Bipolar Junction Transistor Notes | EduRev

: Bipolar Junction Transistor Notes | EduRev

 Page 1


1
Chapter  #  5
Bipolar Junction Transistor
Bipolar Transistor
A Bipolar Transistor essentially 
consists of a pair of PN Junction 
Diodes that are joined back-to-
back. This forms a sort of a 
sandwich where one kind of 
semiconductor is placed in 
between two others.
There are two kinds of Bipolar 
sandwich, the NPNand PNP 
varieties. The three layers of the 
sandwich are conventionally 
called the Collector, Base, and 
Emitter. The reasons for these 
names will become clear later 
once we see how the transistor 
works. 
Some of the basic properties exhibited by a Bipolar Transistor are immediately 
recognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairly 
thin some interesting effects become possible that allow us to use the Transistor as an 
amplifier or a switch
Bipolar device 
Unipolar device
Bipolar Junction Transistor
Page 2


1
Chapter  #  5
Bipolar Junction Transistor
Bipolar Transistor
A Bipolar Transistor essentially 
consists of a pair of PN Junction 
Diodes that are joined back-to-
back. This forms a sort of a 
sandwich where one kind of 
semiconductor is placed in 
between two others.
There are two kinds of Bipolar 
sandwich, the NPNand PNP 
varieties. The three layers of the 
sandwich are conventionally 
called the Collector, Base, and 
Emitter. The reasons for these 
names will become clear later 
once we see how the transistor 
works. 
Some of the basic properties exhibited by a Bipolar Transistor are immediately 
recognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairly 
thin some interesting effects become possible that allow us to use the Transistor as an 
amplifier or a switch
Bipolar device 
Unipolar device
Bipolar Junction Transistor
2
Page 3


1
Chapter  #  5
Bipolar Junction Transistor
Bipolar Transistor
A Bipolar Transistor essentially 
consists of a pair of PN Junction 
Diodes that are joined back-to-
back. This forms a sort of a 
sandwich where one kind of 
semiconductor is placed in 
between two others.
There are two kinds of Bipolar 
sandwich, the NPNand PNP 
varieties. The three layers of the 
sandwich are conventionally 
called the Collector, Base, and 
Emitter. The reasons for these 
names will become clear later 
once we see how the transistor 
works. 
Some of the basic properties exhibited by a Bipolar Transistor are immediately 
recognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairly 
thin some interesting effects become possible that allow us to use the Transistor as an 
amplifier or a switch
Bipolar device 
Unipolar device
Bipolar Junction Transistor
2 3
Base-collector junction at reverse 
bias
Recombination current in the base
• Some of the free electrons 
crossing the base 
encounter a hole and 
‘drop into it’. As a result 
the base region loses one 
of its positive charges 
(holes) each time this 
happens.
• For particle BJP only 
about 1% of the free 
electrons which try to 
cross base region get 
caught in this way. Hence 
we see a base current, I
B
, 
which is typically around 
one hundred times 
smaller than the emitter 
current, I
E
.
Page 4


1
Chapter  #  5
Bipolar Junction Transistor
Bipolar Transistor
A Bipolar Transistor essentially 
consists of a pair of PN Junction 
Diodes that are joined back-to-
back. This forms a sort of a 
sandwich where one kind of 
semiconductor is placed in 
between two others.
There are two kinds of Bipolar 
sandwich, the NPNand PNP 
varieties. The three layers of the 
sandwich are conventionally 
called the Collector, Base, and 
Emitter. The reasons for these 
names will become clear later 
once we see how the transistor 
works. 
Some of the basic properties exhibited by a Bipolar Transistor are immediately 
recognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairly 
thin some interesting effects become possible that allow us to use the Transistor as an 
amplifier or a switch
Bipolar device 
Unipolar device
Bipolar Junction Transistor
2 3
Base-collector junction at reverse 
bias
Recombination current in the base
• Some of the free electrons 
crossing the base 
encounter a hole and 
‘drop into it’. As a result 
the base region loses one 
of its positive charges 
(holes) each time this 
happens.
• For particle BJP only 
about 1% of the free 
electrons which try to 
cross base region get 
caught in this way. Hence 
we see a base current, I
B
, 
which is typically around 
one hundred times 
smaller than the emitter 
current, I
E
.
4
Page 5


1
Chapter  #  5
Bipolar Junction Transistor
Bipolar Transistor
A Bipolar Transistor essentially 
consists of a pair of PN Junction 
Diodes that are joined back-to-
back. This forms a sort of a 
sandwich where one kind of 
semiconductor is placed in 
between two others.
There are two kinds of Bipolar 
sandwich, the NPNand PNP 
varieties. The three layers of the 
sandwich are conventionally 
called the Collector, Base, and 
Emitter. The reasons for these 
names will become clear later 
once we see how the transistor 
works. 
Some of the basic properties exhibited by a Bipolar Transistor are immediately 
recognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairly 
thin some interesting effects become possible that allow us to use the Transistor as an 
amplifier or a switch
Bipolar device 
Unipolar device
Bipolar Junction Transistor
2 3
Base-collector junction at reverse 
bias
Recombination current in the base
• Some of the free electrons 
crossing the base 
encounter a hole and 
‘drop into it’. As a result 
the base region loses one 
of its positive charges 
(holes) each time this 
happens.
• For particle BJP only 
about 1% of the free 
electrons which try to 
cross base region get 
caught in this way. Hence 
we see a base current, I
B
, 
which is typically around 
one hundred times 
smaller than the emitter 
current, I
E
.
4 5
Example 3.1 p119
Calculate the collector and emitter currents, given the base current and current gain. 
Assume a common-emitter current gain ß = 150 and a base current of i
B
= 15 µA. Also 
assume that the transistor is biased in the forward-active mode.   
mA 25 . 2 A µ 15 150 = × = =
B F C
i i ß
mA 27 . 2 µA 15 151 ) 1 ( = × = + =
B F E
i i ß
9934 . 0
150 1
150
1
=
+
=
+
=
F
F
F
ß
ß
a
Read More
Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!