EE 121b - 6 - BJT 3

EE 121b - 6 - BJT 3 - Slide 6-1 EE 121B – Chi On Chui EE...

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Unformatted text preview: Slide 6-1 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 6-2 EE 121B – Chi On Chui Outline • Other Important Effects • Frequency Limitations (Ch. 7) Slide 6-3 EE 121B – Chi On Chui Base Narrowing (or the Early Effect) • W b has been assumed to be independent of the bias voltage • As the C B junction reverse bias is increased (especially for p +-n-p + structure) – W b is indeed shortened – B recombination is thus reduced • This effect is known as – Base Narrowing, – The Early Effect, or – Base-Width modulation ( ) ( ) nC dB aC dB aC CB C x N N N N q V V ε l = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − = 2 1 2 Slide 6-4 EE 121B – Chi On Chui Common Emitter Characteristics and Early Voltage • In normal active mode, V EB is small and rather constant so any change in V EC is reflected on V CB V EC I B I C +- V EB +- I E p + ( E ) n ( B ) p ( C ) I E I C I B + V EB – + V BC – V EB V EC p + ( E ) n ( B ) p ( C ) I E I C I B + V EB – + V BC – V EB V EC V EB +- V EC +- p + ( E ) n ( B ) p ( C ) I E I C I B + V EB – + V BC – V EB V EC p + ( E ) n ( B ) p ( C ) I E I C I B + V EB – + V BC – V EB V EC V EB +- V EC +- E b pB pB b E pB pB C p W D qA L W p L D qA I ∆ csch ∆ ≈ ≈ • β increases with V EC • The family of I C curves – increase almost linearly with V EC – extrapolate to a single voltage V A , called the Early Voltage This drawing is not to scale as | V A | is usually very large Slide 6-5 EE 121B – Chi On Chui Base Punchthrough – The Extreme Base Narrowing • At large C B junction...
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This note was uploaded on 01/17/2012 for the course EE EE 110 taught by Professor Gupta during the Spring '09 term at UCLA.

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EE 121b - 6 - BJT 3 - Slide 6-1 EE 121B – Chi On Chui EE...

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