course230B5

course230B5 - Bipolar Fundamentals VBE Emitter Base V CB...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
Bipolar Fundamentals 1E-2 1E-1 I C I C 0 I B 0 Emitter Base Collector SC V BE V CB 1E-4 1E-3 ) I B E c I C 1E-6 1E-5 Current ( A E v E c I B E-8 1E-7 = 9 2 x 0 W B -W E E v ase current = holes injected 0.4 0.6 0.8 1 1.2 1.4 1E-9 1E-8 Emitter-base voltage (V) A E 9 μ m Base current holes injected from base into emitter. • Collector current = electrons injected from emitter into base. 3/9/2010 1 Current Gain β =I C /I B j
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Ideal I-V Characteristics Saturation region Non-saturation region I = B C I C 5 4 E V CB 3 2 V CE V BE B V CE 0 1 E ± aturation region: Saturation region: V CE < V BE -- both E-B diode and C-B diode are forward biased ± Non-saturation region: V E > V E 3/9/2010 2 CE BE -- E-B diode forward biased, C-B diode reverse biased
Background image of page 2
Quasi-Fermi Level [ ] pn n q kT ip n =− 2 exp ( ) / φφ Non-equilibrium: c E - c E A forward biased p-n junction: E v E f q φ n -q p E v E f qV >0 app x n -x p Excess electrons: n p (x=-x p )=(n i 2 /N a )exp(qV app /kT) xcess holes: p =x =(n 2 xp(qV T) N a N d Excess holes: p n (x x n )( n i /N d )exp(qV app /kT) So collector current is inversely proportional to base doping, nd base current is inversely proportional to emitter doping 3/9/2010 3 And base current is inversely proportional to emitter doping.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Excess Electrons in the p-Region 0 1 = + = n n n G R x J q t n dx dn kT dx dn qn kT dx d qn J n n n μ ψ = = n n n n n G R τ 0 = T 2 1 ensity LD kT q nn n ≡= μτ d n dx n n L pp p n 2 0 2 0 −= , where is the minority carrier diffusion ngth W / L = 0.2 0.6 0.8 s electron d e length. Boundary conditions: () q V k T a p p = 0 exp / exp(- x / L ) 0.2 0.4 alized exces s at x=0, and t x=W (ohmic contact). = 0 0.5 1.0 2.0 00 . 511 . 522 . 53 0 x/L Nom a at xW (ohmic contact). 3/9/2010 4 [] [ ] n q V k T W x L WL p p p app n n 1 exp( / ) sinh ( ) / sinh( / ) .
Background image of page 4
Bipolar Design Considerations E C B E Reach-through ¾ Base: --Thin base means higher urrent gain and smaller n + p pp n n + n p B Emitter Base ollector current gain and smaller stored charge, therefore higher performance. -- oping must be high n n + p n + Collector Subcollector Doping must be high enough to keep base resistance low and avoid E-C punchthrough. (b) C (a) ¾ Emitter: ¾ Collector: --Higher collector doping --Doping as high as possible. --Deep emitter gives higher current gain, but difficult to reduces base widening. --Higher doping also means higher B-C capacitance. 3/9/2010 5 control thin base width. --Need highly doped sub-C to minimize resistance.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Intrinsic-Base Resistance ± Intrinsic base sheet W resistance E B 1 B 2 L ( ) Rq p x x d x Sbi p p W B = () () μ 0 1 ± One base contact only W 1 ± Two-sided base contact B 1 E B 2 r bx r bi r L R Sbi = 3 C r W L R = 1 12 3/9/2010 6
Background image of page 6
Emitter-Collector Punchthrough ± E-C punchthrough when base width becomes so small that V BC tarts to affect the potential barrier punchthrough starts to affect the potential barrier of E-B diode. (like drain-induced barrier lowering in MOSFET.) hen operated near ± When operated near punchthrough, collector current is no longer controlled by V BE .
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 8
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 32

course230B5 - Bipolar Fundamentals VBE Emitter Base V CB...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online