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Lecture_14 - PN Junctions Summary p-n junction diodes...

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1 Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) p-n junction diodes •Depletion approximation •Forward current •Other currents in diodes •Generation-recombination •Recombination •Photocurrent •Tunneling Practical diode structures •p-i-n diodes •Schottky diodes •Ohmic contacts •Heterostructure diodes •Heterojunction Band Lineup Single and Double Heterojunctions PN Junctions Summary Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Optical absorption in depletion region generates extra electrons and holes in excess of thermal generation 1. Under reverse bias Carriers are swept out by electric field to give reverse current of one electron for every generated electron-hole pair Note: electrons go in one direction and holes go in other; there is only one charged particle passing any given point, so there is only one electronic charge traveling around the circuit 2. Zero bias or forward bias Generated electrons and holes drift due to the built-in field. This produces a reverse short-circuit current or increased forward bias voltage in open circuit or resistive circuit, as utilized for photovoltaic operation Note: there is also a finite probability that carriers will diffuse in opposite direction, leading to reduced photocurrent at higher forward biases Photodiodes/Photocurrent
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2 Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Photogeneration and Photocurrent in pn Diodes Optical absorption, electric field, drift and diffusion currents Current-voltage (I-V) characteristic with increasing illumination Electrons and holes outside the depletion region can diffuse the “wrong” direction, decreasing quantum efficiency. It is thus desirable to generate all photocarriers in the depletion region V Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Possible for electrons or holes to tunnel directly across the bandgap in diode structures if the electric field is such that the bottom of conduction band at one point in space is lower than top of the valence band at another point in space This is not normally a significant issue in optoelectronic devices (except sometimes in the case of avalanche diodes) Tunneling is very important in optoelectronics, as in electronics, for the formation of Ohmic contacts and p/n contact in multi-junction solar cells Reverse bias tunneling current
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3 Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Forward bias tunneling current Forward bias I V Tunneling current Ideal diode current electron tunneling Electrons tunnel from the n- region to p-region until the conduction band on the n- side moves above the valence band on the p-side, V f = Φ n + Φ p Φ n + Φ p Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010) pn homojunction used for some optoelectronic devices, including some light emitting diodes and photodetectors Modern layered epitaxial growth enables many complex
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  • Winter '10
  • Harris,J
  • J. S. Harris, Prof. J. S., EE243. Semiconductor Optoelectronic Devices, EE243. Semiconductor Optoelectronic

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