A Metal Oxide Semiconductor Field-effect Transistor (MOSFET, MOS-FET, or MOS FET) functions as a field-effect transistor (FET with an insulated gate) wherein the conductivity of the device is regulated by voltage.
It serves the purpose of signal switching or amplification. The capability to modulate conductivity based on the applied voltage finds utility in amplifying or switching electronic signals.
MOSFETs have surpassed bipolar junction transistors (BJTs) in prevalence across both digital and analog circuits.
The silicon dioxide constitutes the Gate of the MOSFET, facilitating isolation by obstructing direct charge flow from the gate to the conducting channel.
MOSFET remains the predominant transistor in digital circuits, with numerous instances—ranging from hundreds of thousands to millions—incorporated in memory chips or microprocessors.
Given their ability to be fabricated with either p-type or n-type semiconductors, complementary sets of MOS transistors enable the creation of switching circuits boasting remarkably low power consumption, in the configuration of CMOS logic.
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Why Choose MOSFET Over BJT?
MOSFETs are especially advantageous in amplifiers due to their nearly infinite input impedance, which allows the amplifier to capture nearly all of the incoming signal. The primary benefit is that they require virtually no input current to control the load current, making MOSFETs preferable to BJTs.
Structure:
A MOSFET is a four-terminal device comprising Source (S), Drain (D), Gate (G), and Body (B) terminals. The Body (B) is often connected to the source terminal, effectively reducing the number of terminals to three. Its operation involves varying the width of a channel through which charge carriers (electrons or holes) flow.
Charge carriers enter the channel at the source and exit via the drain. The channel width is modulated by the voltage on an electrode called the Gate, situated between the source and the drain. This Gate is insulated from the channel by an extremely thin layer of metal oxide.
A metal-insulator-semiconductor field-effect transistor (MISFET) is a term nearly synonymous with MOSFET. Another equivalent term is IGFET, which stands for insulated-gate field-effect transistor.
Different Types of MOSFET
MOSFET operates in two modes:
1. Depletion Mode: This mode requires the Gate-Source voltage (VGS) to turn the device “OFF.” A depletion-mode MOSFET functions like a “Normally Closed” switch.
2. Enhancement Mode: This mode requires a Gate-Source voltage (VGS) to turn the device “ON.” An enhancement mode MOSFET functions like a “Normally Open” switch.
Based on the working principle, MOSFETs are classified as:
– P-Channel Depletion MOSFET
– P-Channel Enhancement MOSFET
– N-Channel Depletion MOSFET
– N-Channel Enhancement MOSFET
P-Channel MOSFET
In a P-Channel MOSFET, the drain and source regions are heavily doped p+ areas, and the substrate is n-type. The current is carried by positively charged holes, hence the name p-channel MOSFET.
When a negative gate voltage is applied, the electrons beneath the oxide layer experience a repulsive force and are pushed into the substrate. The depletion region is then populated by bound positive charges associated with donor atoms.
The negative gate voltage also attracts holes from the P+ source and drain regions into the channel region.
N-Channel MOSFET
In an N-Channel MOSFET, the drain and source regions are heavily doped N+, while the substrate is p-type. The current flows due to the movement of negatively charged electrons, hence it is called an n-channel MOSFET.
When a positive gate voltage is applied, the holes beneath the oxide layer experience a repulsive force, pushing them downward into the bound negative charges associated with the acceptor atoms. This positive gate voltage also attracts electrons from the N+ source and drain regions into the channel, forming an electron-rich channel.
MOSFET Working Operation
The operation of a MOSFET relies on the MOS capacitor, which is a crucial component of the MOSFET. The semiconductor surface beneath the oxide layer, located between the source and drain terminals, can be inverted from p-type to n-type by applying positive or negative gate voltages.
When a positive gate voltage is applied, the holes under the oxide layer are repelled and pushed into the substrate. This creates a depletion region populated by bound negative charges associated with the acceptor atoms. As electrons accumulate, a conductive channel is formed. The positive voltage also draws electrons from the n+ source and drain regions into the channel.
If a voltage is applied between the drain and source, current flows freely between them, with the gate voltage regulating the electrons in the channel. Conversely, if a negative voltage is applied, a hole channel will form under the oxide layer.
N-Channel MOSFET
In an N-Channel MOSFET, the drain and source regions are heavily doped N+, while the substrate is p-type. The current flows due to the movement of negatively charged electrons, hence it is called an n-channel MOSFET.
When a positive gate voltage is applied, the holes beneath the oxide layer experience a repulsive force, pushing them downward into the bound negative charges associated with the acceptor atoms. This positive gate voltage also attracts electrons from the N+ source and drain regions into the channel, forming an electron-rich channel.
MOSFET Working Operation
The operation of a MOSFET relies on the MOS capacitor, which is a crucial component of the MOSFET. The semiconductor surface beneath the oxide layer, located between the source and drain terminals, can be inverted from p-type to n-type by applying positive or negative gate voltages.
When a positive gate voltage is applied, the holes under the oxide layer are repelled and pushed into the substrate. This creates a depletion region populated by bound negative charges associated with the acceptor atoms. As electrons accumulate, a conductive channel is formed. The positive voltage also draws electrons from the n+ source and drain regions into the channel.
If a voltage is applied between the drain and source, current flows freely between them, with the gate voltage regulating the electrons in the channel. Conversely, if a negative voltage is applied, a hole channel will form under the oxide layer.
