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ch12 - andDevices Topic12: Fall2007 Learning Objectives...

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    Topic 12 : Photogenerated Excess                Carriers in Semiconductors                       Fall 2007 ECE 3500 - Semiconductor Materials  and Devices
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    Learning Objectives In this topic you will learn: How light is absorbed and emitted in a  semiconductor.  How excess carriers affect the conductivity of a  semiconductor. The difference between transient and steady state  excess carrier concentrations. What quasi-Fermi levels are and why they are useful. The Einstein relation and the balance between drift  and diffusion at thermal equilib. How non-uniform doping can result in built-in electric  fields in semiconductors. The continuity equation and its applications.
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    Excess Carriers in Semiconductors Until now, our discussion has primarily been focussed on  thermal equilibrium conditions , i.e. the semiconductor is in the dark and held at a uniform temperature. We are now going to shine light on the semiconductor   and learn how to determine the carrier concentrations under these conditions.
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    Optical Absorption in Semiconductors Light is absorbed by a material by raising an electron from a low energy level to a higher energy level. In the case of semiconductors, the usual form of optical absorption is when a
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    Optical Absorption (cont’d) In the case of a photon having energy much larger than the bandgap, the electron is raised to a very high energy level in the band where it is unstable. It rapidly  thermalizes  down to the bottom of the conduction band (in a time of about 10 -14  s !!).
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    Optical Absorption (cont’d)
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    Optical Absorption (cont’d) Since optical absorption is a purely random process, the intensity of light that is being absorbed by a material drops off  exponentially  as a function of distance into the material, i.e.  I = I 0  e -    x where     is the optical absorption coefficient of the material (units: cm -1 ) α α
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    Optical Absorption (cont’d) The wavelength of photons absorbed by a semiconductor depends on its bandgap.
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    Photoconductivity Light shining on a semiconductor with E ph  > E g  increases the carrier density in the bands. This results in the material exhibiting a higher conductivity. This phenomenon is called  photoconductivity . σ μ μ ph n p q n p = + ( )
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    Luminescence Luminescence is the opposite of optical absorption. It is the  emission  of light   from a semiconductor due to  the  recombination  of electrons and holes. 
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    Luminescence (cont’d) Luminescence can take various forms depending upon what is stimulating the light emission.
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