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Unformatted text preview: 1 Physical Structure • Consists of 3 alternating layers of n and ptype semiconductor called emitter ( E ), base ( B ) and collector ( C ). • Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses baseemitter junction and exits through emitter. • Carrier transport in the active base region directly beneath the heavily doped ( n + ) emitter dominates iv characteristics of BJT. Transport Model for npn Transistor • Narrow width of the base region causes coupling between the two back to back pn junctions. • Emitter injects electrons into base region, almost all of them travel across narrow base and are removed by collector • Baseemitter voltage v BE and basecollector voltage v BC determine currents in transistor and are said to be positive when they forwardbias their respective pn junctions. • The terminal currents are collector current ( i C ), base current ( i B ) and emitter current ( i E ). • Primary difference between BJT and FET is that i B is significant while i G = 0. npn Transistor: Forward Characteristics Forward transport current is I S is saturation current ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BE v S I F i C i A 9 10 A 18 10 − ≤ ≤ − S I V T = kT/q =0.025 V at room temperature Base current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BE v F S I F F i B i β β 500 20 ≤ ≤ F β Emitter current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = + = 1 exp T V BE v F S I B i C i E i α . 1 1 95 . ≤ + = ≤ F F F β β α is forward commonemitter current gain is forward common base current gain In this forward active operation region, F B i C i β = F E i C i α = npn Transistor: Reverse Characteristics Reverse transport current is ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = − = 1 exp T V BC v S I E i R i ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BC v R S I R R i B i β β 20 ≤ ≤ R β Emitter current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − − = 1 exp T V BC v R S I C i α 95 . 1 ≤ + = ≤ R R R β β α is reverse commonemitter current gain is reverse common base current gain Base current is given by Base currents in forward and reverse modes are different due to asymmetric doping levels in emitter and collector regions. npn Transistor: Complete Transport Model Equations for Any Bias ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − = 1 exp exp exp T V BC v R S I T V BC v T V BE v S I C i β ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜...
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This note was uploaded on 10/11/2011 for the course ECE 322 taught by Professor Staff during the Spring '08 term at Boise State.
 Spring '08
 STAFF
 Transistor

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