Course Name: Semiconductor Physics
Course Number: PHYS 473
Bipolar Junction Transistors
(BJT)
Transistor

A three lead semiconductor device that acts as:
(1) Switches
(2) Amplifier.
(3) tuning device
(4) oscillators
Bipolar Junction Transistor (BJT)

A bipolar junction transistor (BJT) is a type of transistor that relies on
the contact of two types of semiconductor for its operation.
(1) NPN
(2) PNP
BJT types


A transistor consisting of two n- and one p-type layers of material called NPN
transistor.
A transistor consisting of two p- and one n-type layers of material called PNP
transistor.
1
Configurations
1: Common Emitter Configuration (CE)
2: Common Base Configuration (CB)
2
3: Common Collector Configuration (CC)
3
NPN BJT: How it works
NPN Transistor in a Complete Circuit
(1)
(2)
A simplified structure of the npn transistor


Transistor consists of two pn-junctions:

emitter-base junction (EBJ)

collector-base junction (CBJ)
Operating mode depends on biasing.

active mode – used for amplification

cutoff and saturation modes – used for switching.
Current Flow
Current Flow


Forward bias on emitter-base junction will cause current to flow.
This current has two components:

electrons injected from emitter into base

holes injected from base into emitter.
Current Flow
Kirchhoff's Current Law (KCL) is: at any point
Sum of input currents = Sum of output currents
I E  I B  IC
IC
CE current gain= 
IB
Valueof  is usually between 20 to 200
IC
CB current gain   
IE
Valueof  is usually less thanunity, i.e.   1
Example-1

An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate
the base current ( I B ) required to switch a resistive load of 4mA.

Solution:
Given :   200
I C  4mA  4 103 A
IB  ?
IC
current gain= 
IB
4 103 A 4 103 A 4 103 A
 IB 




200
200
2 102
IC
 2 103 2 A  2 105 A  20 106 A
 20 A 1  106
 I B  20  A
Example-2


If the collector current Ic= 7.95mA and the emitter current Ie=8mA, then
calculate current gain alpha.
Solution:
Given : I C  7.95 mA  7.95 103 A
I E  8mA  8 103 A
 ?
IC
current gain= 
IE
I C 7.95 103 A
 

 0.994
3
IE
8 10 A
   0.994
Input characteristics
output I-V characteristics
I-V Characteristics of BJT common emitter
configuration
Modes of Operation

Depending on the bias condition on its two junctions, the BJT can operate
in one of three possible modes:

cut-off (both junctions reverse biased)

active (the EBJ forward-biased and CBJ reversed)

saturation (both junctions forward biased)
Summary of Bipolar Junction Transistors






The Bipolar Junction Transistor (BJT) is a three layer device constructed form
two semiconductor diode junctions joined together, one forward biased and one
reverse biased.
There are two main types of BJT, the NPN and the PNP transistor.
Transistors are "Current Operated Devices" where a much smaller Base current
causes a larger Emitter to Collector current, which themselves are nearly equal,
to flow.
The most common transistor connection is the Common-emitter
configuration.
Requires a Biasing voltage for AC amplifier operation.
The output characteristics curves can be used to find either Ib, Ic or β to which
a load line can be constructed to determine a suitable operating point, Q with
variations in base current determining the operating range.