MOSFET
Metal Oxide Semiconductor
Field Effect Transistor
Introduction
• MOSFETs are the most common type of Field Effect Transistor
• MOSFETs are used in switching or amplifying electronic signals
• MOSFETs are voltage-controlled device
• MOSFETs are three-terminal devices with Source (S), Drain (D), and Gate (G) terminals.
• MOSFETs have a very high switching speed
• There are 2 classes of MOSFETS : Enhancement and Depletion type
• These 2 classes are further divided as N-channel and P-channel
Types of MOSFET
N-channel
Enhancem
ent
MOSFET
N-channel Enhancement MOSFET
It has P-type semiconductor material as body/substrate.
Two N-type wells are diffused on the body/substrate.
Two PN junctions are formed creating 2 depletion regions.
A metal contact is created at the bottom and a terminal is taken out known as body/ substrate terminal.
Same metal contacts are created on the top of N-type making source and drain terminals.
There is a layer of Silicon Dioxide in between the 2 N-type well which act as a dielectric.
A metal contact is created on the top of SiO2 layer and a terminal is taken out called Gate.
Generally Source and body terminal are internally connected and is grounded.
N-channel Enhancement MOSFET
Apply VGS between gate and source
terminal keeping gate at high potential
Electrons will push towards the gate
terminal and accumulate there
On increasing VGS electron-hole pairs are
formed pushing holes below and
accumulating more electrons near Gate
When VGS reaches a threshold voltage, an
n-type channel is formed
N-channel Enhancement MOSFET
Now on applying VDS keeping the drain at high
potential, the drain current will flow
On increasing the value of VDS, the drain current
will increase
VDS at which the channel is pinched-off and the
drain current stops rising is called pinch-off
voltage or saturation voltage
At a pinch of voltage, the drain current is called the
saturation current
Now to increase saturation current further increase
VGS
N-channel
Enhancemen
t MOSFET V-I
Characteristi
cs
N-channel
Depletion
MOSFET
N-channel
Depletion MOSFET
• N-type Channel is already present
• Thickness of channel increases on applying VGS keeping gate at
higher potential
• Now on applying VDS keeping drain at hight potential, drain current
will flow
• On increasing the value of VDS, drain current will increase
• On increasing VDS further the N-channel towards drain will become
narrow thus decreasing drain current
• VDS at which channel is pinched-off and drain current stop rising is
called pinch-off voltage or saturation voltage
• At pinch of voltage drain current is called saturation current
• Now to increase saturation current further increase VGS
N-channel
Depletion
MOSFET V-I
Characteris
tics
Important equations region wise
P-channel
Enhancem
ent
MOSFET
P-channel Enhancement MOSFET
• It has N-type semiconductor material as body/substrate
• Two P-type wells are diffused on the body/substrate
• Two PN junctions are formed creating 2 depletion regions
• A metal contact is created at the bottom and a terminal is taken out known as body/ substrate terminal
• Same metal contacts are created on the top of P-type making source and drain terminals
• There is a layer of Silicon Dioxide in between the 2 P-type well which act as a dielectric
• A metal contact is created on the top of SiO2 layer and a terminal is taken out called Gate
• Generally Source and body terminal are internally connected and is grounded
P-channel
Enhancement
MOSFET
• Apply VGS between gate and source terminal keeping gate at
low potential
• Holes will push towards the gate terminal and accumulate
there
• When VGS reaches a threshold voltage, an p type channel is
formed
• Now on applying VDS keeping drain at low potential, drain
current will flow
• On increasing the value of VDS, drain current will increase
• VDS at which channel is pinched-off and drain current stop
rising is called pinch-off voltage or saturation voltage
• At pinch of voltage drain current is called saturation current
• Now to increase saturation current further make VGS more
negetive
P-channel
Enhancemen
t MOSFET V-I
Characteristi
cs
P-channel
Depletion
MOSFET
P-channel
Depletion
MOSFET
• P-type Channel is already present
• Apply VDS keeping drain at low potential, drain
current will flow
• On increasing the value of VDS, drain current
will increase
• On increasing VDS further the P-channel
towards drain will become narrow thus
decreasing drain current
• VDS at which channel is pinched-off and drain
current stop rising is called pinch-off voltage or
saturation voltage
• At pinch of voltage drain current is called
saturation current
• Now to increase saturation current further
increase VGS
P-channel
Depletion
MOSFET V-I
Characteris
tics
