Chap5 - 1 Physical Structure • Consists of 3 alternating layers of n and p-type semiconductor called emitter E base B and collector C •

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Unformatted text preview: 1 Physical Structure • Consists of 3 alternating layers of n- and p-type 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 base-emitter junction and exits through emitter. • Carrier transport in the active base region directly beneath the heavily doped ( n + ) emitter dominates i-v 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 • Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor and are said to be positive when they forward-bias 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 common-emitter 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 common-emitter 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.

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Chap5 - 1 Physical Structure • Consists of 3 alternating layers of n and p-type semiconductor called emitter E base B and collector C •

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