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