UNIT:2
Transistors
Basics of Transistors: PNP and NPN transistors, relationship between α and β, Transistor
configuration & characteristics for CB, CE, CC, load line and biasing methods of transistor,
Transistor as an amplifier: CE amplifier. Overview of FET and MOSFET
A Transistor is a three terminal semiconductor device that regulates current or voltage
flow and acts as a switch or gate for signals.
Why Do We Need Transistors?
Suppose that you have a FM receiver which grabs the signal you want. The received signal
will obviously be weak due to the disturbances it would face during its journey. Now if this
signal is read as it is, you cannot get a fair output. Hence we need to amplify the signal.
Amplification means increasing the signal strength.
This is just an instance. Amplification is needed wherever the signal strength has to be
increased. This is done by a transistor. A transistor also acts as a switch to choose between
available options. It also regulates the incoming current and voltage of the signals.
Constructional Details of a Transistor
The Transistor is a three terminal solid state device which is formed by connecting two
diodes back to back. Hence it has got two PN junctions. Three terminals are drawn out of
the three semiconductor materials present in it. This type of connection offers two types of
transistors. They are PNP and NPN which means an N-type material between two Ptypes
and the other is a P-type material between two N-types respectively.
The construction of transistors is as shown in the following figure which explains the idea
discussed above.
Ms. Hinal Surati
,The three terminals drawn from the transistor indicate Emitter, Base and Collector
terminals. They have their functionality as discussed below.
Emitter
The left hand side of the above shown structure can be understood as Emitter.
This has a moderate size and is heavily doped as its main function is to supply a
number of majority carriers, i.e. either electrons or holes.
As this emits electrons, it is called as an Emitter.
This is simply indicated with the letter E.
Base
The middle material in the above figure is the Base.
This is thin and lightly doped.
Its main function is to pass the majority carriers from the emitter to the collector.
This is indicated by the letter B.
Collector
The right side material in the above figure can be understood as a Collector.
Its name implies its function of collecting the carriers.
This is a bit larger in size than emitter and base. It is moderately doped.
This is indicated by the letter C.
The symbols of PNP and NPN transistors are as shown below.
The arrow-head in the above figures indicated the emitter of a transistor. As the collector
of a transistor has to dissipate much greater power, it is made large. Due to the specific
Ms. Hinal Surati
,functions of emitter and collector, they are not interchangeable. Hence the terminals are
always to be kept in mind while using a transistor.
In a Practical transistor, there is a notch present near the emitter lead for identification.
The PNP and NPN transistors can be differentiated using a Multimeter. The following figure
shows how different practical transistors look like.
Transistor Biasing
As we know that a transistor is a combination of two diodes, we have two junctions here.
As one junction is between the emitter and base, that is called as Emitter-Base junction
and likewise, the other is Collector-Base junction.
Biasing is controlling the operation of the circuit by providing power supply. The function
of both the PN junctions is controlled by providing bias to the circuit through some dc
supply. The figure below shows how a transistor is biased.
By having a look at the above figure, it is understood that
The N-type material is provided negative supply and P-type material is given
positive supply to make the circuit Forward bias.
Ms. Hinal Surati
, The N-type material is provided positive supply and P-type material is given
negative supply to make the circuit Reverse bias.
By applying the power, the emitter base junction is always forward biased as the emitter
resistance is very small. The collector base junction is reverse biased and its resistance
is a bit higher. A small forward bias is sufficient at the emitter junction whereas a high
reverse bias has to be applied at the collector junction.
The direction of current indicated in the circuits above, also called as the Conventional
Current, is the movement of hole current which is opposite to the electron current.
Operation PNP Transistor
The operation of a PNP transistor can be explained by having a look at the following figure,
in which emitter-base junction is forward biased and collector-base junction is reverse
biased.
The voltage VEE provides a positive potential at the emitter which repels the holes in the P-
type material and these holes cross the emitter-base junction, to reach the base region.
There a very low percent of holes recombine with free electrons of N-region. This provides
very low current which constitutes the base current IB. The remaining holes cross the
collector-base junction, to constitute collector current IC, which is the hole current.
As a hole reaches the collector terminal, an electron from the battery negative terminal fills
the space in the collector. This flow slowly increases and the electron minority current
flows through the emitter, where each electron entering the positive terminal of VEE, is
replaced by a hole by moving towards the emitter junction. This constitutes emitter current
IE .
Hence we can understand that −
The conduction in a PNP transistor takes place through holes.
Ms. Hinal Surati
Transistors
Basics of Transistors: PNP and NPN transistors, relationship between α and β, Transistor
configuration & characteristics for CB, CE, CC, load line and biasing methods of transistor,
Transistor as an amplifier: CE amplifier. Overview of FET and MOSFET
A Transistor is a three terminal semiconductor device that regulates current or voltage
flow and acts as a switch or gate for signals.
Why Do We Need Transistors?
Suppose that you have a FM receiver which grabs the signal you want. The received signal
will obviously be weak due to the disturbances it would face during its journey. Now if this
signal is read as it is, you cannot get a fair output. Hence we need to amplify the signal.
Amplification means increasing the signal strength.
This is just an instance. Amplification is needed wherever the signal strength has to be
increased. This is done by a transistor. A transistor also acts as a switch to choose between
available options. It also regulates the incoming current and voltage of the signals.
Constructional Details of a Transistor
The Transistor is a three terminal solid state device which is formed by connecting two
diodes back to back. Hence it has got two PN junctions. Three terminals are drawn out of
the three semiconductor materials present in it. This type of connection offers two types of
transistors. They are PNP and NPN which means an N-type material between two Ptypes
and the other is a P-type material between two N-types respectively.
The construction of transistors is as shown in the following figure which explains the idea
discussed above.
Ms. Hinal Surati
,The three terminals drawn from the transistor indicate Emitter, Base and Collector
terminals. They have their functionality as discussed below.
Emitter
The left hand side of the above shown structure can be understood as Emitter.
This has a moderate size and is heavily doped as its main function is to supply a
number of majority carriers, i.e. either electrons or holes.
As this emits electrons, it is called as an Emitter.
This is simply indicated with the letter E.
Base
The middle material in the above figure is the Base.
This is thin and lightly doped.
Its main function is to pass the majority carriers from the emitter to the collector.
This is indicated by the letter B.
Collector
The right side material in the above figure can be understood as a Collector.
Its name implies its function of collecting the carriers.
This is a bit larger in size than emitter and base. It is moderately doped.
This is indicated by the letter C.
The symbols of PNP and NPN transistors are as shown below.
The arrow-head in the above figures indicated the emitter of a transistor. As the collector
of a transistor has to dissipate much greater power, it is made large. Due to the specific
Ms. Hinal Surati
,functions of emitter and collector, they are not interchangeable. Hence the terminals are
always to be kept in mind while using a transistor.
In a Practical transistor, there is a notch present near the emitter lead for identification.
The PNP and NPN transistors can be differentiated using a Multimeter. The following figure
shows how different practical transistors look like.
Transistor Biasing
As we know that a transistor is a combination of two diodes, we have two junctions here.
As one junction is between the emitter and base, that is called as Emitter-Base junction
and likewise, the other is Collector-Base junction.
Biasing is controlling the operation of the circuit by providing power supply. The function
of both the PN junctions is controlled by providing bias to the circuit through some dc
supply. The figure below shows how a transistor is biased.
By having a look at the above figure, it is understood that
The N-type material is provided negative supply and P-type material is given
positive supply to make the circuit Forward bias.
Ms. Hinal Surati
, The N-type material is provided positive supply and P-type material is given
negative supply to make the circuit Reverse bias.
By applying the power, the emitter base junction is always forward biased as the emitter
resistance is very small. The collector base junction is reverse biased and its resistance
is a bit higher. A small forward bias is sufficient at the emitter junction whereas a high
reverse bias has to be applied at the collector junction.
The direction of current indicated in the circuits above, also called as the Conventional
Current, is the movement of hole current which is opposite to the electron current.
Operation PNP Transistor
The operation of a PNP transistor can be explained by having a look at the following figure,
in which emitter-base junction is forward biased and collector-base junction is reverse
biased.
The voltage VEE provides a positive potential at the emitter which repels the holes in the P-
type material and these holes cross the emitter-base junction, to reach the base region.
There a very low percent of holes recombine with free electrons of N-region. This provides
very low current which constitutes the base current IB. The remaining holes cross the
collector-base junction, to constitute collector current IC, which is the hole current.
As a hole reaches the collector terminal, an electron from the battery negative terminal fills
the space in the collector. This flow slowly increases and the electron minority current
flows through the emitter, where each electron entering the positive terminal of VEE, is
replaced by a hole by moving towards the emitter junction. This constitutes emitter current
IE .
Hence we can understand that −
The conduction in a PNP transistor takes place through holes.
Ms. Hinal Surati
