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Lec8 - Subject_07:Outline...

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Subject_07: Outline Recombination and Generation in Semiconductors * Recombination and generation of carriers * Direct and indirect semiconductors * Minority-carrier lifetimes
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• An important feature of semiconductors is that free carriers may be  DESTROYED  or  CREATED  in a number of  different processes * In  RECOMBINATION  electrons and holes combine to  ANNIHILATE  each other while in     GENERATION  an electron hole pair is  CREATED  by suitable excitation * The simplest process is  BAND-TO-BAND  recombination in which a free electron in     the conduction band drops to fill a hole state in the valence band  In the  OPPOSITE  process an electron-hole pair is  GENERATED  when an electron   is excited from the valence band to the conduction band Recombination and Generation of Carriers • SCHEMATIC ILLUSTRATIONS OF BAND-TO-BAND RECOMBINATION AND GENERATION IN A SEMICONDUCTOR • IN RECOMBINATION THE  EXCESS  ENERGY OF THE ELECTRON  IS CARRIED AWAY IN THE FORM OF A  PHOTON  WHOSE ENERGY  MATCHES THE SEMICONDUCTOR GAP ENERGY • IN GENERATION  ADDITIONAL  ENERGY IS REQUIRED TO EXCITE  THE ELECTRON INTO THE CONDUCTION BAND AND THIS MAY BE  SUPPLIED BY A  PHOTON  OR  THERMAL  EXCITATION RECOMBINATION E c E v PHOTON GENERATION E c E v PHOTON HEAT
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Recombination and Generation of Carriers •  ANOTHER  important source of recombination and generation in semiconductors is the existence of so-called  R-G   (recombination-generation)  CENTERS * These are  IMPURITY LEVELS  associated with lattice defects or special impurity     atoms which give rise to energy levels that lie close to the  CENTER  of the energy gap  The density of such impurities is normally  LOW  compared to the dopant density   in most semiconductors * When  RECOMBINATION  is mediated by R-G centers the excess electron energy is    emitted as  HEAT  transferred directly to the crystal lattice so that these processes     are referred to as  NON-RADIATIVE  recombination R-G CENTERS  INTRODUCED BY SOME COMMON IMPURITIES IN SILICON R-G CENTER RECOMBINATION MECHANISM R-G CENTER GENERATION MECHANISM Au Cu Mn Cr Fe E c E v E c E v E c E v HEAT HEAT
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Recombination and Generation of Carriers • An important process that arises in  HEAVILY  doped regions of semiconductor devices is  AUGER   RECOMBINATION * In this process recombination of an electron-hole pair is caused by the  COLLISION  of    electrons in the conduction band  The electron left in the conduction band absorbs the excess energy of the   electron that recombines with a hole  The highly energetic electron in the conduction band then progressively returns
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This note was uploaded on 02/28/2011 for the course EE 2 taught by Professor Vis during the Winter '07 term at UCLA.

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Lec8 - Subject_07:Outline...

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