The document provides a comprehensive overview of bipolar junction transistors (BJTs) and field-effect transistors (FETs), including their structures, operation principles, and applications. It discusses different types of BJTs, such as NPN and PNP, and various FETs like junction field-effect transistors (JFETs) and metal-oxide-semiconductor field-effect transistors (MOSFETs), emphasizing their characteristics and operational regions. Additionally, it highlights the advantages and disadvantages of each type of transistor in electronic circuit applications.

1. Bipolar Transistor
• A transistor is basically a Si or Ge crystal containing three
separate regions.

2. Architecture of BJTs
• The bipolar junction transistor (BJT) is constructed
with three doped semiconductor regions separated by
two pn junctions
• Regions are called emitter, base and collector

3. Bipolar Transistor
• The depletion layers do not have the same width,
because different regions have different doping
levels.

4. Biasing
• If both the junctions are forward biased free electrons
enter the emitter and collector of the transistor, joins at
the base and come out of the base.
• If both the junction are reverse biased then small
currents flows through both junctions only due to
thermally produced minority carriers and surface
leakage.

5. Biasing
• When the emitter junction is forward biased and
collector junction is reverse biased then one
expect large emitter current and small collector
current but collector current is almost as large as
emitter current.

6. Transistor Applications
• Switching
• Amplification
• Variable Resistor
• Impedance
Matching
• Voltage Regulation

7. Basic circuits of BJT

8. Operation of BJTs
• BJT will operates in one of following
four region
– Cutoff region (for digital circuit)
– Saturation region (for digital circuit)
– Linear (active) region (to be an amplifier)
– Breakdown region (always be a disaster)

9. Operation of BJTs

10. DC Analysis of BJTs
• Transistor Currents:
IE
= IC
+ IB
• alpha (αDC
)
IC
= αDC
IE
• beta (βDC
)
IC
= βDC
IB
– βDC
typically has a value between 20 and
200

11. DC Analysis of BJTs
• DC voltages for the biased transistor:
• Collector voltage
VC
= VCC
- IC
RC
• Base voltage
VB
= VE
+ VBE
– for silicon transistors, VBE
= 0.7 V
– for germanium transistors, VBE
= 0.3 V

12. Common Base Connection of BJT

13. Common Base Connection of BJT

14. Common Emitter Connection of BJT

15. Common Emitter Connection of BJT

16. Common Collector Connection of BJT

17. Common Collector Connection of BJT

18. Comparision

19. Junction Field Effect Transistor (JFET)
• A field effect transistor is a voltage controlled device
• i.e. the output characteristics of the device are controlled by
input voltage.
• There are two basic types of field effect transistors:
• Junction field effect transistor (JFET)
• Metal oxide semiconductor field effect transistor (MOSFET)

20. Junction Field Effect Transistor (JFET)
• A JFET is a three terminal semiconductor device in which current
conduction is by one type of carrier i.e. electrons or holes.
• The current conduction is controlled by means of an electric field
between the gate and the conducting channel of the device.
• The JFET has high input impedance and low noise level.

21. Components of FETs
N-channel JFET - the middle of the opposite phases of the specimen will be
diffused with the P-type semiconductor.
Channel: This is the area in which majority charge carriers flow. When the majority charge
carriers are entered in FET, then with the help of this channel only they flow from source to
drain.
Source: Source is the terminal through which the majority charge carriers are introduced in
the FET.
Drain: Drain is the collecting terminal in which the majority charge carriers enter and thus
contribute in the conduction procedure.
Gate: Gate terminal is formed by diffusion of a type of semiconductor with another type of
semiconductor. It basically creates depletion region which controls the flow of carrier from
source to drain.

22. Junction Field Effect Transistor (JFET)
• The bar forms the conducting channel for the charge carriers.
• If the bar is of p-type, it is called p-channel JFET as shown in fig.1(i) and if the bar is
of n-type, it is called n-channel JFET as shown in fig.1(ii).
• The two pn junctions forming diodes are connected internally and a common
terminal called gate is taken out.
• Other terminals are source and drain taken out from the bar as shown in fig.1.
• Thus a JFET has three terminals such as , gate (G), source (S) and drain (D).
fig.1(ii).
fig.1(i).

23. Junction Field Effect Transistor (JFET)
Case-i:
▪When a voltage VDS
is applied between drain
and source terminals and voltage on the gate is
zero as shown in fig.
▪ The two pn junctions at the sides of the bar
establish depletion layers.
▪The electrons will flow from source to drain
through a channel between the depletion layers.
▪The size of the depletion layers determines the
width of the channel and hence current
conduction through the bar.

24. Junction Field Effect Transistor (JFET)
Case-ii:
• When a reverse voltage VGS is applied between gate and
source terminals, as shown in fig, the width of depletion
layer is increased.
• This reduces the width of conducting channel, thereby
increasing the resistance of n-type bar.
• Consequently, the current from source to drain is
decreased.
• On the other hand, when the reverse bias on the gate is
decreased, the width of the depletion layer also
decreases.
• This increases the width of the conducting channel and
hence source to drain current.

32. Junction Field Effect Transistor (JFET)

33. Mainly the JFET operates in ohmic, saturation, cut-off and break-down
regions.
Ohmic Region:
If VGS = 0 then the depletion region of the channel is very small and
in this region the JFET acts as a voltage controlled resistor.
Pinched-off Region:
This is also called as cut-off region.
The JFET enters into this region when the gate voltage is large
negative, then the channel closes i.e.no current flows through the
channel.
Saturation or Active Region:
In this region the channel acts as a good conductor which is
controlled by the gate voltage (VGS).
Breakdown Region:
If the drain to source voltage (VDS) is high enough, then the channel of
the JFET breaks down and in this region uncontrolled maximum
current passes through the device.

34. Transfer Characteristics

35. The drain current ID
flowing through the channel can be calculated as follows.
ID
= IDSS
(1-(VGS
/VP
))2
Where
ID
= Drain current
IDSS
= maximum saturation current
VGS
= gate to source voltage
VP
= pinched-off voltage
The drain-source resistance is equal to the ratio of the rate of change in drain-source
voltage and rate of change in drain current.
RDS
= Δ VDS
/ Δ ID
= 1/gm
Where
RDS
= drain-source resistance
VDS
= drain to source voltage
ID
= drain current
gm
= Transconductance

36. Junction Field Effect Transistor (JFET)

37. p-channel JFET
A p-channel JFET operates in the same manner as an
n-channel JFET except that channel current carriers will be
the holes instead of electrons and polarities of VGS
and
VDS
are reversed.

38. Junction Field Effect Transistor (JFET)
Applications
▪ Switch, chopper, buffer, oscillatory circuits, cascade amplifiers
Advantages
▪ High impedance
▪ Low power consumption devices
▪ JFET can be fabricated in a smaller size, and as a result, they occupy
less space in circuits due to their smaller size.
Disadvantages
▪ It has a low gain-bandwidth product
▪ The performance of JFET is affected as frequency increases due to
feedback by internal capacitance.

39. MOSFETs are electronic devices used to switch or amplify voltages
in circuits.
It is a voltage controlled device
It is a four-terminal device with source (S), drain (D) and gate
Terminal (G) and body (B) terminals.
The body is frequently connected to the source terminal, reducing
the terminals to three.
Metal Oxide Silicon Field Effect Transistors(MOSFET)

40. Depletion Mode
The depletion-mode MOSFET is equivalent to a “Normally Closed” switch
The transistor requires the Gate-Source voltage (VGS) to switch the device “OFF”.
Enhancement Mode
The enhancement-mode MOSFET is equivalent to a “Normally Open” switch.
The transistor requires a Gate-Source voltage(VGS) to switch the device “ON”.

41. The depletion-mode MOSFET

42. The depletion-mode MOSFET
• The lightly doped P-type substrate contains two heavily doped N-type material
thus forming source and drain.
• A thin layer of SiO2
is deposited over the surface and holes are then cut through
SiO2
.
• Metals are deposited through holes which resultantly forms drain and source
terminal.
• A metal plate is also deposited in between the source and drain terminal which
acts as gate terminal for the device.

43. The depletion-mode MOSFET Working
•In a depletion type or D-MOS, a channel for conduction is already
constructed physically.
•Due to this, current flows in between the source and
drain without any gate bias voltage. This means that the channel
conducts even when VGS
= 0.

44. The depletion-mode MOSFET Working
▪ DE-MOSFET has the ability to work at both positive and negative gate potential.
▪ When the MOSFET is operated with 0 gate voltage it is said that the device is
operating in E-mode.
▪ In a DE-MOSFET when the gate potential is made negative with respect to the
substrate, it causes repulsion of negative charge carriers out of the initially formed
channel.
▪ This increases the channel resistance which resultantly reduces the drain current.

45. The depletion-mode MOSFET Working
▪In a DE-MOS, more negative the gate voltage, the less the drain current that
flows through the channel.
▪In the case when the gate terminal is made positive with respect to the
substrate, more number of electrons gets attracted towards the channel.
▪Thus, causing more current to flow through the channel.
▪A pinch-off condition also arises in DE-MOS when a much negative gate
voltage is applied.

46. The depletion-mode MOSFET Working

47. The enhancement-mode MOSFET

48. The enhancement-mode MOSFET
•E - MOSFET does not have a channel at the time of construction as
seen in the above figure 1.
•In E-MOS, a positive gate to source voltage is required for the channel
to induce electrically. It requires large positive gate voltage for its
operation.

49. The enhancement-mode MOSFET
• Consider a case when a positive drain to source voltage is applied and
the gate terminal is at 0 potential.
• E-MOS does not conduct. Due to this reason, it is called normally-off MOSFET.
• In this case, the P-type substrate and the two N regions behave as two PN junctions
connected back to back and P substrate provides the resistance.
• In this condition, both junctions cannot be forward bias simultaneously leading to
very small drain current which is a reverse leakage current.
• Let us now move further and consider the case when the gate is made
somewhat positive with respect to the source.
• The minority charge carriers of p-type substrate i.e., electrons get attracted by the
positive potential of the gate.

50. The enhancement-mode MOSFET
▪ These negative carriers accumulate or gather at the surface of the substrate just below the
gate terminal.
▪ Any further increase in the VGS
will cause more electrons to deposit under the gate.
▪ Since dielectric is used so these electrons cannot be able to flow across the insulating layer of
SiO2
.
▪ Thus they accumulate at the surface of the substrate itself.
▪ Thus, an N-channel is made between source and drain by the accumulation of minority
charge carriers.
▪ Thus, drain current ID
flows through the channel.
▪ The flow of drain current depends on the channel resistance which in turn depends on
the charge carriers attracted towards the positive gate terminal.
▪ So, by the above discussion, we can conclude that ID
is controlled by the gate potential VGS
. It
is called enhancement MOSFET as the channel conductivity is enhanced by the positive gate
potential.

51. The enhancement-mode MOSFET

52. Operating Regions of MOSFET
Cut-Off Region
The cut-off region is a region in which there will be no conduction and as a result, the
MOSFET will be OFF.
In this condition, MOSFET behaves like an open switch.
Ohmic Region
The ohmic region is a region where the current (IDS)increases with an increase in the
value of VDS.
When MOSFETs are made to operate in this region, they are used as amplifiers.
Saturation Region
In the saturation region, the MOSFETs have their IDS constant in spite of an increase
in VDS and occurs once VDS exceeds the value of pinch-off voltage VP.
Under this condition, the device will act like a closed switch through which a
saturated value of IDS flows.
As a result, this operating region is chosen whenever MOSFETs are required to
perform switching operations.

53. Metal Oxide Silicon Field Effect Transistors(MOSFET)
MOSFET BJT
There are two types of MOSFET
and they are named: N-type or
P-type
BJT is of two types and they are
named as: PNP and NPN
MOSFET is a voltage-controlled
device
BJT is a current-controlled device
The input resistance of MOSFET
is high.
The input resistance of BJT is
low.
Used in high current applications Used in low current applications

54. MOSFET applications
✔ Radiofrequency applications use MOSFET amplifiers extensively.
✔ MOSFET behaves as a passive circuit element.
✔ Power MOSFETs can be used to regulate DC motors.
✔ MOSFETs are used in the design of the chopper circuit.
Advantages of MOSFET
• MOSFETs operate at greater efficiency at lower voltages.
• Absence of gate current results in high input impedance producing high
switching speed.
Disadvantages of MOSFET
✔MOSFETs are vulnerable to damage by electrostatic charges due to the thin
oxide layer.
✔Overload voltages make MOSFETs unstable.