DELD_SE_Information Technology

1. Transistor
• The name "Transistor" came from the word transfer resistor. These were the
semiconductor devices which replaced the Vaccum tubes which lead to lot of
developments in Semiconductor technology.
• What is a Transistors?
– It is a semiconductor device having two junctions and three terminals.
• The transistors have three leads or terminals:
– EMITTER
– BASE
– COLLECTOR
• Emitter (E) is a heavily doped region of the device and is a supplier of majority
charge carriers to the base.
• Base (B) is made thin and is lightly doped. This is done to reduce the
recombination process.
• Collector (C) is moderately doped and collects majority carriers through base.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

2. Transistor
• Transistors are heart of today's circuits. Transistors are
the device which are primarily used for two purposes
in today electronic circuits:
• Amplifier: it can amplify the current or voltage of the
input. Amplification is the process of increasing the
strength of the signal by using external source.
• Switch: The transistors are used in various modern
electronic circuits. It can be used as switch
to "ON" or "OFF" the flow of the current in the circuit.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

3. Transistor
Current flow in the opposite direction of the electrons flow;
same direction as holes
e e e
I
h h h
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

4. Types
• Two Types of Transistors
– Bipolar Junction Transistor
– Field Effect Transistor
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

5. Bipolar Junction Transistor
 The bipolar junction transistor (BJT) has three separately
doped regions and contains two pn junctions.
 Bipolar transistor is a 3-terminal device.
 Emitter (E)
 Base (B)
 Collector (C)
 The basic transistor principle is that the
voltage between two terminals controls the
current through the third terminal.
 Current in the transistor is due to the flow of
both electrons and holes, hence the name
bipolar.10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

6. BJT Structures
 There are two types of bipolar junction transistor: npn and
pnp.
 The npn bipolar transistor contains a thin p-region between
two n-regions.
 The pnp bipolar transistor
contains a thin n-region
sandwiched between two p-
regions.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

7. A Biased n-p-n Transistor
Base (B) is used as the terminal to control the
flow of current between the Collector (C) and
Emitter (E).
The VEE supply is used to forward bias emitter
base junction.
EB Forward Biased and CB Reverse Biased
Due to very less recombination
process, the base current
IB = IC - IE flows.
Apply Kirchhoff current law,
IE = IB + IC where
IE = Emitter Current
IB = Base Current
IC = Collector Current.
• As a result significant current flows, once the
potential barrier is exceeded. The majority
charge carriers diffuse from emitter into base
and this results in emitter current IE as indicated
in the above diagram. Once these reach the
base, very few electrons or hole undergo
recombination process and rest diffuse through
collector base junction due to potential on the
collector side.
• In a PNP the diffused holes reach the collector
and at the same time an electron from the
emitter enters into the positive pole of
VEE thereby creating a hole in the emitter. Thus
the current in PNP is caused by holes and the
current in external circuit by electrons.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

8. The operation of BJT depends upon the biasing of the two
junctions.
 Active
Operating range of the amplifier.
Base-Emitter Junction forward biased.
Collector-Base Junction reverse biased
Output (collector) current is linearly depend
upon the input (emitter) current.
Used as Switch
 Cutoff
The amplifier is basically off. There is voltage
but little current.
Both junctions reverse biased, very small
saturation current pass across the junctions.
3 Regions of Operation
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

9. 3 Regions of Operation
Saturation
The amplifier is full on. There is little voltage
but lots of current.
Both junctions forward biased
Importance in Transistor Switch
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

10. Transistor Configuration
• Transistors can be used in any of the
configurations
– Common Base (CB)
– Common Emitter (CE)
– Common Collector (CC)
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

11.  The Emitter is common to both input
(base-emitter) and output (collector-
emitter).
 Since Emitter is grounded, VC = VCE
 With decreasing VC (VCE), the junction
B-C will become forward biased too.
 The current IC quickly drops to zero
because electrons are no longer
collected by the collector
Common-Emitter Configuration - npn
Node B
0V
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

12. Transistor as a Switch
• A small Positive voltage at Base, Positive voltage to collector and
Negative voltage to emitter is being applied.
• The voltage at Base is slightly positive than that of Emitter and the
voltage at Collector is more positive than that of Base region.
• Due to this arrangement the base region attracts electrons from the
emitter region and the collector attracts the electrons from the
base region. So, the electrons flow from emitter to collector and as
we know that the flow of electrons is opposite to that flow of
current and hence the current flows from Collector to Emitter.
• From this we can conclude that the transistor acts as switch
facilitating the flow of current from the Collector to Emitter.
• Now take the case that the Base region is not connected to voltage
supply, so the there would not be any charge flow from Collector to
Emitter and hence the circuit acts as switch blocking the flow of
current from the Collector to Emitter.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

13. Transistor as a Switch
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

14. Field Effect Transistor (FET)
• FET consists of a bar of n or p type of silicon with
ohmic contacts at the two ends.
• Current flows along the length of the bar when a
voltage supply is connected between the ends.
• It is referred to as an n-channel or p-channel
device depending upon the type of material used.
• In n-channel, the current flow is due to the flow
of electrons (majority carriers)
• In p-channel, the current flow is due to the flow
of holes
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

15. FET
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

16. Field Effect Transistor (FET)
• FET is another 3 terminal
semiconductor device which can
be used as a fast operating
switch.
• A Field effect transistor has two
layers of semiconductor material,
as shown in figure. The two layers
are such that they form
a channel through which
electricity flows.
• FET has three terminals, namely
– Gate
– Source
– Drain.
10/20/2018
Prepared by Ms. K. D. Patil, SCOE,
Kopargaon

17. FET
• Gate is the channel that modulates the conductivity.
• Source is a terminal where the majority carriers enter the channel.
• Drain is the terminal where the majority carriers leave the channel.
• The voltage connected gate, interferes with the current flowing
from source to drain. Hence the gate controls the flow of current in
the channel, i.e. between source and drain. By increasing or
decreasing voltage at gate, the current in the channel can be
controlled.
• The Field effect transistors are of two types :
– Junction Field Effect Transistor (JFET)
– Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
10/20/2018 Prepared by Ms. K. D. Patil, SCOE, Kopargaon