INTRODUCTION TO MOSFETs

MOSFET
A Metal Oxide Semiconductor Field effect transistors,
commonly known as MOSFET, are semiconductor switching
devices that have three terminals which are the gate, drain and
source.
It is used for switching or amplifying signals.

They are unipolar transistors, this means they depend on one

type of charge carriers which can either be holes or electrons.

Its low operating power requirements, switching speed, smaller

size and high-frequency operations made them a better
alternative to bipolar devices. Their switching speed capability
exceeds several hundred kHz.
MOSFET

• MOSFETs are particularly useful in amplifiers due to their input

impedance being nearly infinite which allows the amplifier to capture
almost all the incoming signal.
• The main advantage is that it requires almost no input current to
control the load current and that’s why we choose MOSFET over BJT.
 Structure and operations

• It is a four-terminal device with Source (S), Drain (D), Gate (G), and
body (B) terminals.
• The body (B) is frequently connected to the source terminal,
reducing the terminals to three.
• It works by varying the width of a channel along which charge
carriers flow (electrons or holes).
• The charge carriers enter the channel at the source and exit via the
drain.
• The width of the channel is controlled by the voltage on an
electrode called Gate which is located between the source and the
drain.
• It is insulated from the channel near an extremely thin layer of
metal oxide.
The classification of MOSFET based on the construction and the material
• The line between the
drain (D) and source (S)
connections show the
semiconductive channel of
the transistor. If the line is
unbroken then it represents
the Depletion type of
MOSFET which is normally
in an ON state as the drain
current can flow with no
gate biasing potential.
 Operation of MOSFET
• Based on the construction, MOSFET can be classified into
NMOS and PMOS.
Lets us consider NMOS.
• NMOS consists of a lightly doped p-substrate, and two
heavily doped n-type regions called as Source and drain.
The source and drain terminals are interchangeable.
• The gate terminal is completely insulated from the p-type
substrate by a silicon-oxide layer.
• When a negative voltage is applied to the gate terminal,
an electric field is set up through the silicon-oxide layer.
• This electric field attracts the holes (positively charged
majority carriers) towards the silicon-oxide layer and
repels electrons (negatively charged minority carriers)
away from it.
• At this point, the region below the silicon-oxide layer lacks
electrons.
• This region of charge accumulation is known as
accumulation region Accumulation of holes below silicon
dioxide layer
• At this point, if we apply a small
positive gate voltage, holes are
pushed away from the oxide. At the
same time, the weak electric field will
not be able to attract electrons
toward the oxide region.

• Thus the region below the silicon

dioxide layer lack any charged
particles. This region of operation is
called the depletion region.
Region below Silicon-di-oxide lacks charged particle.
• At this point, if we increase the applied positive
voltage, the electric field becomes strong enough
to pull the electrons towards the silicon-oxide
region.
• At a certain voltage, the concentration of
electrons becomes so high that the region close
to the silicon dioxide insulation becomes n-type.
• This region of operation is known as the inversion
region.
• At this point, the inversion region connects the
heavily doped source and drain regions and
electrons can flow from the drain to the source
through the inversion region.
• The gate voltage required to set up an inversion
layer underneath the silicon-dioxide layer of the Region below Silicon-di-oxide lacks charged particle.
MOSFET is called as the threshold voltage.
A MOSFET is a voltage-controlled FET
which has a metal oxide gate electrode.
Three terminal voltages can be seen in the
figure which are:
•VGS: Gate to Source voltage
•VDS: Drain to Source voltage
•VBS: Body to Source voltage.
The insulated gate acts as one plate of a capacitor. This increases the overall
input resistance of the MOSFET, therefore, making it almost infinite.
This property of the MOSFET makes it a viable for electronic switches or for
making logic gates as with no bias and a high input resistance little or no input
current is required.
Operation of n-channel MOSFET

• When VDS = 0 and VGS = 0, MOSFET remains in

the cutoff region and no current flows between
source and drain.
• When VDS = 0 and 0 < VGS < Vt, the depletion
region is formed.
• When VDS = 0 and VGS > Vt, the inversion region
is formed and MOSFET will be ready to conduct.
• At this point of VDS is increased, current flows
from drain to source and the IDS ∝ VDS.
• Increasing VGS further increases the electron
density in the channel and also increases the
conductivity between the source and drain.
 • A MOSFET is seen to exhibit three operating regions. Here, we
will discuss those regions.
• 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.
Operating • Ohmic Region

Regions of • 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.
MOSFET • 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.
Different Types of MOSFET
1.) Depletion Mode: The transistor requires the Gate-
Source voltage (VGS) to switch the device “OFF”. The
depletion-mode MOSFET is equivalent to a “Normally Closed”
switch.
2.) Enhancement Mode: The transistor requires a Gate-
Source voltage(VGS) to switch the device “ON”. The
enhancement-mode MOSFET is equivalent to a “Normally
Open” switch.
Now with respect to the working principle, MOSFET is
classified as follows:
• P-Channel Depletion MOSFET
• P-Channel Enhancement MOSFET
• N-Channel Depletion MOSFET
• N-Channel Enhancement MOSFET
Depletion Mode MOSFET
• This type of MOSFET is the least used transistor
mode and is normally in an ON state when there is
no gate bias voltage.
• This means that the channel would only conduct
when VGS=0.
• The straight unbroken line in the symbol suggests
that the depletion-mode MOSFET is considered a
normally closed device that conducts when the
conductive channel is closed.
• In the n-channel depletion mode, when a negative
gate-source voltage (-VGS) is applied, it depletes
the conductive channel of its free electrons,
therefore, switching the transistor “OFF”.
• Likewise in a p-channel depletion MOS transistor
a positive gate-source voltage, +VGS depletes the
conductive channels of its free holes, therefore,
turning the transistor “OFF”.
•
• Hence, if an n-channel depletion mode is used and the gate-
source voltage is positive this would suggest that more
electrons are allowed to roam freely, therefore, allowing more
current to flow and vice versa would hold for p-channel
depletion mode MOSFET.
• In a p-channel depletion mode MOSFET –VGS would mean
fewer electrons are allowed to flow freely, therefore, increasing
the total number of holes. This would mean less current would
flow and this type of MOSFET would be considered a “normally-
closed” switch.
Enhancement Mode MOSFET (eMOSFET)
• The eMOSFET’s working principle is opposite to that of
the depletion-mode MOSFET.
• In this type of transistor, the conducting channel is only
lightly doped or sometimes not even doped which makes
it non-conductive.
• Therefore, the device is normally in an OFF state which
means it is non-conducting when the gate-bias voltage
VGS=0.
• Its symbol consists of broken channel lines which
illustrate that it is an open normally non-conducting
channel.
• When the n-channel is the conductive channel
the eMOSFET will only allow the drain current to
flow when a gate voltage (VGS) is applied at the
gate terminals.
• The applied voltage needs to be greater than
the threshold voltage V(TH) level in which the
conductance takes place.
• This would make the semiconductor device a
transconductance device.
• When a positive voltage is applied to the gate in
an n-channel device, more electrons are
attracted towards the oxide layer present around • For a p-type eMOSFET, the device is in
the gate. an “OFF” state when VGS=0 and the
• This enhances the thickness of the channel conductive channel is open.
which allows more current to flow.
• When a negative gate voltage (-VGS)
• The increase in the positive voltage at the gate is applied to the p-type eMOSFET, the
results in a decrease in channel resistance.
channel conductivity increases
• This causes the drain current ID through the
channel to increase. therefore turning the transistor “ON”.
• To summarise, for an n-channel eMOSFET, a • This would mean for the transistor to
positive VGS switches the transistor “ON” and a be in an “OFF” state the applied
zero or –VGS switches the transistor “OFF”. voltage at the gate needs to be positive
(+VGS).
• Review:
• Metal Oxide Semiconductor Field Effect Transistor is a voltage controlled unipolar
switching device.
• It has a metal layer at the top, a silicon oxide insulation beneath that and a
semiconductor part at the bottom giving it a vertical structure of p and n layers.
• Based on operation, MOSFETs are classified into Enhancement MOSFET (p-
channel and n-channel) and depletion MOSFET (p-channel and n-channel).
• They have three terminals: Gate, Source and Drain.
• Gate acts as the control terminal and the actual conduction occurs between the
source and the drains.
• Gate is insulated from the semiconductor layer by a thin layer of SiO2.
• Switching times of a MOSFET can be controlled completely by an external gate
drive design.
 Why Biasing required?
• Biasing a transistor is the process of applying a DC
voltage to the transistor's terminals in order to
establish a stable and predictable operating point,
or "bias point," for the transistor.
• The need for biasing a transistor is because
transistors are active devices, meaning that their
operating point is not fixed and can be easily
influenced by changes in temperature and other
environmental factors.
• Without biasing, the transistor would not operate in
a stable and predictable manner, making it difficult
or impossible to use in electronic circuits.
MOSFET BIASING