MOSFET is metal-oxide semiconductor Field Effect
Transistor.
Unipolar device
MOSFET
has majority carriers but no minority carriers. This is a unipolar device
depending only on either electron in n-channel MOSFET or hole in
p-channel MOSFET for conduction. The
E-MOSFET operates only in the enhancement mode and has no depletion
mode. It differs in construction from the D-MOSFET, it has no structural
channel
Construction:
MOSFET has three terminals namely source,
gate, and drain. The substrate is high resistive p-type material. This device has no pn junction structure.
Source and drain:
These
are heavily doped n-type material and these are diffused in substrate.
Silicon dioxide: A thin layer of
silicon dioxide (SiO2) is deposited on the left side of the channel.
Silicon dioxide is the same as glass, which is an insulator or dielectric. Due to SiO2, the MOSFET has very high
input impedance.
Gate: The gate is
metallic and is insulated from the channel by a very
thin silicon dioxide (SiO2) layer. Polycrystalline
silicon is now used for the gate material instead of metal. Another name
for the device is insulated gate FET or
IGFET.
Operation:
VGG-
Supply voltage to gate
VDD-Supply
voltage to drain
VDS-
Potential difference across Drain and Source
VGS-
Potential difference across Gate and Source
Voltage
controlled device
The output drain current is controlled
by gate voltage.
Enhancement-mode device -The
n-channel E-MOSFET is an enhancement-mode device because
a gate voltage greater than the threshold voltage enhances its conductivity thus
pulling more electrons into the channel area. For any gate voltage below the
threshold value, there is no channel.
Positive VGS,
positive VDS
When the gate
is positive, it attracts free electrons into the p region. The free electrons recombine with
the holes next to the silicon dioxide. When the gate voltage is positive enough,
all the holes touching the silicon dioxide are filled, and free electrons begin
to flow from the source to the drain. This current is drain current whose
magnitude depends on channel resistance. The effect is the same as creating a thin
layer of n-type
material next to the silicon dioxide.
Formation of channel- The inversion layer
For an n-channel
device, a positive gate voltage above a threshold value induces a
channel by creating a thin layer of negative charges in the substrate region
adjacent to the SiO2 layer. This inversion layer acts like an n
channel connects the source to the drain
Threshold
voltage
The minimum
VGS that
creates the n-type
inversion layer is called the threshold voltage VT.
When VGS
is less than VT, the
drain current is zero. When VGS is
greater than VT,
an n-type
inversion layer connects the source to the drain and the drain current can flow
which when the device is conducting
When
VGS becomes
positive, Drain current ID will increase
following the square-law equation:
Drain Characteristics
This characteristics is
Drain source voltage Vs Drain current
There are three regions
of operation
1.
Ohmic
region – The curve is linear E-MOSFET is
equivalent to a resistor.
2.
Saturation
region- The drain current is constant it is equivalent to a current source.
3. Cutoff region - When
VGS is
less than VT,
The drain current is almost zero the
E-MOSFET is off because there is no conducting channel between source and drain.
Channel Length Modulation
In
MOSFET, a nonzero slope exists beyond the saturation point. For the saturation
region, (VDS>VDS(sat)), The effective channel length
decreases and this phenomenon is called Channel
Length Modulation
This
characteristics is gate source voltage VGS VS Drain current ID
Application
1.
The
enhancement-mode MOSFET is widely used in both discrete and integrated
circuits.
2.
In
discrete circuits, the main use is in power switching, which means turning
large currents on and off.
3.
In
integrated circuits, the main use is in digital switching, the basic process
behind modern computers.
