Non-Ideal Behavior of BJT: Base Width Modulation, Punch-Through, and Other Effects - Prof., Study notes of Electrical and Electronics Engineering

Download Non-Ideal Behavior of BJT: Base Width Modulation, Punch-Through, and Other Effects - Prof. and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 Chapter 11-2. Deviations from the ideal Base-width modulation Punch-through Avalanche multiplication and breakdown Others – base resistance, depletion region recombination-general The measured characteristics deviates slightly from the ideal characteristics discussed. We will discuss some of the non- idealities of the BJT characteristics. 2 Review: BJT in “cut-off” E B C pB0nE0 nC0 5 Base width modulation When the reverse bias applied to the C-B junction increases, the C-B depletion width increases and W decreases. Thus, the collector current, IC increases. This is also known as “Early Effect”. More prominent in narrow-base transistors. IC VEC 1µA 2µA 3µA 3 mA 2 mA 1 mA 6 Punch-through Punch-through can be viewed as base width modulation carried to the extreme, i.e., punch-through occurs when W → 0. For C-B voltage beyond punch-through, the E-B barrier lowers and results in large increase in carrier injection from emitter to collector. Large increase in collector currents at high VCE0 occurs due to two reasons: Punch-through or Avalanche multiplication 7 Example 1 Two transistors are identical except that the base doping is different. A. Which transistor will have higher base-width-modulation effect? B. Which one will have higher punch-through voltage? 1019 1016 1016 1019 1015 1016 WB WB Approximate value of punch-through voltage can be obtained by equating the depletion layer width on the base side to the base- width, WB. 2 1 BCB CSi2 1 CBbi(CB) BCB CSi nCBB 2)( )( 2       + ε ≈      − + ε =≈ CBV)NN(N N q VV NNN N q xW