MOSFET (METAL OXIDE
FIELD EFFECT TRANSISTOR)
WHAT IS A MOSFET?
Metal Oxide Silicon Field Effect Transistors commonly known as MOSFETs are
electronic devices used to switch or amplify voltages in circuits. It is a voltage
controlled device and is constructed by three terminals.
The terminals of MOSFET are named as follows:
• Source
• Gate
• Drain
CONSTRUCTION OF MOSFET
• 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.
WORKING PRINCIPLE OF MOSFET
• The ability to change conductivity with the amount of applied voltage is used
for amplifying or switching electronic signals.
• When voltage is applied to the gate, an electrical field is generated that
changes the width of the channel region, where the electrons flow.
• It works almost like a switch and the functionality of the device is based on
the MOS capacitor.
• As the Gate terminal is electrically isolated from the main current carrying
channel between the drain and source, “NO current flows into the gate” and
just like the JFET, the MOSFET also acts like a voltage controlled resistor
where the current flowing through the main channel between the Drain and
Source is proportional to the input voltage.
MOS CAPACITOR
• The MOS structure can be considered a parallel-plate capacitor, with the top plate
being the positive plate, oxide being the dielectric, and Si substrate being the
negative plate. (We are assuming P-substrate.)
• The capacitance of an MOS capacitor varies with the voltage applied to its gate
terminal, with the body typically grounded during this application.
• The flat band voltage is an important term related to the MOS capacitor. It is
defined as the voltage at which there is no charge on the capacitor plates and hence
there is no static electric field across the oxide.
• Depending on the type (doping) of the semiconductor and the voltage applied on the metal
gate, the MOS Capacitor has three modes of operation.
• In one mode, the voltage applied on the metal contact accumulates majority carriers on the
surface of the semiconductor and this is called "Accumulation."
• In the other case, the applied voltage induces minority carriers on the semiconductor
surface. This initially creates a depletion region at the surface (the "Depletion" mode of
operation).
• By increasing the applied voltage VG >>0, the band bending becomes so large. (Ei cross
over the Fermi level) and electrons is greater than holes and we enter the inversion regime
• This final mode is called the "Inversion."
TYPES OF MOSFET
The classification of MOSFET based on the construction and the material used
is given below in the flowchart.
Enhancement Mode
• 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.
• Depletion Mode MOSFET
In this type of MOSFET, channel is
present from the beginning which
means conduction of current is there.
When the voltage across the gate
terminal is either positive or negative,
then the channel conductivity
decreases.
SYMBOL OF P-CHANNEL SYMBOL OF N-CHANNEL
MOSFET MOSFET
OPERATING REGIONS OF MOSFET
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.
• 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.
MOSFET VS BJT
MOSFET BJT
• There are two types of MOSFET and • BJT is of two types and they are
they are named: N-type or P-type 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
ADVANTAGES OF MOSFET
• MOSFETs operate at greater efficiency at lower voltages.
• 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.
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.
THANK YOU
