Nano Science (Lec 11 LED & Solar Cell)

Nano Science (Lec 11 LED & Solar Cell) - MAE 287/EE...

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MAE 287/EE 257 1
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A p-n junction is formed by combining P-type and N-type semiconductors together in very close contact. Normally they are manufactured from a single crystal with different dopant concentrations diffused across it. Creating a semiconductor from two separate pieces of material with a boundary. The most common function of junction is to allow an electric current to pass in one direction while blocking current in the opposite direction. 2
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1. An electric field is created by the charge regions. 2. The electric field creates a electric potential barrier that balances the Fermi levels on p & n sides. The electron energies are higher on the p-side compared to the n-side, and the hole energies are higher on the n-side compared to the p- side. 3. The potential barrier drives the draft currents that cancel the diffusion currents, and stops the net current through the p-n junction. The system reaches dynamic equilibrium. 3
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Semiconductor light-emitting diode (LED) and laser can emit light when voltage and electric current flow through the p-n junction. They produce more light per watt than do incandescent bulbs; this is useful in battery powered or energy-saving devices. They can have a relatively long useful life (35,000 to 50,000 hours, while incandescent light bulbs at 1,000–2,000 hours). They are used extensively for display, illumination, telecommunication, printing, and CD/DVD data storage, etc. 4
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Semiconductor laser or LED have p-n junction diode structure. When a forward voltage bias is applied on the two electrodes, holes and electrons can be "injected" from opposite electrodes of the device into the p-n junction area and recombine to emit photons (light). The diode structure also induces electrons and holes to recombine in the narrow junction region, therefore enhances their recombination rate, leading to higher optical efficiency in devices 5
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Due to very high luminous efficiency and the reliability of LED, it can potentially becoem the light source for illumination. Whereas conventional light sources exhibit an average output of 15 to 100 lumens per watt, the efficiency of white LEDs is predicted to reach more than 300 lumens per watt through continued development. Some of the current LED materials systems exhibit higher luminous performance than most of the conventional light sources, and soon light emitting diodes are expected to be the most efficient light source available. 6
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c E v The combination of an electron and a hole will lead to the emission of a photon The energy gap between the electron and hole will determine the wavelength of the photon. The wavelengths of the emitted lights increase with increasing quantum confinement sizes. h
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Nano Science (Lec 11 LED & Solar Cell) - MAE 287/EE...

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