Working Principles of Depletion and Enhancement MOSFETs
Prepared by
Syed Umar Quadri
Roll No. 05
&
Yahiya Sani khan
Roll no.6
• Metal Oxide Silicon Field Effect
Transistors commonly known as
MOSFET 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
• Body
Types of MOSFET
Symbol of MOSFET
MOSFET Construction
• The p-type semiconductor forms the base of the MOSFET.
• The two types of the base are highly doped with an n-type impurity
which is marked as n+ in the diagram.
• From the heavily doped regions of the base, the terminals source
and drain originate.
• The layer of the substrate is coated with a layer of silicon dioxide
for insulation.
• A thin insulated metallic plate is kept on top of the silicon dioxide
and it acts as a capacitor.
• The gate terminal is brought out from the thin metallic plate.
• A DC circuit is then formed by connecting a voltage source between
these two n-type regions.
Working of Enhancement MOSFET
• In the enhancement mode the applied Gate voltage is always
positive.
• When it crosses the threshold voltage it turns ON.
• The current is generated due to the movement of majority
carriers.
• In N channel majority carriers are electrons and in P channel
majority carriers are holes.
• The source is connected to the negative terminal.
• When the electrons move from source to drain the positive
charges formed below the dielectric because of the repulsive
force from gate combine with each other
Working of Enhancement MOSFET
Working of Enhancement MOSFET
• When the applied gate voltage is increased the
number of majority carriers becomes more than the
minority carriers below the dielectric medium.
• So the majority carriers overcomes the
recombination of holes and electrons and the
majority carrier move from source to drain in the
channel, which forms the current.
• Thus the gate voltage controls the concentration of
the majority carriers which is responsible for the
formation of the channel.
Working of Depletion MOSFET
• The depletion MOSFET is ON by default.
• The source and drain terminals are physically connected.
• When the gate terminal is connected to the negative terminal and source to
the positive terminal, the electrons gets repelled below the dielectric layer.
• The positive charged carrier from the source gets combined with the
majority carrier the electrons in the N type and thus depletion layer is
formed and the channel resistance increases and the current flow decreases.
• Thus the increase in gate voltage decreases the drain current.
• They are inversely proportional.
• When the negative voltage is further increased it reaches the pinch off
mode.
• When the gate is connected to the positive terminal and the source terminal
it operates in the enhancement mode.
Working of Depletion MOSFET
V-I Characteristics of MOSFET
V-I Characteristics of MOSFET
• Cut off region:
No current flows through it and the MOSFET is off.
• Ohmic region:
Drain current increases when the drain source voltage increases. Used as
amplifier in this region.
• Saturation region:
Drain current is constant for drain source voltage. Used as switch in this
region. This occurs when the drain source voltage reaches pinch off
voltage.
• Depletion mode:
The MOSFET is ON by default. When negative voltage is applied to the
gate terminal it operates in the depletion mode and when positive voltage
is applied, it operates in the enhancement mode.
• Enhancement mode:
When positive voltage is applied to the gate terminal, it starts conducting
MOSFET as a Switch
• MOSFETs are commonly used as
switches. The circuit below shows the
configuration of MOSFET when it is
used as a switch.
• In the circuit arrangement, an
Enhancement-mode N-channel
MOSFET is used to switch a simple
lamp “ON” and “OFF.” The input fate
voltage Vgs is adjusted to an
appropriate positive voltage to switch
“ON” the device and the voltage level
is set to a negative value or zero to
turn it “OFF.”
MOSFET Applications
• Radiofrequency applications use MOSFET
amplifiers extensively.
• MOSFET behaves as a passive circuit element.
• Power MOSFETs can be used to regulate DC
motors.
• MOSFETs are used in the design of the
chopper circuit.
Advantages & Disadvantages
Advantages of MOSFET:
• MOSFETs operate at greater efficiency at lower
voltages.
• Absence of gate current results in high input
impedance producing high switching speed.
Disadvantages of MOSFET:
• MOSFETs are vulnerable to damage by
electrostatic charges due to the thin oxide layer.
• Overload voltages make MOSFETs unstable.
Summary: Characteristics for both N-channel
and P-channel type MOSFET
References
• https://byjus.com/physics/mosfet/
• https://www.learnelectronicswithme.com/202
0/07/mosfet-types-construction-working-and.
html
THANK YOU !
