MOSFET

TRANSISTOR
 What are transistors?
 How do they work?

 What are the types of transistors?

 Semiconductor device -maintain

and regulate the voltage and
current level.
 Switch and an amplifier

 Electronic devices
 FET
 FET is a Field Effect Transistor.
It is a three-terminal device that uses the electric field to regulate and
maintain the flow of current. The three terminals are :
1. Gate

2. Drain

3. Source

 Current flows between two terminals, drain and source.

 The flow of current can be controlled by applying external voltage between gate and
source.
 This external voltage generates electric field in device.
 By controlling electric field and voltage, we can regulate the flow of current.
 Voltage Controlled Device.
 Applications
 Integrated circuits, oscillators and buffer amplifiers. Small in size, used in ICs.
 Two types of FET:-

 1. JFET ( Junction Field Effect Transistor)

 2. IG-FET (Insulated Gate Field Effect Transistor)

 Important Terminologies

• Transistor : It is a kind of semiconductor device which can

generate electric current or voltage on its own.

• Gate Terminal : The current conduction between drain and

source is controlled by applying voltage in the gate terminal.

• Threshold voltage : Maximum amount of voltage required

for the formation of channel.

• p-channel : Channel made with p-type semiconductor.

• n-channel : Channel made with n-type semiconductor.

• Saturation : Level in which something becomes constant.

  MOSFET Basics
MOSFET -Metal Oxide Semiconductor Field Effect
Transistor
In MOSFET, the gate terminal is separated from
the channel using the insulating layer. This
insulating layer is formed from the oxide layer of
the semiconductor. The insulating layer of
MOSFET is formed from SiO2.

MOSFET is also classified into two types:-

1. Depletion type
2. Enhancement type
When we provide external voltage in the channel, it can either increase or
decrease the amount of charge carriers in the channel.
 If the number of charge carriers increases, it is known as enhancement type
MOSFET.
 If the number of charge carriers decrease then it is known as depletion type
MOSFET.
 MOSFET Types
 Enhancement type and depletion type MOSFET is further
classified into p-channel and n-channel MOSFET.
Enhancement Type MOSFET

•In this type of MOSFET, no channel is present from

the beginning and hence no current flows.

•But when the +ve voltage more than the threshold

voltage is applied, it leads to an enhancement of a
channel between the drain and source due to the gate
voltage and thus it results in the conductivity of device.

•They are mostly used in digital applications.

Depletion Type MOSFET

•In this type of MOSFET, channel is present from the

beginning which means conduction of current is there.

•-ve voltage is applied to reduce the flow of current

and this reduces the width of the channel.

•Not used commonly in devices.

WORKING PRINCIPLE OF MOSFET
 MOSFET is a type of transistor in which conductivity depends
upon the semiconductor channel across the drain and source
terminal. This semiconductor channel may be p-channel or n-
channel depending upon the configuration of the MOSFET.
 A MOSFET consists of three terminals- drain, source and
gate. By applying some voltage across gate and source, there
forms a inversion layer or a channel between the drain and
source if the voltage applied is threshold voltage. (Threshold
voltage is the minimum required voltage for the conduction of
current) . If the applied voltage is less than the threshold
voltage, no channel is formed. Hence current cannot flow in
the MOSFET. This situation is called as Cutoff region (OFF).
 And after a certain level of voltage, the current becomes
constant in the MOSFET. This condition is called as
saturation point. MOSFET is a voltage controlled device so the
thickness of channel and the amount of current depends upon
the voltage applied across gate and source. If more voltage
applied, width of channel increases and more amount of
current able to flow through the device.

 MOSFET Construction
 MOSFET is a transistor which is used as switch or amplifier and in many other applications. The
basic construction of MOSFET can be explained as below:-
• Substrate : MOSFET is constructed on a silicon wafer that is it acts as a base of the device.

• SiO2 : A thin layer of insulating material is formed with SiO2 for the exchange of electrons and
holes.

• Gate Terminal : A gate terminal is formed on the insulating layer. This controls the flow of
current between the drain and source with the help of gate voltage.

• Source and drain terminals : These are created on the either side of the gate. These are
basically doped regions.

• Channel : Region between the gate, drain and source is known as channel which controls the
flow of charge among them.


 Characteristics of MOSFET

 MOSFETs have majorly two characteristics:-

1. Drain characteristics
2. Transfer characteristics
 Enhancement Type MOSFET

 1. Drain characteristics of Enhancement Type MOSFET

 Characteristics between output current and output voltage. O/P i ->

ID, O/P v -> VDS , control variable = VGS. Here we will plot a graph
between ID and VDS for various levels of VGS.

 Case 1: VGS1 > VT

 (Here VT is the threshold voltage)

 Veff = VGS1 - VT
• will directly affect the width of the n-channel

• significant amount of drain current will flow through it.

 When VDS is increased to a certain level, drain current becomes

constant. This is called pinch-off condition.
 Case 2: VGS1 > VGS2

 conductivity of 1 > conductivity of 2

 R2> R1
 slope of 2 < slope of 1
 Now there are few regions in this graph:-
1. Saturation region : Region in which drain current is constant.

2. Triode region : Left area of locus of VDS saturation.

3. Cut-off region : When it can't achieve the threshold voltage,

MOSFET remains OFF. Current is 0 amp

 With increasing voltage, current flowing through the terminals

increases with voltage. You can see this in the graph. VGS more
than the threshold voltage is the condition of flowing current.
 Transfer Characteristics of Enhancement Type
MOSFET

 Characteristics between
output current and input voltage.

 V = VT (Threshold Voltage)
• When the value of voltage across gate and source
is less than the threshold voltage
 Working of N-channel Enhancement Type MOSFET
 In such kind of MOSFET, we have to make gate terminal more
+ve, hence +ve charges will accumulate in the gate and will
attract -ve charges in the body. Electrons will be drifted
towards the surface and the region near the surface will
become less p type. +ve charges are pushed down. So above
region will become n type and thus a channel is formed.
 If we will increase the voltage between gate and source, then
the width of channel will increase. And if the voltage is more
than a particular voltage, the channel width is sufficient to
allow flow of current. And this particular voltage is called
Threshold voltage. And the resultant current is known as drain
current.
 Working of N- channel Depletion Type MOSFET
• In this type of MOSFET, three terminals are present- drain,
source and gate.

• Same as other MOSFETs, it also contains a thin insulating

layer made up of gate oxide to avoid direct contact with the
metal.

• Here, channel is present from the beginning. By applying a

+ve voltage it creates a depletion region reducing the charge
carriers and results in decrement of current.

• MOSFET is generally ON.

• Drain and source terminal is made up of n-type

semiconductor.
 Depletion Type MOSFET
 Drain characteristics : ID V/S VDS for various VGS.

Transfer Characteristics : ID V/S VGS for

fixed VDS.
Difference Between Enhancement Type and Depletion Type MOSFET

Enhancement Type
Depletion Type MOSFET
MOSFET

1. By applying external voltage in 1. By applying external voltage in

the channel, if the amount of the channel, if the amount of
charge carriers increases, it is charge carriers decreases, it is
known as enhancement type known as depletion type
MOSFET. MOSFET.

2. No channel is present in the 2. Channel is present from the

beginning. beginning.

3. There is no such threshold

3. There is a threshold voltage.
voltage.

4. It does not produce current in 4. It can produce current without

the absence of VGS. any gate voltage.
 Uses of MOSFET
• Used in digital logic circuits.

• It is used as Amplifiers

• They are used in Integrated circuits due to small

size.

• It is also used in Microprocessors

• Used in Power electronics

Switching characteristics for both N channel and P channel MOSFET
in tabular form

MOSFET VGS < 0 VGS = 0 VGS > 0

P-channel
ON OFF OFF
enhancement type

N-channel
OFF OFF ON
enhancement type

P-channel
ON ON OFF
depletion type

N-channel
OFF ON ON
depletion type
 Applications of MOSFET
• Amplifiers: MOSFETs are used as an amplifiers in order
to amplify weak signals.

• Switching power supplies: They are used as switches

because they can alter power supply efficiently.

• Digital logic gates: They are used to build logic

gates such as NAND, NOR etc.

• Voltage regulators: They are used as voltage

regulators because they can control the amount of
voltage.

• Memory devices: They are used in memory cells.

 Advantages of MOSFET
• High Switching Speed : It can change its state rapidly from ON and OFF position.

• Low Power Consumption : Useful for battery operated devices.

• High input impedance : They use minimum input current.

• Low noise : They produce low noise and do not cause much disturbance.

 Disadvantages of MOSFET
• Breakdown: MOSFETS have a very thin layer of gate oxide so on applying high voltage it
can cause to breakdown of entire device.

• Temperature: Some functions of MOSFET are altered because of varying temperatures.

• Voltage capacity: They have a limited voltage capacity.

• Expensive: Complex manufacturing of MOSFETS can lead to high cost of overall

electronic device.
 Thank you…….