UNIT-V

Bipolar Junction
Transistor
 Content
• BJT - Characteristics and sBiasing
• Fundamentals of BJT& Operation
• I/P and O/P Characteristics CB configurations.
• I/P and O/P Characteristics CE configurations.
• I/P and O/P Characteristics CC configurations.
 BJT-
• Introduction
Bipolar Junction Transistor is an active device .
• It works as a current-controlled current source.
• It’s basic action is control of current at
one terminal by controlling current applied at other
terminal.
• Current conduction is due to two types of charge carriers i.e
due to both Holes and Electrons. (hence the name Bipolar)
• It consists of Input junction and Output junction. It is a Bipolar
junction transistor.
• Transistor means Transfer Resistor.
• Signals transfer from Low resistance path to High Resistance
path (Transfer resistor ).
 BJT
• Transistor consists of three terminals they are
Emitter, Base, Collector.
• Emitter is a heavily doped region,
• Base is lightly doped region and
• Collector is moderately doped region.

• Transistors are divided into two types - They are

PNP and NPN transistors.
• Arrow placed on the emitter of the transistor indicates the
direction of current flow.
• The direction of current flow is always from ‘p’ region to ‘n’
region.
NPN and PNP Transistor
Symbols
 BJT
• Transistors can act as either an insulator or a conductor by the
application of a small signal voltage.
• The transistor's ability to change between these two states
enables it to have two basic functions:
"switching" (digital electronics) or "amplification" (analog
electronics).
• It is used as amplifier and oscillator circuits, and as a switch
in
digital circuits.

• It has wide applications in computers, satellites and other

modern communication systems.
• The principle of operation of the two transistor types PNP and
NPN, is exactly the same the only difference being in their
biasing and the polarity of the power supply for each type
 BJT – Regions of
• Operation
Bipolar transistors have the ability to operate within three
different regions:
1. Active Region - the transistor operates as an amplifier and I c
= β.Ib
2. Saturation Region - the transistor is "fully-ON" operating as a
switch and Ic = I(saturation)
3. Cut-off Region - the transistor is "fully-OFF" operating as a
switch and Ic = 0
•In CB configuration,
Active Region -
VB > VE & VC > VB

Cut-off Region –
VE > VB & VC > VB

Saturation Region –
VB > VE &
VB > VC
 Transistor
• Construction
In an NPN transistor, a layer of P-type material is sandwiched
between two layers of N-type material.
• The BJT consists of a silicon (or germanium) crystal in which
a thin layer of N-type silicon is sandwiched between two
layers of P-type silicon which forms PNP transistor.
• The emitter is heavily doped so that it can inject a large
number of charge carriers into the base.
• The base is lightly doped and very thin. It passes most of the
injected charge carriers from the emitter into the collector.
• The collector is moderately doped.
 Transistor
• Biasing
Usually the emitter-base junction is forward biased (F.B.)
and the collector-base junction is reverse biased (R.B.).
 Transistor working
• The forward bias applied(NPN)
to the emitter base junction of an
NPN transistor causes a lot of electrons from the emitter
region to cross over to the base region.
• As the base is lightly doped with P-type impurity, the number
of holes in the base region is very small and, hence, the
number of electrons that combine with holes in the P-type base
region is also very small.
• Hence, a few electrons recombine with holes to constitute a
base current IB.
• The remaining electrons (more than 95%) cross over into the
collector region to constitute a collector current I C.
 Transistor working
• (NPN)
Thus, the base and collector current summed up gives the
emitter current, i.e., IE = (IC + IB).
• In the external circuit of the NPN bipolar junction transistor,
the magnitudes of the emitter current IE, the base current IB,
and the collector current IC are related by IE = IC + IB.
 Transistor working
• The forward bias applied(PNP)
to the emitter-base junction of a PNP
transistor causes a lot of holes from the emitter region to cross
over to the base region as the base is lightly doped with N-
type impurity.
• A few holes combined with electrons to constitute a base
current IB.
• The remaining holes (more than 95%) cross over into the
collector region to constitute a collector current I C.
• Thus, the collector and base current when summed up gives
the emitter current, i.e.,
IE = (IC + IB)
• In an npn transistor current conduction is due to majority
charge carriers i.e electrons.
• Emitter current is a combination of hole current and electron
current.
 Transistor
• Configuration
Common base (CB) configuration,
• Common emitter (CE) configuration,
• Common collector (CC) configuration.
 Transistor
• Input characteristics
characteristics • Output
characteristics
• Input characteristics are • Output characteristics
those curves which are are those curves which are
drawn between Input drawn between output
voltage vs input current voltage vs out put current by
by keeping output keeping input current as
voltage as constant constant
 Transistor
characteristics
Input characteristics Output characteristics
Name of
Input Input Output Output Output Input
the
Voltage Current voltage Voltage Current Current
configur
X-axis Y-axis Constant X-axis Y-axis Constant
ation

CE
VBE
IB VCE VCE
IC IB

CB
VEB
IE VCB VBC
IC IE

CC
VBC
IB VEC VCE
IE IB
 CB
configuration

• Base is common ,
• Emitter is input terminal and
• Collector is output terminal.
• Emitter base junction (I/P) Junction is forward biased.
• Collector base Junction (O/P) junction is reverse biased.
 CB - Input
• characteristics
Input characteristics:
• Input characteristics represents forward characteristics of
emitter base diode for various collector voltages VCB as
constant.

• The collector base voltage VCB is kept constant at zero volt and
the emitter current IE is increased from zero in suitable equal
steps by increasing VEB.
• This is repeated for higher fixed values of VCB
• A curve is drawn between IE and VEB at constant VCB.
 CB - Input
• When VCB = 0 andcharacteristics
the emitter base junction is forward biased, the junction
behaves as a forward biased diode so that emitter current I E increases
rapidly with small increase in VEB.
• When VCB is increased keeping VEB constant, the width of the base region
will decrease.
• This effect results in an increase of IE. (as Rate of recombination ↓ & IB is
reduced)
• Therefore, the curves shift towards the left as VCB is increased.

• This curve shows the relationship between input voltage

(VBE) to input current (IE) for various levels of output
voltage(VCB).
Input characteristics
 CB - Input
∆𝑉
𝑅𝑖 𝑂ℎ𝑚
• characteristics
Input Resistance:

∆�
= 𝑠
𝐵𝐸
• 𝐼 �of the slope of the
Input Resistance is the reciprocal
characteristic curve.
• Input Resistance is Low in Common Base configuration.
 CB - Output
• characteristics
To determine the output characteristics, the emitter current I E
is kept constant at a suitable value by adjusting the emitter-
base voltage VEB.
• Then VCB is increased in suitable equal steps and the collector
current IC is noted for each value of IE. This is repeated for
different fixed values of IE.
• The curves of VCB vs IC are plotted for constant values of IE.
 CB - Output
• characteristics
From the characteristics, it is seen that for a constant value of
IE, IC is independent of VCB and the curves are parallel to the
axis of VCB.
• and IC ≅ 𝐼𝐸
• Also, IC flows even when VCB is equal to zero.

• This curve shows the relationship between output voltage

(VCB) to output current (IC) for various levels of input
current (IE).
Output Characteristics
 Output
• There are threecharacteristics
different regions in transistor Output
characteristics.
1. Saturation Region
2. Active Region
3. Break Down Region.
 CB - Output
• characteristics
Saturation Region: In this region both I/P and O/P Junctions
are forward biased.
• A small Change in VCB results a large change in IC.

• Active region: In active region I/P Junction is forward biased

and O/P is reverse biased.
• In this region collector current IC is independent of collector
voltage VCB and Depends only on emitter current.

• Cut-off Region: In this both I/P & O/P Junctions are reverse
biased.
• The Region below IE = 0 is cutoff region.
 CB-
1. Input ResistanceConclusions
of CB configuration is very Low.
2. Output Resistance of CB Configuration is very High.
3. Current Gain is less than unity.
4. It is used for high frequency circuits.

• In CB configuration current gain varies between 0.9 to

0.985(α).
 CE -
Configuration

• Emitter is common,
• Base is input terminal and
• Collector is output terminal.
 CE – Input
• Characteristics
Input Characteristics:
• The collector-to-emitter voltage is kept constant at zero volt,
and the base current IB is increased from zero in equal steps by
increasing VBE.
• The value of VBE is noted for each setting of IB.
• This procedure is repeated for higher fixed values of VCE,

• This curve shows the relationship between input voltage

(VBE) to input current (IB) for various levels of output
voltage(VCE).
Input Characteristics
 CE – Input
• When VCE = 0,Characteristics
the emitter-base junction is forward biased and
behaves as a forward biased diode.
• Hence, the input characteristic for VCE = 0 is similar to that of
a forward-biased diode.
• When VCE is increased, the width of the depletion region at
the reverse-biased collector-base junction will increase. Hence,
the effective width of the base will decrease.
• This effect causes a decrease in the base current I B.
• Hence, to get the same value of IB as that for VCE = 0, VBE
should be increased.
• Therefore, the curve shifts to the right as VCE increases.
 CE – Input
• Characteristics
Reciprocal of the slope of the curve is Input Resistance and it
is medium.
• Input Resistance is about 750 Ohms.
• Rin=∆ VBE / ∆ IB .
 CE – Output
• Output Characteristics:
•
Characteristics
The base current I is kept constant at a suitable value by
B
adjusting the base-emitter voltage, VBE.
• The magnitude of the collector-emitter voltage VCE is
increased in suitable equal steps from zero and the collector
current IC is noted for each setting VCE.
• This is repeated for different fixed values of IB.
• This curve shows the relationship between output voltage
(VCE) to output current (IC) for various levels of input
current (IB).
Output Characteristics
 CE – Output
• When IB is keptCharacteristics
constant,
• As VCE increases, VCB also increases,
• We can write VCE = VCB + VBE
• As VCB increases, depletion region of Base Collector junction
increases, effective Base width decreases, IB decreases.
• As VCE increases, IC increases
• We have IC = β IB
• IC not only depends on IB but also VCE
 CE – Output
• we have, Characteristics

• For larger values of VCE, due to early effect, a very small

change in α is reflected in a very large change in β.
0.9
• For example, when α 8
= 0.98, 1−0.9 = 49.
β = 8
0.98
• If α increases to 0.985, then β = 5
1−0.98
= 66.
5
• Here, a slight increase in α by about 0.5% results in an
increase in β by about 34%.
• Hence, the output characteristics of CE configuration show a
larger slope when compared with CB configuration.
 CE – Output
• In active regionCharacteristics
of CE configuration transistor will work as an
Amplifier.
• Here Input junction is forward biased and Collector Base
junction is Reverse biased.
• Transistor output current IC responds to the input current IB.
• IC varies with changes in VCE when IB is kept constant.
• If VCE is increased continuously then depletion region in CB
junction increases.
• It increases IC Rapidly and operates the transistor in the active
Region.
 CE – Output
•
Characteristics
In Cut-off region of CE configuration both Emitter Base junction
and Collector Base junction are reverse biased.
• It is the region below the curve IB = 0.
• Current is due to minority charge carriers, called as Reverse
saturation current.
• The transistor is virtually an open circuit between collector and
emitter.

• In Saturation region of CE configuration both junctions are

forward biased.
• Current is maximum.
• An increase in the base current IB does not cause a corresponding
large change in IC.
• In saturation region, current IC = VCC / RL.
 CE – Output
• Characteristics
Output Resistance: R =∆ V / ∆ I
out CE C .
• Reciprocal of the slope of the curve is output Resistance and it is
medium.
• Output Resistance is about 45 Kilo Ohms.

• Conclusion:
• It has medium Input Resistance and medium Output Resistance.
Hence CE configuration is preferred for Amplification purpose.

• Application: CE configuration is used for Amplification

purpose.
 CC -
Configuration

• Collector is common,
• Base is input terminal and
• Emitter is output terminal.
 CC – Input
• Characteristics
Input Characteristics: To determine the input characteristics,
VEC is kept at a suitable fixed value.
• The base-collector voltage VBC is increased in equal steps and
the corresponding increase in IB is noted.
• This is repeated for different fixed values of VEC.

• This curve shows the relationship between input voltage

(VBC) to input current (IB) for various levels of output
voltage(VEC).
 Comparison of CB,CE and CC
Property Configurations
Common Base Common Emitter Common Collector
configuration configuration configuration

Input Low (Around Medium High (750

Resistance 100Ω) (Around 750 Ω) kilo Ω )
Output
High Medium Very low
Resistance
Current Gain <1 High High
Voltage Gain Around 150 Around 500 <1
Phase shift 00 or 3600
between input No phase shift 1800 No phase
and output shift
For high
For Audio For impedance
Applications frequency
amplification matching
circuits
FIELD EFFECT
TRANSISTOR
 Content
• s
Construction & Working of JFET,
• JFET characteristics,
• FET Parameters,
• Construction & Working of MOSFET,
• MOSFET characteristics (Enhancement and depletion mode)
• Comparison of JFET & MOSFET
• Biasing of JFET - Self bias and fixed bias.
• Small signal Analysis of
– Common Source,
– Common Drain and
– Common Gate amplifier configurations
 Field Effect
• Transistor
FET is a Unipolar Device.
• FET is a Voltage controlled Device.
• Current conduction is either by holes or by
electrons.
• Terminals are
• Source (S)
• Gate (G)
• Drain (D)

• “Field-Effect”
– Electric field controls the conduction path of Output circuit,
hence the name Field Effect Transistor.
 FET BJT
• Field Effect Transistor • Bipolar Junction Transistor
• Terminals • Terminals
• Source (S) • Emitter (E)
• Gate (G) • Base (B)
• Drain (D) • Collector (C)
• Unipolar Device • Bipolar Device
• Conduction is due to either • Conduction is due to Both
Electrons or Holes Electrons and Holes
• Voltage controlled Device • Current controlled Device
• n-channel & p-channel • npn & pnp
• Very High Input Impedance • Very Low Input Impedance
• Easy to fabricate • Complicate to fabricate
 FET BJT
•More Stable to Temperature •Less Stable to
changes compared to BJT Temperature
•Negative Temperature changes compared to FET
coefficient •Positive Temperature
•Gain Bandwidth product is coefficient
less compared to BJT •Gain Bandwidth product
•Costlier is
more compared to FET
•Cheaper
 Advantages of FET over
• BJT
Its operation depends on the flow of majority charge carriers
only. Hence it is a unipolar device.
• It has High input impedance.
• It is less affected by radiation.
• It has better thermal stability.
• It is less noisy compared with BJT.
• In IC form it is easy to fabricate rather than BJT.
• It is small in size than BJTs

• Disadvantage: Relatively small gain band width product.

 FET
1. The Field effect transistor is abbreviated as FET, it is an another semiconductor device
like a BJT which can be used as an amplifier or switch.
2. The Field effect transistor is a voltage operated device. Whereas BJT is a current
controlled device. Unlike BJT a FET requires virtually no input current.
3. This gives it an extremely high input resistance, which is its most important advantage
over a BJT.
4. FET is also a three terminal device, labeled as source, drain and gate.
5. The source can be viewed as BJT’s emitter, the drain as collector, and the gate as the
counter part of the base.
6. The material that connects the source to drain is referred to as the channel.
7. FET operation depends only on the flow of majority carriers ,therefore they are called
uni-polar devices. BJT operation depends on both minority and majority carriers.
8. As FET has conduction through only majority carriers it is less noisy than BJT.
9. FETs are much easier to fabricate and are particularly suitable for ICs because they
occupy less space than BJTs.
10. FET amplifiers have low gain bandwidth product due to the junction capacitive effects
and produce more signal distortion except for small signal operation.
11. The performance of FET is relatively unaffected by ambient temperature changes. As
it has a negative temperature coefficient at high current levels, it prevents the FET
from thermal breakdown. The BJT has a positive temperature coefficient at high
current levels which leads to thermal breakdown.
FET
 FET
•
Family
Field Effect Transistor is broadly divided in to two categories. They
are
1. Junction Field Effect Transistor [JFET]
2. Metal Oxide Semiconductor Field Effect Transistor
[MOSFET]
• Junction Field Effect Transistor further subdivided into
a. n-channel JFET.
b. p-channel JFET.
• MOSFET is further subdivided in to
a. Enhancement MOSFET
b. Depletion MOSFET
• Enhancement MOSFET is further subdivided into n channel & p
channel MOSFET.
• Depletion MOSFET is further subdivided into n channel & p
channel MOSFET.
 JFET
Construction

• There are Three terminals:

– Source (S) and Drain (D)are connected to n-channel
– Gate (G) is connected to p-type material.
• JFET -
Analogy
 JFET-
• Construction
It consists of an N-type bar which is made of silicon. Ohmic
contacts (terminals), made at the two ends of the bar, are called
source and drain.
• Source (S): This terminal is connected to the negative pole of the
battery. Electrons which are the majority carriers in the N-type bar
enter the bar through this terminal.
• Drain (D): This terminal is connected to the positive pole of the
battery. The majority carriers leave the bar through this terminal.
• Gate (G): Heavily doped P-type silicon is diffused on both sides of
the N-type silicon bar by which PN junctions are formed. These
layers are joined together and called the gate G.
• Channel: The region of the N-type bar between the depletion region
is called the channel. Majority carriers move from the source to
drain when a potential difference VDS is applied between the source
and drain.
 JFET-
Construction
• Conventional current at Source is denoted as IS
• Conventional current at Drain is denoted as ID
• Conventional current at Gate is denoted as IG
• For n-channel JFET,
– Drain to Source voltage, VDS is Positive
– Gate to Source voltage, VGS is Negative

• Bias Voltages (n-channel)

– VDD : Drain Junction Must be always forward
Biased.
– VGG : Gate Junction Must be always reverse Biased.
 JFET-Working
• In a reverse-biased p-n junction, The current carriers have
diffused across the junction, leaving only uncovered positive
ions on the n side and negative ions on the p side and forms a
Electric Field.
• As the reverse bias across the junction increases, the thickness
of the region of immobile uncovered charges also increases.
• The conductivity of this region is nominally zero because of
the unavailability of current carriers.

• Hence we see that the effective width of the channel will

become progressively decreased with increase in reverse
bias.
 JFET-Working
• Accordingly, for a fixed VDS, the ID will be a function of the
reverse-biasing voltage across the gate junction.
• The term field effect is used to describe this device because the
mechanism of current control is due to the increasing reverse
bias, i.e. the field associated with the region of uncovered
charges.

Operating Conditions:
• When VGS = 0 and VDS = 0
• When VGS is Decreased from Zero and VDS = 0
• When VGS = 0 and VDS is Increased from Zero
• When VGS is Negative and VDS is Increased
 JFET-Working
1. When VGS = 0 and VDS = 0
•When no voltage is applied between drain and source, and gate
and source, the thickness of the depletion regions around the PN
junction is uniform.
2. When VGS is Decreased from Zero and VDS = 0
•In this case, the PN junctions are reverse biased and, hence, the
thickness of the depletion region increases.
•As VGS is decreased from zero, the reverse-bias voltage across
the PN junction is increased and, hence, the thickness of the
depletion region in the channel also increases until the two
depletion regions make contact with each other. In this condition,
the channel is said to be cut-off.
•The value of VGS which is required to cut off the channel is
called the cut off voltage VC.
 JFET-Working
3. When VGS = 0 and VDS is Increased from Zero
•Drain is positive with respect to the source with VGS = 0. Now
the majority carriers (electrons) flow through the N-channel
from source to drain.
•Therefore, the conventional current ID flows from drain to
source.
•The magnitude of the current will depend upon
– conductivity of the channel.
– length (L) of the channel
– cross-sectional area (A) of the channel at B
– The magnitude of the applied voltage VDS

𝜌 is the
• When VGS = 0 and VDS is Increased from Zero
 JFET-Working
• As VDS is increased, the cross-sectional area of the channel
will be reduced.
• At a certain value VP of VDS, the cross-sectional area at B
becomes minimum. At this voltage, the channel is said to be
pinched off and the drain voltage VP is called the pinch-off
voltage.

• Above the pinch-off voltage, at a constant value of VDS, ID

increases with an increase of VGS.
• Hence, a JFET is suitable for
use as a voltage amplifier,
• PINCH OFF Region:
• It is also called as saturation region or constant current region.
• Channel is occupied with depletion region and is Limited
• The depletion region is more towards the drain and less towards
the source,
• With this limited number of carriers are only allowed to cross this
channel from source to drain causing a current that is constant in
this region.
• To use FET as an amplifier it is operated in this saturation region.
• In this drain current remains constant at its maximum value I DSS.
• The drain current in the pinch off region depends upon the gate to
source voltage and is given by the relation

• This is known as shokley’s relation.

• BREAK DOWN Region:
• In this region, the drain current increases rapidly as the drain to
source voltage is increased.
• It is due to avalanche effect at the gate to source junction.
• The avalanche break down occurs at progressively lower value of
VDS, because the reverse bias gate voltage adds to the drain
voltage, thereby increasing effective voltage across the gate
junction
• This causes
– 1. The maximum saturation drain current is smaller
– 2. The ohmic region portion decreased.
• It is important to note that the maximum voltage VDS which can
be applied to FET is the lowest voltage which causes available
break down.
 JFET
• characteristics
Curves that explains the relationship between voltage and
current are characteristic curves.
• FET characteristics are of two types
1. Drain characteristics or Static characteristics
2. Transfer characteristics
 JFET - Drain
Characteristics
 JFET – Transfer
• Characteristics
These curves establishes a relationship between I D and VGS
keeping VDS as constant.
• When VGS = 0, Drain current is maximum and is equal to IDSS.
• As VGS is increased channel width Reduces and drain current
ID reduces.
• At one Particular Gate voltage, Channel is pinched off.
• The maximum reverse gate voltage where the channel is
pinched off is known as pinch off voltage.
 JFET – Transfer
• Characteristics
In transfer characteristic, V is maintained as constant at a
DS
suitable value greater than the Reference Voltage.
• Gate voltage is increased till ID is reduced to Zero.
• The shape of the transfer characteristic is half parabola .
 JFET –
Characteristics

JFET – Transfer Characteristics JFET – Drain Characteristics

 MOSFET
• Metal Oxide Semiconductor Field Effect Transistor
• MOSFET’s operate the same as JFET’s but have a gate
terminal that is electrically isolated from the conductive
channel.
• Field Effect Transistor whose Gate input is electrically
insulated from the main current carrying channel and is
therefore called an Insulated Gate Field Effect Transistor
(IGFET).
• It has a “Metal Oxide” Gate electrode which is electrically
insulated from the main semiconductor n-channel or p-
channel by a very thin layer of insulating material usually
silicon dioxide, commonly known as glass.
 MOSFET
• Principle: By applying a transverse electric field across an
insulator deposited on the semiconducting material, the
thickness and, hence, the resistance of a conducting channel of
a semiconducting material can be controlled.

• In a depletion MOSFET, the controlling electric field reduces

the number of majority carriers available for conduction,
• In enhancement MOSFET, application of electric field causes
an increase in the majority carrier density in the conducting
regions of the transistor.
 MOSFET
• Metal-Oxide-Semiconductor FET or another name Insulated-
Gate FET (IGFET)
• Metal: for the drain, source and gate connections to the
proper surface- in particular the gate terminal and the control
to be offered by the surface area of the contact.
• Oxide: Silicon dioxide insulating layer.
• Semiconductor: the basic structure on which the n- and p-type
regions are diffused.
 MOSFET
• MOSFETs are available in two
basic forms:
• Depletion Type – the transistor
requires VGS to switch the device
“OFF”.
– The depletion mode MOSFET is
equivalent to a “Normally Closed”
switch.
• Enhancement Type – the
transistor requires VGS to switch
the device “ON”.
– The enhancement mode MOSFET is
equivalent to a “Normally Open”
switch.
 Depletion
MOSFET
Modes in N-channel DE-MOSFET :
1.Depletion Mode:
 VDS is +ve and VGS -ve
2.Enhancement mode:
 VDS is +ve and VGS +ve

1) Depletion Mode :
– VDS is +ve and VGS –ve
•When G is –ve w.r.t S
•Negative gate induces +ve charge into the N-channel
•The free electrons are repelled away from the channel
•Channel is depleted of free electrons and depletion layer is
formed
 Depletion
• As VGS MOSFET
increases ,Channel
resistance increases and ID
reduces

• At a sufficient high value of VGS

the channel is totally depleted of
free electrons and ID becomes
zero
 Enhancement
2) Enhancement mode MOSFET
:
•VDS is +ve and VGS = 0
 Drain current flows even if VGS =0
 DE MOSFET is called normally ON MOSFET

•Enhancement mode :
•VDS is +ve and VGS = +ve
•Due to positive gate voltage, negative charges are induced into
the channel.
•As VGS increases, negative charges increases, the channel
resistance reduces and thus conductivity of channel increases
•Thus, increases drain current ID
Depletion
MOSFET
 Enhanc
• Construction ement -
MOSFET
 Enhanc
• Working: ement -
•
1) V >0 and V =0
DS GS
MOSFET
VDS and VGS both positive

2) VDS>0 and VGS>0

1) VDS +ve and VGS = 0

• When VDS is applied, it
try to force electrons from S
to D but P- region doesn't
allow to pass through it.
• At VGS = 0, ID=0
• Thus, it is also called as
Normally OFF MOSFET
 Enhanc
2)V +ve and V +ve
DS GS ement -
GS
MOSFET
•When V is applied , G induces
negative charges in P-type substrate
adjacent to Sio2 layer.
•The induced negative charges are
formed by attracting free electrons from
Source
•When VGS is increased further , more
negative charges are induced and a thin
layer of electrons are formed under
SiO2 layer from S to D
•This thin electron layer acts as layer
of N-Channel called as N-type
inversion layer
 Enhanc
• The minimum V at which
GS ement - is formed is called
inversion layer
threshold voltage V
•
MOSFET
GSth

Through this layer the current, I starts flowing from D to S as

D
electrons flow from S to D
• Thus,
• When VGS < VGSth,
• When VGS >= VGSth,
ID = 0
• Above certain value of VDS, the drain current, ID becomes constant
ID flows
and reaches it saturation value
 Enhanc
ement -
MOSFET

Darin Characteristics Transfer Characteristics

 Summar
• Depletion MOSFET y
• Physical channel is constructed and ID is due to VDS
• Used in both Depletion and Enhancement modes

• Enhancement MOSFET
• No Physical channel is constructed.
• Gate voltage develop channel of charge carriers and current ID
flows when VDS is applied.
• This device is used to enhance the conductivity of channel
 Comparison of MOSFET with
• JFETtypes of MOSFETs, the
1. In enhancement and depletion
transverse electric field induced across an insulating layer
deposited on the semiconductor material controls the
conductivity of the channel. In the JFET, the transverse electric
field across the reverse-biased PN junction controls the
conductivity of the channel.
• 2. The input resistance of a MOSFET is very high in the order
of 1010 to 1015 W. The input resistance of a JFET is of the
order of 108 W.
• 3. The output characteristics of the JFET are flatter than those
of the MOSFET and, hence, the drain resistance of a JFET is
much higher than that of a MOSFET.
• 4. JFETs are operated only in the depletion mode. The
depletion-type MOSFET may be operated in both depletion
and enhancement mode.
 Comparison of MOSFET with
• JFETare easier to fabricate.
5. Comparing to JFET, MOSFETs
• 6. A MOSFET is very susceptible to overload voltage and
needs special handling during installation. It gets damaged
easily if it is not properly handled.
• 7. A MOSFET has zero offset voltage. As it is a symmetrical
device, the source and drain can be interchanged. These two
properties are very useful in analog signal switching.
• 8. Special digital CMOS circuits are available which involve
near-zero power dissipation and very low voltage and
current requirements. This makes them most suitable for
portable systems.