EEL501 Fundamental of Electrical and

Electronics Engineering

Bipolar Junction Transistor (BJT)

Content

• Constructional details of PNP and NPN transistors

• Working of a transistor: Charge transport phenomenon, Transistor
amplifying action, Relation between different currents in a transistor,
Simple problems
• Configuration of transistor (CB, CE and CC)
• Behavior of BJT in Active, Cut off and Saturation regions: Transistor
as a switch, Transistor as an amplifier.
Bipolar Junction Transistors

• The transistor is a three-layer semiconductor device consisting of

either two n- and one p- type layers of material or two p- and one n-
type layers of material.
• The former is called an npn transistor, while the latter is called a pnp
transistor
• So, there are two types of BJT-
i) pnp transistor ii) npn transistor
Bipolar Junction Transistors

In each transistor following points to be noted-

i)There are two junction, so transistor can be considered as two diode
connected back to back.
ii)There are three terminals.
iii)The middle section is thiner than other.
Naming of Transistor Terminals
• Transistor has three section of doped semiconductor.
• The section one side is called “emitter” and the opposite side is
called “collector”.
• The middle section is called “base”.

Transistor
symbol
Naming of Transistor Terminals

1) Emitter:
•The section of one side that supplies carriers is called emitter.
•Emitter is always forward biased wr to base so it can supply carrier.
•For “npn transistor” emitter supply holes to its junction.
•For “pnp transistor” emitter supply electrons to its junction.
Naming of Transistor Terminals

2) Collector:
•The section on the other side that collects carrier is called collector.
•The collector is always reversed biased with respect to base.
•For “npn transistor” collector receives holes to its junction.
•For “pnp transistor” collector receives electrons to its junction.
Naming of Transistor Terminals

3) Base:
•The middle section which forms two pn junction between
emitter and collector is called Base.
Key Points

• The transistor has three region named emitter, base and collector.
• The Base is much thinner than other region.
• Emitter is heavily doped so it can inject large amount of carriers into the
base.
• Base is lightly doped so it can pass most of the carrier to the collector.
• Collector is moderately doped.
Key Points

• The junction between emitter and base is called emitter-base

junction(emitter diode) and junction between base and collector is called
collector-base junction(collector diode).
• The emitter diode is always forward biased and collector diode is reverse
biased.
• The resistance of emitter diode is very small(forward) and resistance of
collector diode is high(reverse).
Transistor Operation

1) Working of npn transistor:

Forward bias Is applied to
emitter-base junction and reverse
bias is applied to collector- base
junction.
The forward bias in the emitter-
base junction causes electrons to
move toward base. This constitute
emitter current, IE
Transistor Operation

1) Working of npn transistor:

As this electrons flow toward p-type base, they try to recombine with
holes. As base is lightly doped only few electrons recombine with
holes within the base.
These recombined electrons constitute small base current.
The remainder electrons crosses base and constitute collector current.
Transistor Operation

2) Working of pnp transistor:

 Forward bias is applied to emitter-
base junction and reverse bias is
applied to collector- base junction.
The forward bias in the emitter-base
junction causes holes to move
toward base. This constitute
emitter current, IE
Transistor Operation

2) Working of pnp transistor:

As this holes flow toward n-type base, they try to recombine with
electrons. As base is lightly doped only few holes recombine with
electrons within the base.
These recombined holes constitute small base current.
The remainder holes crosses base and constitute collector current.
Transistor Symbol
Transistor Operating Modes

• Active Mode
Base- Emitter junction is forward and Base- Collector junction is
reverse biased.
• Saturation Mode
Base- Emitter junction is forward and Base- Collector junction is
forward biased.
• Cut-off Mode
Both junctions are reverse biased.
Transistor Cofiguration

Transistor can be connected in a circuit in following three ways-

Common Base

Common Emitter

Common Collector
Common Base Connection
• The common-base terminology is derived from the fact that the base is
common to both the input and output sides of the configuration.

• First Figure shows common base npn configuration and second

figure shows common base pnp configuration.
Common Base Connection

• Current amplification factor () :

The ratio of change in collector current to the change in emitter current
at constant VCB is known as current amplification factor,.

  IC at constantVCB
IE

Practical value of  is less than unity, but in the range of 0.9 to 0.99
Expression for Collector Current
 Total emitter current does not reach the collector terminal, because a
small portion of it constitute base current. So,
IE  IC  IB
Also, collector diode is reverse biased, so very few minority carrier
passes the collector-base junction which actually constitute leakage
current,ICBO .
So, collector current constitute of portion of emitter current I E
and leakage current ICBO .
I C   IE ICB 0
Expression for Collector Current
Characteristics of Common Base Configuration

• Input Characteristics:
VBE v/s IE characteristics is called input
characteristics.
IE increases rapidly with VBE . It means
input resistance is very small.
IE almost independent of VCB.
Characteristics of Common Base Configuration

Output Characteristics:

VBC vs IC characteristics is
called output characteristics.
IC varies linearly with VBC
,only when VBC is very small.
As, VBC increases, IC becomes
constant.
Common Base Configuration

• Input Resistance: The ratio of change in emitter-base voltage to the

change in emitter current is called Input Resistance.
VBE
ri 
IE

• Output Resistance: The ratio of change in collector-base voltage to

the change in collector current is called Output Resistance.

VBC
r0 
IC
Common Emitter Connection
• The common-emitter terminology is derived from the fact that the emitter is
common to both the input and output sides of the configuration.

• First Figure shows common emitter npn configuration and second figure shows
common emitter pnp configuration.
Common Emitter Connection

• Base Current amplification factor ( ) :

• In common emitter connection input current is base current and output current is
collector current.
• The ratio of change in collector current to the change in base current is known as
base current amplification factor, .
IC

IB
•Normally only 5% of emitter current flows to base, so amplification
factor is greater than 20. Usually this range varies from 20 to 500.
Relation Between  and 
Expression for Collector Current
Characteristics of Common Emitter Configuration

Input Characteristics:
• VBE v/s IB Characteristics is called input
characteristics.
IB increases rapidly with VBE .
It means input resistance is very small.
 IE almost independent of VCE.
IB is of the range of micro amps.
Characteristics of Common Emitter Configuration

• Output Characteristics:

VCE vs Ic characteristics is
called output characteristics.
 IC varies linearly with VCE
,only when VCE is very small.
 As, VCE increases, IC becomes
constant.
Common Emitter Configuration

• Input Resistance: The ratio of change in emitter-base voltage to

the change in base current is called Input Resistance.
V
ri  BE
I B

• Output Resistance: The ratio of change in collector-emitter voltage

to the change in collector current is called Output Resistance.

VCE
r0 
IC
Common Collector Configuration

• The common-collector terminology is derived from the fact that the collector
is common to both the input and output sides of the configuration.

• First Figure shows common collector npn configuration and second figure
shows common collector pnp configuration.
Common Collector Configuration

• Current amplification factor ( ):

• In common emitter connection input current is base current and output current is
emitter current.
• The ratio of change in emitter current to the change in base current is known as
current amplification
• factor in common collector configuration.   I E
I B
• This circuit provides same gain as CE configuration as.
I E  IC
Relation Between α and γ
Expression for Collector Current
Comparison of Transistor Connection
Transistor as an amplifier in CE conf.

• Figure shows CE amplifier for npn transistor.

• Battery VBB is connected with base in-order to make base forward biased,
regardless of input ac polarity.
• Output is taken across Load R
Transistor as an amplifier in CE conf.

• During positive half cycle input ac will keep the emitter- base junction more
forward biased. So, more carrier will be emitted by emitter, this huge current
will flow through load and we will find output amplified signal.
• During negative half cycle input ac will keep the emitter-base junction less
forward biased. So, less carrier will be emitted by emitter. Hence collector
current decreases.
• This results in decreased output voltage (In opposite direction).