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Lec17 - Subject_16:Outline BipolarJunctionTransistors...

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Subject_16: Outline Bipolar Junction Transistors * Introductory concepts * BJT electrostatics * BJT operation  Band bending * BJT currents & current factors
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Introductory Concepts • Bipolar junction transistors (BJTs) consist of two  BACK-TO-BACK  pn junctions connected by a common  BASE * The thickness of this base is  SMALLER  than the minority carrier diffusion length     while the outer regions are referred to as the  EMITTER  and the  COLLECTOR  The emitter is typically doped  MORE  heavily than the collector   In a  pnp  transistor the emitter and the collector are  p-TYPE  and the base is  n-   TYPE  while in a  npn  transistor the emitter and the collector are  n-TYPE  and the   base is  p-TYPE E C B E C B pnp  BJT AND ITS CIRCUIT SYMBOL npn  BJT AND ITS CIRCUIT SYMBOL p+ p E C B n n + n E C B p
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• The discussion of BJT electrostatics is simplified by noting that the BJT may be viewed as two  BACK-TO-BACK  pn junctions * For simplicity we focus here on the specific case of a  pnp  transistor and we note that     in real devices the  DOPING  is typically  DIFFERENT  in the different transistor regions  Usually N AE  ( EMITTER  doping) >> N DB  ( BASE  doping) > N AC  ( COLLECTOR  doping)  At  EQUILIBRIUM  we may therefore draw the  CHARGE-DENSITY  diagram of   the BJT as shown below BJT Electrostatics p + n p EMITTER BASE COLLECTOR W B W CB W EB DEPLETION REGION  WIDTHS  IN A pnp BJT WITH N AE  >> N DB  >> N AC ρ (x) ρ (x) x W B CHARGE-DENSITY  DIAGRAM FOR THE BJT IN THE DEPLETION APPROXIMATION
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• Given the charge-density diagram we may use the  POISSON EQUATION  to determine the variation of the electric  field and the electrostatic potential as we did previously for pn junctions * Note the  DIFFERENT  sign of the electric field in the two depletion regions  This arises due to the different  DIRECTIONS  in which these electric fields point * Also note that due to the high doping density in the  COLLECTOR  the built-in potential     drop is  LARGER  at the collector-base side than at the emitter-base side     ELECTRIC-FIELD  VARIATION IN A TYPICAL pnp BJT POTENTIAL  VARIATION IN A TYPICAL pnp BJT E(x) E(x) x EMITTER BASE COLLECTOR V(x) V(x) x EMITTER BASE COLLECTOR BJT Electrostatics
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BJT Electrostatics  In BJTs we must distinguish between the  THICKNESS  of the base which we term the  BASE WIDTH  W B  and the  EFFECTIVE  width W obtained after accounting for the effects of depletion
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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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Lec17 - Subject_16:Outline BipolarJunctionTransistors...

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