Lec14

Lec14 - Subject_13:Outline NonIdealBehaviorinpnJunctions

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Subject_13: Outline Non-Ideal Behavior in pn Junctions * Reverse-bias breakdown  Avalanche and Zener mechanisms  Breakdown electric field * R-G current  Forward and reverse biasing
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Reverse-Bias Breakdown •  The description we have developed thus far for the pn junction is based on a number of  IDEAL  approximations  that may  FAIL  under real operation conditions * The rectifier-equation predicts that in reverse-bias mode an essentially  NEGLIGIBLE    current should flow for  ALL  reverse-bias voltages * Experimental devices reveal a perfectly reproducible  BREAKDOWN  however when     the reverse-bias voltage is increased beyond some critical value  When breakdown occurs a  LARGE  reverse-bias current flows in the junction V a  (V) I (mA) FORWARD BIAS REVERSE BIAS V a  (V) I (mA) FORWARD BIAS REVERSE BIAS > 20 V I-V CHARACTERISTIC FOR AN IDEAL  pn JUNCTION I-V CHARACTERISTIC FOR A TYPICAL  pn JUNCTION
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Reverse-Bias Breakdown •  Reverse-bias breakdown may occur due to  TWO  different mechanisms depending on the doping  DENSITY  in the  junction  * In typical junctions with  MODERATE  doping the breakdown is due to the increased    importance of  IMPACT IONIZATION  under conditions of strong reverse bias * In order to understand this process we need to consider the properties of carriers     that  DRIFT  across the depletion region  These carriers are  ACCELERATED  by the electric field but also  LOSE  energy as  they  COLLIDE  with the semiconductor crystal  E v E c COLLISION WITH LATTICE • pn JUNCTION UNDER  MODERATE  REVERSE BIAS • ELECTRONS AND HOLES THAT  DRIFT  ACROSS THE  DEPLETION REGION DO SO WHILE  LOSING  ENERGY TO  THE LATTICE IN  INELASTIC  COLLISIONS • AS THE REVERSE-BIAS VOLTAGE IS INCREASED THE  DRIFTING CARRIERS BECOME  MORE  ENERGETIC AND  SO LOSE  MORE  ENERGY IN COLLISIONS WITH THE  LATTICE
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Reverse-Bias Breakdown •  As the reverse-biased voltage is increased the amount of energy exchanged in each inelastic scattering event  INCREASES * For sufficiently high reverse bias this energy may induce electron-hole pair creation     by means of  IMPACT IONIZATION * The  ADDITIONAL  carriers created by this ionization are then accelerated by the     electric field and in turn cause  FURTHER  impact ionization  The net effect is an  AVALANCHING  of current in the reverse-bias direction and  this effect is thus referred to as  AVALANCHE BREAKDOWN • ILLUSTRATION OF THE  AVALANCHE EFFECT  IN A STRONGLY  REVERSE BIASED pn JUNCTION • THE BREAKDOWN DOES  NOT  OCCUR  AT  A CRITICAL VOLTAGE BUT 
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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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Lec14 - Subject_13:Outline NonIdealBehaviorinpnJunctions

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