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Lecture_16 - 1 EE243. Semiconductor Optoelectronic Devices...

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Unformatted text preview: 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Prof. J. S. Harris 1 Pizza Lunch for all “Survivors” CISX-Patio Friday Noon March 12 Final Exam Wednesday, March 17 7-10 PM Room 380X Please go to AXESS for online course evaluation EE243. Semiconductor Optoelectronic Devices (Winter 2010) Prof. J. S. Harris 2 A large variety of semiconductor materials, structures and devices are used as photodetectors in optical receivers. The most important for communications are: • pn, p-i-n and Schottky Barrier Photodiodes • Avalanche Photodiodes • Metal-Semiconductor-Metal (MSM) Photodiodes • Photoconductors Equally important optical “detectors”, but structurally completely different and not used for optical communications include: • Charge-Coupled Devices (CCDs) • CMOS Imagers • Photocathodes • Solar Cells 6. Semiconductor Detectors 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Prof. J. S. Harris 3 Optical Absorption Band-to-Band Impurity/Defect- to-Band Quantum Well or Dot Intersubband (QWIP or QDIP) defect transitions are much weaker and usually parasitic thus avoided except for far IR detectors. Intraband QW or QD transitions are now being used in place of narrow bandgap semiconductors because of better semiconductor processing technology in more established materials (GaAs vs HgCdTe). Photocurrent results from the fow oF carriers created by absorption of photons. Most detectors rely on band-to-band absorption. Impurity or EE243. Semiconductor Optoelectronic Devices (Winter 2010) Prof. J. S. Harris 4 Optical Absorption in Semiconductors Since the carrier collection regions (depletion regions) are ~1 μm, the absorption coefFcient needs to be ~10 4 cm-1 to achieve high efFciency. This only occurs for direct bandgap materials above the bandgap. Optimally choose a bandgap just less than the photon energy so as to have identical thermal and photon energies for generation. I = I inc exp − α x ( ) The photon ¡ux passing through an absorbing medium is 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Prof. J. S. Harris 5 Optical absorption creates extra pairs of electrons and holes in excess of the thermal equilibrium concentration. If this is in the depletion region, then under the built-in Feld, or adding to it with reverse bias, the carriers are swept out by the electric Feld to give a reverse current of one electron for every generated electron-hole pair. Because electrons and holes have opposite charge, they move in opposite directions and there is only one particle passing any given point, so there is current of only one electronic charge, not two. This drift of charge increases the nominal reverse current of the diode in the short circuit condition or pushes the diode into forward bias if in the open circuit condition (i.e. a forward current that just balances the reverse photocurrent to provide ZERO net current). The latter is the operating region for photovoltaic or solar cell operation....
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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_16 - 1 EE243. Semiconductor Optoelectronic Devices...

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