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Optoelectronics is the study and application of electronic devices that source, detect and control light, usually considered a sub-field of photonics. In this context, light often includes invisible forms of radiation such as gamma rays, rays, ultraviolet and infrared, in addition to visible light. Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers, or instruments that use such devices in their operation.
P-n junctions are an integral part of several optoelectronic devices. These include photodiodes; solar cells light emitting diodes (LEDs) and semiconductor lasers. In this section, we discuss the principle of operation of these devices and derive equations for key parameters.
Light absorption and emission:
A large number of optoelectronic devices consist of a p-type and n-type region, just like a regular p-n diode. The key difference is that there is an additional interaction between the electrons and holes in the semiconductor and light. This interaction is not restricted to optoelectronic devices. Regular diodes are also known to be light sensitive and in some cases also emit light. The key difference is that optoelectronic devices such as photodiodes, solar cells, LEDs and laser diodes are specifically designed to optimize the light absorption and emission, resulting in a high conversion efficiency.
Light absorption and emission in a semiconductor is known to be heavily dependent on the detailed band structure of the semiconductor. Direct bandgap semiconductors, i.e semiconductors for which the minimum of the conduction band occurs at the same wavevector, k, as the maximum of the valence band, have a stronger absorption of light as characterized by a larger absorption coefficient. They are also the favored semiconductors when fabricating light emitting devices. Indirect bandgap semiconductors, i.e. semiconductors for which the minimum of the conduction band does not occur at the same wavevector as the maximum of the valence band, are known to have a smaller absorption coefficient and are rarely used in light emitting devices.
The photo-generated carriers cause a photocurrent, which opposes the diode current under forward bias. Therefore, the diode can be used as a photodetector - using a reverse or even zero bias voltage - as the measured photocurrent is proportional to the incident light intensity. The diode can also be used as a solar cell - using a forward bias – to generate electrical power.
The primary characteristics of a photodiode are the responsivity, the dark current and the bandwidth. The responsivity is the photocurrent divided by the incident optical power. The maximum photocurrent in a photodiode equals.
This photocurrent is obtained by integrating the generation rate over the absorption region with thickness, d. The photocurrent is further reduced if photo-generated electron-hole pairs recombine within the photodiode instead of being swept into the regions where they are majority carriers.
The dark current is the current through the diode in the absence of light. This current is due to the ideal diode current, the generation/recombination of carriers in the depletion region and any surface leakage, which occurs in the diode. The dark current obviously limits the minimum power detected by the photodiode, since a photocurrent much smaller than the dark current would be hard to measure.
Solar cells are typically illuminated with sunlight and are intended to convert the solar energy into electrical energy. The solar energy is in the form of electromagnetic radiation, more specifically "black-body" radiation as described in section The sun’s spectrum is consistent with that of a black body at a temperature of 5800 K. The radiation spectrum has a peak at 0.8 eV. A significant part of the spectrum is in the visible range of the spectrum (400 - 700 nm). The power density is approximately 100 mW/cm2.
Only part of the solar spectrum actually makes it to the earth's surface. Scattering and absorption in the earth's atmosphere, and the incident angle affect the incident power density. Therefore, the available power density depends on the time of the day, the season and the latitude of a specific location.
Light emitting diodes are p-n diodes in which the recombination of electrons and holes yields a photon. This radiative recombination process occurs primarily in direct bandgap semiconductors where the lowest conduction band minimum and the highest valence band maximum occur at k = 0, where k is the wavenumber. Examples of direct bandgap semiconductors are GaAs, InP, and GaN while most group IV semiconductors including Si, Ge and SiC are indirect bandgap semiconductors.
The radiative recombination process is in competition with non-radiative recombination processes such as trap-assisted recombination. Radiative recombination dominates at high minority-carrier densities. Using a quantum well, a thin region with a lower bandgap, positioned at the metallurgical junction, one can obtain high carrier densities at low current densities. These quantum well LEDs have high internal quantum efficiency as almost every electron injected in the quantum well recombines with a hole and yields a photon.
Laser diodes :
Laser diodes are very similar to LEDs since they also consist of a p-n diode with an active region where electrons and holes recombine resulting in light emission. However, a laser diode also contains an optical cavity where stimulated emission takes place. The laser cavity consists of a waveguide terminated on each end by a mirror. As an example, the structure of an edge-emitting laser diode is shown.Photons, which are emitted into the waveguide, can travel back and forth in this waveguide provided they are reflected at the mirrors.
OMICS publishing group is a pioneer in organizing over 300 scientific international conferences and hosts over 400 leading-edge peer reviewed Open Access Journals around the globe. The conference organised by OMICS group and support from various visual communication scientific associations which are involved in the research and advancements in their respective field and also got the support from over 20,000 editorial board members. OMICS Publishing Group journals have over 3 million readers and the fame and success of the same can be attributed to the strong editorial board which contains over 30000 eminent personalities that ensure a rapid, quality and quick review process. OMICS Group signed an agreement with more than 1000 International Societies to make healthcare information Open Access.
The Optics is an academic journal hosted by OMICS publishing GROUP – a pioneer in open access publishing and is listed among the top 10 journals in Optics. The Journal of Lasers, Optics & Photonics is done in omics group. Each year research scientists have noticed a rise in the number of congresses being held in this field. The Journals provides a unique platform to researchers and scientist to explore the advanced and latest research developments in the field of Optics, thus bridging the gap between researchers and young scientists. OMICS publishing group is a pioneer in organizing over 300 scientific international conferences around the globe.
List of major conferences on Optodevices:
1. 2nd International Conference and Exhibition on Lasers, Optics & Photonics
2. 3rd International Conference and Exhibition on Lasers, Optics and Photonics
3. OP14 — SPIE Optics + Photonics 2014
4. Optical Engineering + Applications 2014 - Part of SPIE Optics + Photonics
5. 6th EPS-QEOD Europhoton Conference — "Solid State, Fibre, and Waveguide Coherent Light Sources"
6. LIP2014 — 10th Int. Conf. on Laser-light and Interactions with Particles
7. ELI Beamlines Summer School 2014
8. Photonics Prague 2014 on Photonics, Devices and Systems VI
9. 5th International Conference on Optical Communication Systems
10. 33rd European Conference on Laser Interaction with Matter
List of major Associations and Societies on Optodevices:
1. The Fiber Optic Association
2. American Optometric Association
3. SPIE - the international society for optics and photonics
4. IEEE Photonics Society
5. American Society for Photobiology
6. European Optical Society
7. Photonics Industry & Technology Development Association
8. Society of Photographic Science and Technology of Japan
9. Korea Assn. for Photonics Industry Development
10. IEEE Lasers & Electro-Optics Society
List of major companies on Optodevices:
1. Axsys Technologies, Inc., United States
2. Carl Zeiss AG, Germany
3. Cary Instruments, United States
4. CyberOptics Corporation, United States
5. II-VI Incorporated
6. KLA Tencor
7. Meade Instruments
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This page was last updated on 10th Oct, 2014
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