121B_1_EE121B_Slide05 - BJT3

121B_1_EE121B_Slide05 - BJT3 - EE 121B Principles of...

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Slide 5-1 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui EE 121B Principles of Semiconductor Device Design Bipolar Junction Transistors - III Professor Chi On Chui Electrical Engineering Department University of California, Los Angeles Email: chui@ee.ucla.edu Slide 5-2 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Outline Non-Ideal Effects Base Narrowing / Base-Width Modulation / Early Effect Base Punchthrough Avalanche Breakdown High-Level Injection Access Resistance Gummel Model
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Slide 5-3 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Base Narrowing If the base has relatively low doping and if the depletion region is not small relative to the neutral base width, a narrowing of the base results as | V BC | is increased pC BJ x W W = C I = n E n n C L W n L D Aq I csch Neutral Base pC x ( ) () aB dC aB dC BC biC pC N N qN N V V x + = ε 2 Note that I C is a function of the neutral base width W . Up to now we have assumed that W is a constant independent of voltage. However, as we see on the left, W' is a weak function of V BC through the depletion region width x pC . (For now we ignore the depletion at the other end as we are interested in the effect of V CE on I C with V BE constant.) Slide 5-4 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui -0.5 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 Vce (Volts) Ic (mA) Ib = 10 μA Ib = 8 μA I C -V CE or Common Emitter Characteristic 0.4 0.405 0.41 0.415 0.42 00 . 511 . 52 VCE VBE -1.5 -1 -0.5 0 0.5 VBC V BC V BE V BC forward V BC reverse Saturation V BC > 0 In forward active operation at a particular base current bias, V BE stays constant so any change in V CE is a change in V BC V CE I B I C + - V BE + - I E V CE = V CB + V BE
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Slide 5-5 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Base Narrowing We are particularly interested in the base width narrowing effect at fairly high | V BC |. We want to know the change in current for an incremental change in the V CE . pC BJ x W W = E n C n W D Aq I Neutral Base pC x () aB dC aB dC BC biC pC N N qN N V V x + = ε 2 The first equality comes from our observation that, for V CE > V BE , almost all of the voltage change occurs across the BC junction. BC C CE C V I V I ( ) BC C BC C CE C V W I V W W I V I = ln This is a very slowly varying function of V CE and is often treated as a constant 1/V A where | V A | is called the Early Voltage Slide 5-6 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Early Voltage If the base is low doping relative to the collector and if the depletion region is not small relative to the base width, narrowing of the base results in a higher current gain at higher | V BC | (why?) The transistor IV curves are therefore spread further apart at higher V CE .
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121B_1_EE121B_Slide05 - BJT3 - EE 121B Principles of...

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