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Chapter (3)Transistors and Integrated Circuits
B I P O L A R J U N C T I O N T R A N S I S T O R
BJT in contrast to the "unipolar" FET
• Both minority and majority carries play significant roles.• Permits greater gain better high-frequency performance.
Alloy Structure consist of
• Collector (C) n-type chip less than 1 mm square• Base (B) p-type thinner than this paper• Emitter n-type alloyed to the base • The result is a pair of pn junctions separated by a base
region, npn junction transistor.
Chapter (3)Transistors and Integrated Circuits
Planar Structure
npn BJT• (n) grown upon a heavily doped (n+) substrate.• oxidation of the surface • window opened to diffuse impurities
(P) into the crystal to form the pn junction.• A smaller window for emitter in opened to diffuse emitter region (n).
Biasing Parameters
BJT C E B VEB VCB iE iC iB
Chapter (3)Transistors and Integrated Circuits
Operation• The emitter junction forward biased
VEB reduced the barrier at emitter electrons injection into B where they are
minority carriers.• The collector junction is reverse biased
VCB increase the barrier at C
• B is very thin most electron pass from E to C
• The net result transfer electron from E to C.• Large RL insertion in C large voltage
voltage amplification • Variation of the base current iB large iC
current amplification • Switching operation used in digital signal
processing.
Chapter (3)Transistors and Integrated Circuits
DC Behavior• E forward C reverse biased. • iE consist electron across
np J holes from B.
• almost unity iE nearly electrons.• varies from 0.90 to 0.999 where a typical value is 0.98.• Most of these electrons in B
diffuse to C B is very narrow. • iC consist of iE and a very small current due to thermally generated minority carrier ICBO CBO
KTevCBO IeI 1/
iC = -iE + ICBO
iB = -iE -iC
Chapter (3)Transistors and Integrated Circuits
Common - Base Characteristics
• CB configuration
• B common input E output C.
• The emitter-base section forward-biased
diode.
Input Characteristics
• Input characteristics Fig.( b ) similar to
Fig. (a) diode characteristics
• The effect of VCB small
• +VCB emitter open-circuited IE = 0
C section reverse-biased junction
Chapter (3)Transistors and Integrated Circuits
Output Characteristics
• The collector
characteristic reverse
bias diode iE = - 5 mA,
iC = - iE + 5 mA.
• The slope of the curves
in Fig C due to an
effective increase in
as VCB increases.
• always less 1
The common base
configuration is not good for
practical current amplification
Chapter (3)Transistors and Integrated Circuits
Chapter (3)Transistors and Integrated Circuits
Chapter (3)Transistors and Integrated Circuits
Input and output CE Characteristics
iB depends on VBE only .
iB = 0 iC = iCEO
= 0.98 = 49,
A very small increase in iB large
increase in iC
A very small increase in much
greater change in .Practice Problem 6-3
Chapter (3)Transistors and Integrated Circuits