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Lec10 - Subject_09:Outline ThepnJunction *Thepnjunction

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Subject_09: Outline The pn Junction * The pn junction * Carrier diffusion in the pn junction * pn junction built-in voltage * The depletion approximation
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The pn Junction • We begin our discussion of semiconductor  DEVICES  by looking at the properties of the  pn JUNCTION   * The pn junction is a  TWO-TERMINAL  semiconductor device that is formed by     bringing n-type and p-type semiconductors into contact with each other  In practice such junctions are formed by  DIFFUSING  suitable dopants into a   semiconductor substrate that is already n-type or p-type doped p-TYPE n-TYPE p-TYPE DOPANTS A pn JUNCTION MAY BE FORMED BY  DIFFUSING ACCEPTORS  INTO A SUBSTRATE THAT IS    UNIFORMLY  DOPED WITH  DONORS x N A , N D N D N A p-TYPE N A  > N D n-TYPE N D  > N A AFTER DIFFUSION THE  SURFACE  REGION IS HEAVILY p-TYPE  DOPED WHILE BELOW THIS THE  SUBSTRATE ITSELF REMAINS  n-TYPE  DOPED x
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The pn Junction • For an analysis of the properties of pn junctions it is the  NET  doping density that is important  * The point within the junction where this net doping density is zero is referred to as     the  METALLURGICAL JUNCTION * When modeling pn junctions it is often sufficient to assume that the doping profiles    within the pn junction change  ABRUPTLY  at some interface  DOPING VARIATIONS AS A FUNCTION OF POSITION IN A  DIFFUSED  pn JUNCTION N - N D x ACTUAL PROFILE ABRUPT APPROXIMATION VARIATION OF THE  NET  DOPING PROFILE AS A FUNCTION OF POSITION
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Carrier Diffusion in the pn Junction • The spatial variation of the net doping density that is present in pn junctions can be expected to give rise to strong  DIFFUSION CURRENTS * In order to understand how these currents arise consider an  IDEALIZED  situation     where a p-type and a n-type semiconductor are brought into contact with each other    thus allowing the  EXCHANGE  of carriers between the two materials  Holes will diffuse into the n-type material leaving behind  IMMOBILE  ionized   acceptors while electrons will diffuse into the p-type material leaving behind   IMMOBILE  ionized donors  - h - h - h - h - h - h - h - h + e + e + e + e + e + e + e + e p-TYPE n-TYPE p-TYPE n-TYPE - h - h - - - h - h - - + e + e + + + e + e + + h e 1. 2.
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Carrier Diffusion in the pn Junction • Since the carriers that diffuse across the junction rapidly recombine with the majority carriers a so-called 
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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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Lec10 - Subject_09:Outline ThepnJunction *Thepnjunction

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