Lec13

Lec13 - Subject_12:Outline ***

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Subject_12: Outline Rectification in pn Junctions * The rectifier equation * Electron and hole currents * Carrier concentrations
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The Rectifier Equation •  We would now like to develop a  QUANTITATIVE  analysis that allows us to compute the  CURRENT  that flows  through a pn junction in response to the application of an  EXTERNAL  voltage * We begin by noting that a pn junction may be broken up into two  QUASI-NEUTRAL    regions that surround the  DEPLETION  region  For the sake of the analysis here we will assume that the  ELECTRIC FIELD  in  these quasi-neutral regions is equal to  ZERO • IN OUR PREVIOUS TREATMENT OF THE ELECTROSTATICS  IN THE pn JUNCTION WE SAW THAT THE ELECTRIC FIELD IS  ZERO  OUTSIDE THE  DEPLETION REGION • WE THEREFORE ASSUME THAT THE ELECTRIC FIELD IS  ALWAYS EQUAL TO ZERO IN THE QUASI-NEUTRAL REGIONS • IN CONTRAST THE ELECTRIC FIELD WITHIN THE  DEPLETION REGION IS  NOT  EQUAL TO ZERO BUT VARIES  AS THE EXTERNAL VOLTAGE BIAS IS CHANGED   p-TYPE QUASI-NEUTRAL REGION: E = 0 n-TYPE QUASI-NEUTRAL REGION: E = 0 DEPLETION REGION  0 E(x) x -x p x n
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The Rectifier Equation •  Since the electric field is equal to zero the electron and hole  DRIFT  currents that flow in the quasi-neutral regions  must  ALSO  be equal to zero * In Subject_11 we obtained expressions for the  MINORITY-CARRIER  concentrations    under the influence of an external bias  ) 2 . 12 ( dx p d qD J J J n p diffusion P drift P P - = + = ) 1 . 12 ( dx n d qD J J J p n diffusion N drift N N = + = ) 9 . 11 ( , exp 1 exp ) ( 2 p p p B a D i n D L L x T k qV N n x p τ = - - = ) 10 . 11 ( , exp 1 exp ) ( 2 n n n B a A i p D L L x T k qV N n x n = - - =
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The Rectifier Equation •  By combining Equations 11.9,11.10, 12.1, and 12.2 we thus obtain expressions for the electron and hole  CURRENT DENSITIES  in the junction * Note that in these equations the  DEFINITION  of direction x is  DIFFERENT  for the two    situations ) 3 . 12 ( exp 1 exp 2 - -
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Lec13 - Subject_12:Outline ***

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