# Chap5 - 1 Physical Structure • Consists of 3 alternating layers of n and p-type semiconductor called emitter E base B and collector C •

This preview shows pages 1–2. Sign up to view the full content.

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

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 1 Physical Structure • Consists of 3 alternating layers of n- and p-type semiconductor called emitter ( E ), base ( B ) and collector ( C ). • Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter. • Carrier transport in the active base region directly beneath the heavily doped ( n + ) emitter dominates i-v characteristics of BJT. Transport Model for npn Transistor • Narrow width of the base region causes coupling between the two back to back pn junctions. • Emitter injects electrons into base region, almost all of them travel across narrow base and are removed by collector • Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor and are said to be positive when they forward-bias their respective pn junctions. • The terminal currents are collector current ( i C ), base current ( i B ) and emitter current ( i E ). • Primary difference between BJT and FET is that i B is significant while i G = 0. npn Transistor: Forward Characteristics Forward transport current is I S is saturation current ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BE v S I F i C i A 9 10 A 18 10 − ≤ ≤ − S I V T = kT/q =0.025 V at room temperature Base current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BE v F S I F F i B i β β 500 20 ≤ ≤ F β Emitter current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = + = 1 exp T V BE v F S I B i C i E i α . 1 1 95 . ≤ + = ≤ F F F β β α is forward common-emitter current gain is forward common- base current gain In this forward active operation region, F B i C i β = F E i C i α = npn Transistor: Reverse Characteristics Reverse transport current is ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = − = 1 exp T V BC v S I E i R i ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − = = 1 exp T V BC v R S I R R i B i β β 20 ≤ ≤ R β Emitter current is given by ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − − = 1 exp T V BC v R S I C i α 95 . 1 ≤ + = ≤ R R R β β α is reverse common-emitter current gain is reverse common- base current gain Base current is given by Base currents in forward and reverse modes are different due to asymmetric doping levels in emitter and collector regions. npn Transistor: Complete Transport Model Equations for Any Bias ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − = 1 exp exp exp T V BC v R S I T V BC v T V BE v S I C i β ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜...
View Full Document

## This note was uploaded on 10/11/2011 for the course ECE 322 taught by Professor Staff during the Spring '08 term at Boise State.

### Page1 / 8

Chap5 - 1 Physical Structure • Consists of 3 alternating layers of n and p-type semiconductor called emitter E base B and collector C •

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online