A crucial part of contemporary electronics, the
Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is the basic
building block of many different kinds of integrated circuits (ICs), ranging
from power supply to microprocessors. MOSFETs, one of the most effective and
adaptable semiconductors, are crucial for a variety of applications, including
power management systems, analogue signal processing, and digital circuits.
It is necessary to examine a MOSFET's structure, the function of each terminal,
and the physical principles underlying its operation in order to comprehend its
working principle. The operating concept of the MOSFET is thoroughly examined
in this article, covering everything from its fundamental functions to its uses
in electronics.
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1. Overview of MOSFET
The following terminals make up the MOSFET, a three-terminal device:
The terminal that allows carriers (electrons or holes) to enter the device is
called the source (S).
Drain (D): The terminal that allows carriers to leave the apparatus.
The terminal that regulates the flow of current between the source and drain is
called a gate (G). It is known as a Metal-Oxide-Semiconductor field-effect
transistor because of the thin oxide layer that isolates the gate from the
semiconductor.
Depending on the kind of charge carrier used in current conduction, MOSFETs are
divided into two main categories:
The MOSFET's Basic Structure
A MOSFET's fundamental structure is a semiconductor channel, typically made of
silicon, with the source and drain being formed by regions with opposing
doping. A thin layer of oxide (SiO₂) separates the channel from the gate and
sits between the source and the drain. The oxide layer is covered by the gate
electrode, which is usually composed of metal or polysilicon.
The body (bulk) of an N-channel MOSFET is doped with P-type material, while the
source and drain are doped with N-type material.
The body of a P-channel MOSFET is doped with N-type material, while the source
and drain are doped with P-type material.
2. The Function of MOSFETs
By supplying a voltage to the gate, a MOSFET's main job is to regulate the
current flowing between the source and the drain. The MOSFET's conductivity is
determined by the gate-to-source voltage (Vgs).
3. Threshold voltage
A threshold voltage (Vth) must be exceeded by the voltage provided to the gate
(Vgs) in order for the MOSFET to conduct. In order to invert the kind of
material beneath the gate region and create a conductive channel between the
source and drain, this voltage is the bare minimum needed.
Current can go from the source to the drain of an N-channel MOSFET when Vgs
surpasses Vth because an inversion layer of electrons forms beneath the gate.
4. Operating
Modes
Depending on the Vgs and the voltage between the drain and source (Vds),
MOSFETs typically function in three main modes.
OFF Mode, or Cutoff Mode
The MOSFET is disabled in this mode when the gate-source voltage (Vgs) is lower
than the threshold voltage (Vth). No current flows since there isn't a
conductive route connecting the source and drain. The MOSFET functions
similarly to an open switch.
· Condition: Vgs<VthV_{gs}
< V_{th}Vgs<Vth
·
Current: Ids=0I_{ds} = 0Ids=0
Triode or Linear Mode (ON Mode)
The MOSFET functions in the linear or triode zone when Vgs is higher than Vth
and the drain-source voltage (Vds) is low. With the current flowing from source
to drain reliant on both Vgs and Vds, the MOSFET functions similarly to a
variable resistor in this mode. Analogue amplification is a popular use for
this mode.
· Condition: Vgs>VthV_{gs}
> V_{th}Vgs>Vth and Vds<Vgs−VthV_{ds} < V_{gs} - V_{th}Vds<Vgs−Vth
· Current: Ids=k[(Vgs−Vth)Vds−Vds22]I_{ds}
= k \left[ (V_{gs} - V_{th}) V_{ds} - \frac{V_{ds}^2}{2} \right]Ids=k[(Vgs−Vth)Vds−2Vds2]
saturation mode
In saturation mode, the Vgs is greater than Vth, and Vds exceeds −Vth. In this mode, the
MOSFET is fully "on," and current between the source and drain
becomes relatively independent of Vds. This is the mode in which the MOSFET
operates as a current amplifier.
·
Condition: Vgs>VthV_{gs} > V_{th}Vgs>Vth and Vds>Vgs−VthV_{ds}
> V_{gs} - V_{th}Vds>Vgs−Vth
· Current: Ids=k2(Vgs−Vth)2I_{ds} =
\frac{k}{2} (V_{gs} - V_{th})^2Ids=2k(Vgs−Vth)2
Gate voltage-based current control
The MOSFET's gate voltage (Vgs) regulates its operation. An electric field
produced in the semiconductor by the gate voltage alters the conductivity of
the channel between the source and drain. The MOSFET can now function as a
voltage-controlled resistor as a result.
By attracting electrons to the channel beneath the gate with a positive
voltage, an N-channel MOSFET creates a conductive tunnel. The conductivity of
the channel increases with Vgs, permitting more current to move from the source
to the drain.
When a negative voltage is applied to the gate of a P-channel MOSFET, holes are
drawn to the channel, enabling current to go from the drain to the source.
MOSFET Pinch-off and Saturation
In the saturation mode, a phenomena known as pinch-off takes place as the drain
voltage (Vds) rises. This occurs when the current saturates because the voltage
differential between the gate and the drain is insufficient to sustain the
inversion layer. The gate-source voltage (Vgs) is the primary determinant of
the current in the saturation zone, where it stays largely constant even as Vds
increases further.
Affecting MOSFET Performance Factors
A number of factors affect a MOSFET's performance, including:
Thickness of Gate Oxide
The MOSFET's power consumption and switching speed are directly impacted by the
gate oxide layer's thickness. Faster switching times and reduced gate capacitance
are made possible by a thinner gate oxide, but leakage current may also rise as
a result. In order to balance performance and power efficiency, high-k
dielectrics or ultra-thin gate oxides are frequently used in current MOSFETs.
Length of Channel
The MOSFET's on-resistance and switching properties are determined by the
length of the channel between the source and drain. The channel length has been
reduced in advanced technology nodes in order to boost integration density and
device speed, however this also results in an increase in leakage current and
short-channel effects.
Concentration of Doping
The threshold voltage (Vth) and the general behaviour of the MOSFET are
influenced by the doping concentration of the source, drain, and channel
material. Higher doping levels, for instance, may result in reduced breakdown
voltage and greater junction capacitances in addition to increased current
conduction.
Applications of MOSFETs
Because of its efficiency and adaptability, MOSFETs are widely used in many
different applications. Among the frequent applications are:
Circuits for Digital Logic
The fundamental components of contemporary digital logic circuits, such as
those utilising CMOS (Complementary Metal-Oxide-Semiconductor) technology, are
MOSFETs. NMOS and PMOS transistors are used to create logic gates in CMOS
circuits, which enable high-speed and low-power operation. Microprocessors,
memory chips, and other electronic devices use CMOS technology.
Electronic Power
MOSFETs are frequently found in power electronics, such as power amplifiers,
motor drives, and DC-DC converters. They are perfect for power control in a
range of applications due to their quick switching properties and capacity to
handle large currents and voltages.