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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: [email protected] 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 0 0.5 1 1.5 2 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