BrookeECE51_05_BJTs

BrookeECE51_05_BJTs - Chapter 5 Bipolar Junction...

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Chapter 5 Bipolar Junction Transistors Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Chap 5 - 1
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Chapter Goals Explore physical structure of bipolar transistor Understand bipolar transistor action and importance of carrier transport across base region Study terminal characteristics of BJT. Explore differences between npn and pnp transistors. Develop Transport model for the bipolar device. Define four operation regions of BJT. Explore model simplifications for each operation region. Understand origin and modeling of Early effect. Present SPICE model for bipolar transistor. Provide examples of worst-case and Monte Carlo analysis of bias circuits. Chap 5 - 2
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Bipolar Transistor: 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. Chap 5 - 3
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Transport Model for the 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. Chap 5 - 4
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npn Transistor: Forward Characteristics Forward transport current is I S is saturation current i C = i F = I S exp v BE V T - 1 10 - 18 A I S - 9 A V T = kT/q =0.025 V at room temperature Base current is given by i B = i F β F = I S F v V T - 1 20 ≤β F 500 Emitter current is given by i E = i C + i B = I S α F v V T - 1 0.95 ≤α F = F F + 1 1.0 is forward common-emitter current gain is forward common- base current gain In this forward active operation region, i C i B F i C i E F Chap 5 - 5
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npn Transistor: Reverse Characteristics Reverse transport current is i R =- i E = I S exp v BC V T - 1 i B = i R β R = I S R v V T - 1 0 ≤β R 20 Emitter current is given by i C I S α R v V T - 1 0 ≤α R = R R + 1 0.95 is reverse common-emitter current gain is reverse common-base current gain
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This note was uploaded on 03/29/2009 for the course ECE 51 taught by Professor Martinbrooke during the Fall '08 term at Duke.

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BrookeECE51_05_BJTs - Chapter 5 Bipolar Junction...

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