ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
1
17/05/01
Lecture 11
The Bipolar Junction Transistor
The term Bipolar is because two type of charges
(electrons and holes) are involved in the flow of
electricity
The term Junction is because there are two pn
junctions
There are two configurations for this device
n-type p-type n-type
NPN Symbol
b
e
c
p-type n-type p-type
PNP Symbol
b
c
e
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
2
17/05/01
Lecture 11
NPN and PNP Transistors
NPN is more widely used
– majority carriers are electrons so it operates more quickly
PNP is used for special applications
– we will concentrate on NPN
The terminals of the transistor are labelled Base,
Emitter,and Collector
– The emitter is always drawn with the arrow.
Base
Emitter
Collector
Base
Emitter
Collector
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
3
17/05/01
Lecture 11
Asymmetry of Transistor
Although in the NPN schematic,it looks like the
collector and emitter can be reversed,in reality the
device is very inefficient in reverse connection and
has very little amplification (gain).
n-type p-type n-type
NPN
is really
collector base emitter collector base emitter
NPNquite asymmetric
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
4
17/05/01
Lecture 11
Operation of NPN Transistor
In normal operation,the EB junction is forward biased and the
BC junction is reverse biased
The base region is very thin so the ratio L1:L2 is typically about
150:1
DC DC
n p ne
b
c
Ie Ic
Ib
L1
L2
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
5
17/05/01
Lecture 11
Behaviour
Forward biasing of the EB junction causes a heavy flow of
majority carriers (electrons) from the n-type material into the
base junction and also majority carriers (holes) from the base
region into the emitter region,We denote this current Ie.
The transistor is made so that nearly all the current Ie consists of
a flow of carriers (electrons) from emitter to the base,This is
achieved by making the emitter much more heavily doped than
the base.
DC DC
n p ne
b
c
Ie Ic
Ib
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
6
17/05/01
Lecture 11
Behaviour
The base region is very thin so that most of the electrons
attracted to this region pass straight through it (attracted by the
collector which is positive relative to the base) before there is
much chance of recombination with the base’s holes.
Because of this,the collector current is very nearly equal in
value to Ie,Thus with the current directions shown Ic=-αIe where
α is close to unity (e.g.,α=0.98).
DC DC
n p ne
b
c
Ie Ic
Ib
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
7
17/05/01
Lecture 11
Behaviour
The current that does not go through the collector forms the
base current so that we have Ib=-(1-α)Ie.
From this
Typically β=50 to 200
The parameter β is called the DC current gain and represents
the current amplification of the transistor.
Indeed the use of the transistor as an
amplifier is one of its main applications
Another major application is using the
transistor as a switch
baa =?= 1
b
c
I
I
So what do
they look
like?
School of Computer Science and Electrical Engineering
1
17/05/01
Lecture 11
The Bipolar Junction Transistor
The term Bipolar is because two type of charges
(electrons and holes) are involved in the flow of
electricity
The term Junction is because there are two pn
junctions
There are two configurations for this device
n-type p-type n-type
NPN Symbol
b
e
c
p-type n-type p-type
PNP Symbol
b
c
e
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
2
17/05/01
Lecture 11
NPN and PNP Transistors
NPN is more widely used
– majority carriers are electrons so it operates more quickly
PNP is used for special applications
– we will concentrate on NPN
The terminals of the transistor are labelled Base,
Emitter,and Collector
– The emitter is always drawn with the arrow.
Base
Emitter
Collector
Base
Emitter
Collector
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
3
17/05/01
Lecture 11
Asymmetry of Transistor
Although in the NPN schematic,it looks like the
collector and emitter can be reversed,in reality the
device is very inefficient in reverse connection and
has very little amplification (gain).
n-type p-type n-type
NPN
is really
collector base emitter collector base emitter
NPNquite asymmetric
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
4
17/05/01
Lecture 11
Operation of NPN Transistor
In normal operation,the EB junction is forward biased and the
BC junction is reverse biased
The base region is very thin so the ratio L1:L2 is typically about
150:1
DC DC
n p ne
b
c
Ie Ic
Ib
L1
L2
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
5
17/05/01
Lecture 11
Behaviour
Forward biasing of the EB junction causes a heavy flow of
majority carriers (electrons) from the n-type material into the
base junction and also majority carriers (holes) from the base
region into the emitter region,We denote this current Ie.
The transistor is made so that nearly all the current Ie consists of
a flow of carriers (electrons) from emitter to the base,This is
achieved by making the emitter much more heavily doped than
the base.
DC DC
n p ne
b
c
Ie Ic
Ib
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
6
17/05/01
Lecture 11
Behaviour
The base region is very thin so that most of the electrons
attracted to this region pass straight through it (attracted by the
collector which is positive relative to the base) before there is
much chance of recombination with the base’s holes.
Because of this,the collector current is very nearly equal in
value to Ie,Thus with the current directions shown Ic=-αIe where
α is close to unity (e.g.,α=0.98).
DC DC
n p ne
b
c
Ie Ic
Ib
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs
School of Computer Science and Electrical Engineering
7
17/05/01
Lecture 11
Behaviour
The current that does not go through the collector forms the
base current so that we have Ib=-(1-α)Ie.
From this
Typically β=50 to 200
The parameter β is called the DC current gain and represents
the current amplification of the transistor.
Indeed the use of the transistor as an
amplifier is one of its main applications
Another major application is using the
transistor as a switch
baa =?= 1
b
c
I
I
So what do
they look
like?
