EE 121b - 6 - BJT 3

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

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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: chui@ee.ucla.edu 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 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 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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EE 121b - 6 - BJT 3 - Slide 6-1 EE 121B Chi On Chui EE 121B...

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