release the bandgap energy as phonons. while conserving momentum. This is radiative recombination. electrons at the conduction-band minimum can combine directly with holes at the valence band maximum. and thus do not emit light. light emission from indirect semiconductors is very inefficient and weak. . such as a phonon or a crystallographic defect. There are new techniques to improve the light emission by Optoelectronics Devices 21 indirect semiconductors. so a direct transition across the bandgap does not conserve momentum and is forbidden. Recombination occurs with the mediation of a third body. The energy of the recombination across the bandgap will be emitted in the form of a photon of light. In indirect bandgap semiconductors such as crystalline silicon. which allows for conservation of momentum. The prime example of a direct bandgap semiconductor is gallium arsenide—a material commonly used in laser diodes. instead of photons. As such. also called spontaneous emission. Optoelectronics Devices 22 .Direct And Indirect Band gap Semiconductor contd… E-k diagram illustrating a) Photon absorption in a direct bandgap semiconductor b) Photon absorption in an indirect bandgap semiconductor assisted by phonon absorption and c) Photon absorption in an indirect bandgap semiconductor assisted by phonon emission. The color of the light emitted depends on the bandgap of the material.Light Emitting Diode (LED) What is an LED? A p-n junction diode that emits light under forward-bias conditions due to the energy that is released when electrons and holes recombine. Why they are used? Less power Releases less heat Last longer than incandescent Material Used GaN grown on a Sapphire Substrate Optoelectronics Devices 23 . Optoelectronics Devices 24 . Evolution Of LED Optoelectronics Devices 25 . Solid State Lighting • utilizes light-emitting diodes (LEDs). or polymer lightemitting diodes (PLED) as sources of illumination • "solid state" refers to the fact that light in an LED is emitted from a solid object—a block of semiconductor Optoelectronics Devices 26 . organic light-emitting diodes (OLED). Optoelectronics Devices 27 . vibration. SSL creates visible light with reduced heat generation or parasitic energy dissipation. its solid-state nature provides for greater resistance to shock. and wear. thereby increasing its lifespan significantly.Why Solid State Lighting • Compared to incandescent lighting. however. • In addition. • Example: Blue GaInN LED is pumped with YAG:Ce3+ yellow phosphor. Optoelectronics Devices 28 . • Phosphor density and thickness is chosen such that fraction of blue light is transmitted.Phosphor Configuration in White LEDs • Phosphor absorbs short wavelength emission from LED and down converts it to long wavelength emission. Figure b: • • Thickness uniformity. Non-uniform Distribution. Due to which color variation at different viewing angles is drastically reduced.Typical Arrangements Of Phosphor in White LED Lamps • Placement & arrangement of phosphors are important for : • • • Figure a: • • • Luminous Source Efficiency. Color variation for different view angles. Color rendering Index Of white Led Lamps. Optoelectronics Devices 29 . Optoelectronics Devices 30 .Typical Arrangements Of Phosphor in White LED Lamps contd… • For the configurations a & b. phosphors are closely distributed around the chip due to which: • Light Emitted from phosphor directly impinges on LED chip.This isue is severe. • Contacts & Bonding Metal of LED chip are absorptive at phosphorescence wavelength. • Figure c: • Phosphor layer at large distance from LED chip.This reduces: • Operating Temperature for phosphor which in turn improves lifetime. Absorption & Emission Optoelectronics Devices 31 . Elements Of Laser Optics • Er=E0e-2r2/r02 Where E0=irradiance at the center of the beam(W/m2.lm/m2) Er=irradiance at the distance r from the center Beam Edge=where the irradiance falls to 1/e2 r0= distance to the beam edge • D0=4λ/π*Θ Where D0=waist Diameter λ =wavelength of the radiation Θ =divergence angle Optoelectronics Devices 32 . material looks like transparent. stimulated emission takes place and the applied radiation is amplified.Principle Of Laser Operation • • • For N1<N0 :Absorption takes place For N1=N0 :Called Two-Level Saturation. For N1>N0 :Population Inversion occurs. Optoelectronics Devices 33 . Feedback & Tuning Method In A Laser Optoelectronics Devices 34 . Semi-Conducting Laser Optoelectronics Devices 35 . • Starting Device: Edge emitting LED. Radiation from the edges escape in an elliptical cone pattern.Laser Diode • Laser diode is an LED. • Laser Diode Types: Double heterostructure lasers Quantum well lasers Quantum cascade lasers Distributed feedback lasers Vertical-Cavity Surface Emitting Laser Optoelectronics Devices 36 . with an added optical activity that provides feedback & generates stimulated emission. • Both N-n-P and N-p-P structures show the same behaviour.Double Heterostructure Laser • A type of laser in which a layer of GaAs. Optoelectronics Devices 37 . for example. where N and P represent the wider bandgap semiconductor according to carrier type. is sandwiched between two layers of the ternary compound AlGaAs which has a wider energy gap than GaAs and also a lower refractive index. whereas in the conventional laser a change in wavelength requires a change in layer composition. about 1 to 50 nanometres) to separate the quantum levels of electrons confined therein. • Superior performance characteristics compared to the standard double heterostructure lasers. and result in an improved semiconductor laser requiring fewer electrons and electron holes to reach laser threshold.Quantum Well Laser • quantum well structure would alter the density of states of the semiconductor. • threshold reductions resulting from modification of the density of electron states.g.. • Laser wavelength could be changed merely by changing the thickness of the thin quantum well layers. Optoelectronics Devices 38 . • active layers are thin enough (e. Quantum Well Laser contd. Optoelectronics Devices 39 . a small number of identical quantum wells are often used. very high quantum efficiencies can be achieved with the quantum well laser. • Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructure lasers.. This is called a multi-quantum well laser. • Moreover. • To compensate for the reduction in active layer thickness. since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by the electrons and holes. • Narrow divigent circular beam as compared to elliptical beam in Edge .formed during epitaxial growth. • Improved reliability due to unexposed active regions. • Permits accurate wavelength control in design.Vertical Cavity Surface Emitting Laser (VCSEL) • Optical activity between two layered distributed bragg reflector.Emitting Laser. • Reduced operating current due to small active area. Optoelectronics Devices 40 . Challenges Facing VCSEL Technology • Emission at shorter(<635nm) & Longer wavelengths(>1000nm). • More Robust Performance. Optoelectronics Devices 41 .(>40Gbit/s) • Added Functionality. • Wavelength Tunability. • Higher Bandwidth. • Increasing Single Mode Power. Optoelectronics Devices 42 .e part of the output circuitry . Capacitance less than a picofarad. Insulation resistance of the order of ohms. Electrical Insulation in input & Output Circuitry. Isolation Voltage Strength of several Kilovolts. • • • • 1012 Often called as Optoisolators.Opto Couplers • • An Optocoupler has a source that is optically coupled to a receiver. It has two separate circuits: Input Circuit contains a radiant source. Flux from this source is coupled to a detector i.here called the receiver. Open circuit Voltage Short circuit Current Maximum Power Point Fill Factor . • Solar Cells :A solar cell is a device that transforms the electron traffic across the bandgap into electric current.I.FF= JmVm/JscVoc η =JscVocFF/Ps Optoelectronics Devices 43 .Summary • Radiometry & Photometry: 1W=683 lm at 555nm according to C.P-type.E standard. • Phototransistors as tachometer and optical receiver. • Three layers of Solar Cell :N-type. • Switch a relay by the action of Photoresistors.p-n Junction. Tristate polarity Indicator and logic probe. Optoelectronics Devices 44 . Applications as voltage level sensor . Laser Diode.t source and misalignment in frequency changes its efficiency.. Optoisolators: Combination of photodiode and phototransistor. • • White Light as future light sources. • Optical Sensor’s orientation w. • • Electroluminescence is the conversion of electrical energy into light. LED: radiation emitted when electrons and holes combine.r.Summary contd. Basic Principle Of Laser: Stimulated Emission.Diode with an optical cavity for feedback & stimulated Emission. References: • • • • Optoelectronics by Endel Uiga Devices for Optoelectronics by Wallace B. Leigh Optoelectronic devices and circuits by Samuel Weber. World Of Knowledge : Internet Optoelectronics Devices 45 . Optoelectronics Devices 46 .