Technical Report Writing On:
Different Modes of Operations of MOSFET- An Overview
Submitted by
Name: ANUSHREE DEY
Department: ECE
Paper Name: Nanoelectronics , Paper Code: PE-EC505A
Semester: 5th
Roll Number: 16900322060
Year: 2024
Academy of Technology
Aedconagar, Hooghly-712121
West Bengal, India
Abstract
This document provides a comprehensive overview of the different
modes of operation for Metal-Oxide-Semiconductor Field-Effect
Transistors (MOSFETs). We will explore the fundamental principles
behind MOSFET operation, according to the construction as the
depletion MOSFET and enhancement MOSFET and the operating
regions (triode, saturation, and cut-off). Understanding these modes is
crucial for optimizing MOSFET performance in various electronic
circuits and systems.
Introduction
Metal-oxide field-effect transistors (MOSFETs) are another class of
FETs. They are named so because the metal gate in a MOSFET is
insulated from the semiconductor channel by a very thin oxide layer.
MOSFETs are also referred to as insulated gate field-effect transistors
(GFETs). Like a JFET, a MOSFET is also a three terminal device
where the drain current is controlled by the applied gate voltage.
These concepts include non-ideal effects, small device geometry,
breakdown, threshold voltage adjustment by ion implantation, and
radiation effects. Although there are a multitude of details that
become important when fabricating MOSFETs in ICs, we are able to
consider only a few here MOSFETs are further classified into two
types depending upon their construction and mode of operation,
namely, the depletion MOSFET (or DE-MOSFET) and the
enhancement MOSFET (or E-MOSFET).
In a depletion MOSFET, a channel is physically constructed between
the drain and the source terminals. Depletion MOSFETs are further
classified as N-channel depletion MOSFETs and P-channel depletion
MOSFETs depending on whether the channel material is an N-type
semiconductor or a P-type semiconductor. The construction of an
enhancement MOSFET is similar to that of a depletion MOSFET with
the difference that there is no physical channel between the source
and drain terminals in the enhancement MOSFET. The invention of
enhancement MOSFETs has revolutionized the computer industry and
they are extensively used in digital electronics and computers.
Procedure and Discussion
Fundamentals of MOSFET Operation
MOSFETs are three-terminal semiconductor devices that act as
voltage-controlled switches or amplifiers. They consist of a
semiconductor substrate (usually silicon), a gate oxide layer, a gate
electrode, and two diffusions called the source and drain. The gate
electrode controls the current flow between the source and drain by
modulating the conductivity of a channel beneath the gate. This
control is achieved by applying a voltage to the gate, which creates an
electric field that attracts or repels charge carriers in the channel, thus
modulating its conductivity.
Depletion MOSFETs’
It comprises of a substrate made of a P-type semiconductor material.
Two N+ type regions linked by an N-channel are formed on the
substrate. The source and the drain terminals are formed by
connecting metal contacts to the two N+ regions as shown in the
figure. The gate terminal is connected to the insulating silicon dioxide
(SiO2) layer on top of the N-channel. Therefore, there is no direct
electrical connection between the gate terminal and the channel of a
depletion MOSFET.
The construction of a P-channel depletion MOSFET is similar to that
of an N-channel depletion MOSFET with the difference being that the
substrate is an N-type semiconductor while the channel is a P-type
material.
N-Channel and p-Channel depletion type MOSFET.
Enhancement MOSFETs’
The N-channel enhancement MOSFET functions as follows. When
the gate—source voltage is zero and some positive drain—source
voltage is applied, there is no drain current as there is no channel
available for flow of drain current. Enhancement MOSFETs are also
called normally off MOSFETs as they do not conduct when VGS = 0.
The drain current flows only when a positive voltage is applied to the
gate terminal with respect to the source terminal as this induces a
channel by drawing the electrons (minority carriers) in the P-type
substrate to accumulate near the surface of the SiO2 layer.
The working of the n-channel enhancement MOSFET –
Also, holes in the P-substrate are forced to move away from the edge
of the SiO2 layer. As the SiO2 layer is insulating, it prevents the
electrons from being absorbed at the gate terminal. These electrons
lead to the flow of current between the drain and the source terminals.
As the value of gate—source voltage is increased, more and more
electrons accumulate leading to an enhanced flow of drain current.
The level of gate—source voltage that leads to significant flow of
drain current is referred to as threshold voltage and is denoted by VTH.
For a fixed gate—source voltage, increasing the level of drain—
source voltage leads to initial increase in the drain current, which
eventually saturates due to the reduction in the gate—drain voltage
(VGD) with increase in the drain—source voltage (VDS). Reduction in
the gate—drain voltage reduces the attractive forces for the free
carriers in the induced channel near the drain region, resulting in the
reduction of effective channel width near the drain region. This effect
is referred to as the pinching effect. Pinching effect refers to the
reduction in the width of the channel near the drain region with
increase in the drain—source voltage.
MOSFETs exhibit three primary operating regions: triode/linear
mode, saturation/active mode, and cutoff mode. Each mode
corresponds to a specific range of gate-source voltage (VGS) and
drain-source voltage (VDS). The mode of operation dictates the
MOSFET's behaviour as a switch or an amplifier and determines its
current-voltage characteristics.
Triode Mode
The MOSFET operates as a linear resistor.
Saturation Mode
The MOSFET acts as a current source.
Cut-off Mode
The MOSFET is off and does not conduct any current.
Triode Mode/Linear Mode/Ohmic Mode
In triode mode, the gate voltage (VGS) is sufficiently high to create a
conducting channel between the source and drain. In this mode, the
MOSFET behaves like a variable resistor. As the gate voltage
increases, the channel resistance decreases, allowing more current to
flow through the device. The drain current (ID) in triode mode is
proportional to the difference between the gate-source voltage (VGS)
and the threshold voltage (VTH), and the drain-source voltage (VDS).
Saturation Mode/active mode
Saturation mode is characterized by a strong electric field between the
drain and source. In this mode, the channel is pinched off, meaning
the density of charge carriers near the drain is reduced. The drain
current becomes almost independent of the drain-source voltage (VDS)
but is still dependent on the gate voltage (VGS). This makes the
MOSFET behave like a current source. The MOSFET exhibits its
maximum current gain in this region. It is the most commonly used
mode for amplification and switching applications
Cut-off /subthreshold /weak - inversion
In cut-off mode, the gate voltage (VGS) is below the threshold voltage
(VTH). This results in the absence of a conducting channel between the
source and drain. The MOSFET acts as an open switch, blocking the
flow of current. The current is extremely small and almost negligible
in cut-off region. This mode is used in applications where a MOSFET
is required to act as an ideal switch, for example, in digital circuits.
Conclusion
The different modes of operation of MOSFETs provide a wide range
of functionality, enabling their use in diverse electronic circuits and
systems. Understanding these modes is fundamental to designing and
optimizing MOSFET-based applications. Triode mode region
provides a linear current-voltage relationship, saturation mode region
allows for high gain and current sourcing, and cut-off mode region
acts as an open switch. By carefully selecting the appropriate mode,
designers can achieve the desired performance characteristics for their
circuits. This knowledge forms a crucial cornerstone for leveraging
the capabilities of MOSFETs in various technological advancements.
