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
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.
Channel: Source and
drain are connected to a narrow channel which is n-type region and it is adjacent
to the insulating gate.
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, MOSFET has very high input impedance.
Gate: The gate is
metallic and it is insulated from the channel by
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:
Voltage
controlled device
The output drain current is controlled
by gate voltage.
Mode of operation
1 Depletion-mode
When Gate source Voltage VGS is
negative, the D-MOSFET is operating in the depletion mode.
2Enhancement-mode
When Gate
source Voltage VGS is
positive, the D-MOSFET is operating in the enhancement mode
1.
During VGS=0 and positive VDS
The
result is an attraction for the positive potential at the drain by the free
electrons of the n-channel.
The resulting current is IDSS
VGG-
Supply voltage to gate
VDD-Supply
voltage to drain
VDS- Potential
difference across Drain and Source
VGS-
Potential difference across Gate and Source
2.
During negative VGS and positive VDS (or) Depletion-mode
– Formation of parallel plate capacitor
The gate acts
as one plate of a parallel-plate capacitor and the channel as the other plate.
The silicon dioxide insulating layer is the dielectric. With a negative gate
voltage, the negative charges on the gate repel conduction electrons from the
channel and attract holes from the p-type
substrate. Recombination between electrons and holes will occur and leaving positive ions in channel. The n channel is
depleted of some of its electrons, thus decreasing the channel conductivity. The
drain current is small. At VGS(off), the channel is
totally depleted and the drain current is zero or cut off.
3. During positive VGS and
positive VDS(or) Enhancement-mode
The positive gate voltage increases the
number of free electrons flowing through the channel. The more positive the
gate voltage, the greater the conduction from source to drain. The VDD
supply forces free electrons to flow from source to drain.
When
VGS becomes
positive, ID will
increase following the square-law equation:
Drain Characteristics
There are three regions of operation
1.
Ohmic
region – This is the vertical part of the curve which is linear
2.
Saturation
region- This is the horizontal part of the
curve. The drain current is constant
3.
Cutoff region - The drain current is almost zero
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
Enhancement region
The region of
positive gate voltages on the drain or transfer characteristics
is the enhancement region,
Depletion region
The region
between cutoff and the saturation level of IDSS is the depletion
region.
Application
1.
Depletion
mode MOSFETs are used in high-frequency
front-end
communications circuits as RF amplifiers
2.
The
enhancement-mode MOSFET is widely used in both discrete and integrated
circuits.
Explain Channel Length Modulation (4m)
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
