Chapter42_BJT-Statics

# Chapter42_BJT-Statics - 4.2 BJT Static Analysis 4.2.1...

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4.2 BJT Static Analysis 4.2.1 Calculate Carrier Densities 4.2.2 Calculate Current Densities 4.2.3 Circuit Parameters 4.2.4 Narrow Base Approximations Pierret, Chapter 11, page 389-404 Slides courtesy of Prof. Oliver Brand

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2 BJT – Ideal Static Analysis I Goal: Calculate BJT currents and transistor parameters α 0 , I CB0 , β Assumptions (similar to pn-junction analysis): 1-D model with homogeneous doping, i.e. N A (x), N D (x) = const.; as a result, the junctions are treated as step junctions Low-level injection No R-G currents in the depletion regions Non-degenerate doping, i.e. Boltzmann approximation is valid Depletion layer width << base width No electric field outside the depletion regions Solution Approach (similar to pn-junction analysis): 1. Calculate carrier densities in quasineutral regions 2. Calculate current densities at the edges of the depletion regions
3 BJT – Ideal Static Analysis III – The Model – N E = N A,E D E = D n L E = L n n E0 = n p0 τ E = τ n N B = N D,B D B = D p L B = L p p B0 = p n0 τ B = τ p N C = N A,C D C = D n L C = L n n C0 = n p0 τ C = τ n x’’ x’ x 0 0 W 0 p + p n Emitter Base Collector

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4 4.2.1 Calculate Carrier Densities Approach: Solve steady-state continuity equations in the quasi-neutral regions (no electric field, no carrier generation) of emitter, base, and collector n p (x,t) t = + n p μ n ε x n ε n p x + D n 2 n p x 2 + G n n p n p0 τ n p n t = 0 steady state    = p n μ p ε x μ p ε p n x = 0 no electric field   + D p 2 p n x 2 Diffusion     + G p = 0 no G p n p n0 τ p Thermal R-G    
5 Carrier Density – Emitter I Continuity equation for emitter: General solution: 0 = D E d 2 Δ n E ( x ) d x 2 Δ n E (

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## This note was uploaded on 10/12/2011 for the course ECE 3040 taught by Professor Hamblen during the Fall '07 term at Georgia Tech.

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Chapter42_BJT-Statics - 4.2 BJT Static Analysis 4.2.1...

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