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Resources » Articles/Knowledge Sharing » Education »
Bipolar Junction Transistors
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Hello Friends,
This article is on Bipolar Junction Transistors. The article will be of great help for students pursuing their b tech in electronics and computer science as well as others also. BJT's are most commonly used in electric circuitry these days. I have taken the data from 2,3 books and also scanned some images(circuits) which i couldn't draw. The data has been compressed in form of notes so that it doesnt goes much in detail for you. It will be easy for you remember. Also the article can be used by others as a means to increase their general knowledge, which can also implemented in electronics projects.
Transistors
Invented in 1948 by John Bardeen, Walter Brattain and Willium Shockley at Bell Labs USA.
They got Nobel Prize in 1956, first time given for an engineering device rather than for a concept.
These replaced bulky vacuum tubes.
Later, field effect transistors (FETs), were developed.
Their operation depends on the influence of electric field on the flow of the majority carriers only.
The basic of electronic system nowadays is semiconductor device.
The famous and commonly use of this device is BJTs
(Bipolar Junction Transistors).
It can be use as amplifier and logic switches.
BJT consists of three terminal:
collector : C
base : B
emitter : E
Type of Transistors
Transistor may be of two types:
Bipolar Junction Transistor (BJT)
There are two types of charge carriers: Free electrons and Hole. Hence called bipolar.
It is a current controlled device.
Field Effect Transistor (FET)
Only one type of carriers (free electrons or holes) are responsible for conduction of current.
It is a voltage controlled device.
Construction of BJTs
Two types : PNP and NPN
3 layer semiconductor device consisting:
2 n- and 1 p-type layers of material ? npn transistor
2 p- and 1 n-type layers of material ?pnp transistor
The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material
A single pn junction has two different types of bias:
forward bias
reverse bias
Thus, a two-pn-junction device has four types of bias.
Base is located at the middle and more thin from the level of collector and emitter.
The emitter and collector terminals are made of the same type of semiconductor material, while the base of the other type of material.
Difference between three regions
Doping wise :
E > C > B
Width wise :
C > E > B
The fact that base is lightly doped and thin is responsible for transistor’s action.
Biasing Conditions
Working of a Transistor
Emitter injection ratio, or emitter efficiency is the ratio of the electron current to the total current. ?=0.995.
Base transportation factor is the ratio of number of electrons arriving at the collector to the number of injected electrons. ß’=0.995.
The basic operation will be described using the pnp transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged.
One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.
Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated.
Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.
A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes.
The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.
Transistor current equation
IE=IC+IB
Collector Reverse Leakage Current
ICBO or ICO is 2 – 5 µA for Ge and 0.1 – 1 µA for Si
Collector current in normal operation
It has two components :
I(INJ), due to carriers injected by the emitter into the base,
ICBO
a0 = ?ß’= 0.98
Basic Amplifier
Common Formula's Used:
Three Configurations or modes
Common Base
Common Emmiter
Common Collector
Review
Type of Transistors.
Construction of BJTs.
Biasing Conditions.
Working of a Transistor.
Transistor current equation.
Collector Reverse Leakage Current.
Basic Amplifier.
Three Configurations or modes.
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