ETI 2203 PHYSICAL ELECTRONICS II

Metal-Oxide Semi-Conductor FET

Introduction
A MOSFET is a compact transistor. Transistors are semiconductor devices used to
control the flow of electric current by regulating how much voltage flows through
them.
What makes it different from a BJT is how it allows current to pass through. In
MOSFET, the voltage applied to the gate region determines how much current flows
from drain to source. And, this property gives MOSFETs their name – Metal Oxide
Semiconductor Field Effect Transistors. Interestingly, these transistors can amplify a
signal or let only one specific kind of electric charge carrier through.

Physical Structure

MOSFET is a three-terminal device that determines electric flow in a

closed circuit. Its primary structure terminals are; Source (S), gate (G), and
Drain (D). What it does is dependent on the applied gate voltage.
However, if we consider its base/body (B), then MOSFET is a four-
terminal device. The body (B) is frequently connected to the source terminal,
reducing the terminals to three. It works by varying the width of a channel along
which charge carriers flow (electrons or holes).
The charge carriers enter the channel at the source and exit via the drain.
The width of the channel is controlled by the voltage on an electrode
called Gate which is located between the source and the drain. It is
insulated from the channel near an extremely thin layer of metal oxide.
A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost
synonymous with MOSFET. Another synonym is IGFET for the insulated-gate field-
effect transistor.

Operation

A MOSFET works by either letting current flow across its terminals or not depending
on the voltage applied. It operates on an electrical field effect produced by the voltage
applied across the semiconductor surface adjacent to the metal oxide layer. As a
result, this allows the MOSFET to work as either a p-type or n-type. Essentially,
when positive/negative voltage is applied to the gate (with respect to substrate), it
enhances the number of electrons/holes in the channel and increases conductivity
between source and drain.
The gate electrode controls channel conductivity between the channel at the source
and the drain region within the device. This principle of operation makes the
transistor act as an ideal switch. Thereby allowing us to control how electricity flows
through our circuit. By controlling how much voltage flows through the gate region,
you can determine the drain current in the drain-source channel.
It is important to note that MOSFET transistors and BJT transistors slightly differ. In
BJTs, current flows from collector to emitter, while in MOSFETs, this flow happens
between source and drain. What happens is that when the voltage applied to its gate
terminal exceeds a specific threshold voltage, the current starts flowing through it.

As the Gate terminal is electrically isolated from the main current carrying channel
between the drain and source, “NO current flows into the gate” and just like the
JFET, the MOSFET also acts like a voltage controlled resistor where the current
flowing through the main channel between the Drain and Source is proportional to
the input voltage. Also like the JFET, the MOSFETs very high input resistance can
easily accumulate large amounts of static charge resulting in the MOSFET becoming
easily damaged unless carefully handled or protected.
Types of MOSFETs

MOSFETs are available in two basic forms:

• Depletion Type - the transistor requires the Gate-Source voltage, ( V GS ) to
switch the device “OFF”. The depletion mode MOSFET is equivalent to a
“Normally Closed” switch. For the n-channel depletion MOS transistor, a
negative gate-source voltage, -VGS will deplete (hence its name) the
conductive channel of its free electrons switching the transistor “OFF”.
Likewise for a p-channel depletion MOS transistor a positive gate-source
voltage, +VGS will deplete the channel of its free holes turning it “OFF”.
In other words, for an n-channel depletion mode MOSFET: +VGS means more
electrons and more current. While a -VGS means less electrons and less current.
The opposite is also true for the p-channel types. Then the depletion mode
MOSFET is equivalent to a “normally-closed” switch.
• Enhancement Type - the transistor requires a Gate-Source voltage, ( V GS ) to
switch the device “ON”. The enhancement mode MOSFET is equivalent to a
“Normally Open” switch. For the n-channel enhancement MOS transistor a
drain current will only flow when a gate voltage ( V GS ) is applied to the gate
terminal greater than the threshold voltage ( VTH ) level in which conductance
takes place making it a transconductance device.
The application of a positive (+ve) gate voltage to a n-type eMOSFET attracts
more electrons towards the oxide layer around the gate thereby increasing or
enhancing (hence its name) the thickness of the channel allowing more current
to flow. This is why this kind of transistor is called an enhancement mode
device as the application of a gate voltage enhances the channel.
Increasing this positive gate voltage will cause the channel resistance to
decrease further causing an increase in the drain current, I D through the
channel. In other words, for an n-channel enhancement mode MOSFET: +V GS
turns the transistor “ON”, while a zero or -VGS turns the transistor “OFF”.
Thus the enhancement-mode MOSFET is equivalent to a “normally-open”
switch.
The reverse is true for the p-channel enhancement MOS transistor. When
VGS=0 the device is “OFF” and the channel is open. The application of a
negative (-ve) gate voltage to the p-type eMOSFET enhances the channels
conductivity turning it “ON”. Then for an p-channel enhancement mode
 MOSFET: +VGS turns the transistor “OFF”, while -VGS turns the transistor
“ON”.

The symbols and basic construction for both configurations of MOSFETs are shown
below.
I) P Channel MOSFET Depletion and Enhancement Mode
The drain and source are heavily doped p+ region and the substrate is in n-type. The
current flows due to the flow of positively charged holes, and that’s why known as p-
channel MOSFET. In short, Majority carriers are holes.
When we apply negative gate voltage, the electrons present beneath the oxide layer
experience repulsive force and are pushed downward into the substrate, the depletion
region is populated by the bound positive charges which are associated with the
donor atoms.
The negative gate voltage also attracts holes from the P+ source and drain region into
the channel region.
ii) N-channel MOSFET Enhancement and Depletion Mode

The drain and source are heavily doped N+ region and the substrate is p-type. The
current flows due to the flow of negatively charged electrons and that’s why known
as n-channel MOSFET. The majority carriers are electrons.
When we apply the positive gate voltage, the holes present beneath the oxide layer
experience repulsive force, and the holes are pushed downwards into the bound
negative charges which are associated with the acceptor atoms.
The positive gate voltage also attracts electrons from the N+ source and drain region
into the channel thus an electron-rich channel is formed.

Applications

1. MOSFET as a switch
MOSFET is very useful in electrical power applications for control voltages. It can
easily do this by conducting current flow control through the gate terminal.
Consequently, this makes it either turn on or off (much like a primary switch).
Moreover, this makes MOSFETs ideal devices useful as an interface between power
sources and devices that need to be powered.
2. Amplification applications
MOSFETs can also amplify current passing through it by allowing larger electric
currents to flow through its drain electrode when in its on-state. It makes MOSFET
suitable for applications that require voltage amplification (such as amplifying sound
waves).

3. Dynamic applications
MOSFETs are helpful in applications where they need to change state quickly,
making them valuable devices in circuit designs that require a lot of switching.
4. As voltage-controlled resistors or variable resistors
When MOSFET acts as an off switch in applications, they are suitable devices for
making variable or voltage-controlled resistors.

Advantages of MOSFET over BJTs

The main advantage is that it requires almost no input current to control the load
current and that’s why we choose MOSFET over BJT. Other advantages include;

• MOSFET is useful for making more compact circuits, ICs, CPU, RAM, etc.
• MOSFETs require less power and energy than BJT transistors, making them
more efficient overall. Since there is no base current, the device dissipates very
little energy when in its off state.
• MOSFETs are cheaper to manufacture than BJT transistors. Therefore, they are
the preferred choice when designing domestic and commercial circuits.
• MOSFET can work in high-temperature environments since its gate terminal
does not need insulation as a PNP transistor would. It makes it useful in
applications where temperatures become very high.
• Finally, MOSFETs are useful for voltage amplification since they have two
conducting terminals. This feature allows a more significant flow of current
through the circuit when in its on-state. In contrast, BJTs only require one
terminal to pass an electric current from drain to source terminal or vice versa.
Conduction Characteristics for MOSFET

Transconductance in MOSFETS

Transconductance (gm) is defined as the ratio between the change in output

current and the corresponding change in the input voltage of a MOSFET. The SI
unit of transconductance is Siemens (S). The transconductance value indicates the
sensitivity of MOSFET to input voltage change.

It is expressed as;

It determines MOSFET's amplification capabilities in small-signal applications. A

higher transconductance value enables greater amplification and improved linearity,
making MOSFETs suitable for applications such as audio amplifiers and RF circuits.
The transconductance parameter also influences biasing, stability, and power
efficiency thus helping the PCB designers in achieving optimal performance and
efficiency in their circuit designs.
Factors affecting transconductance
1. Channel Width (W): In a MOSFET, the channel width (W) represents the width
of the semiconductor channel between the source and drain terminals. It is one of the
key geometric parameters of a MOSFET and is typically controlled during the
fabrication process. A wider channel (larger W) allows for a larger current-carrying
capacity and hence a higher transconductance.
2. Biasing Conditions: The biasing conditions determine the voltage difference
between the gate and source (VGS) and the drain current (ID). Different biasing
configurations can alter the MOSFET's transconductance value, affecting the overall
amplifier performance and linearity.
3. Material Properties: The choice of materials used in the MOSFET fabrication
process can influence transconductance. Different materials have distinct electron
mobility and charge carrier properties, which affect the device's transconductance.
The properties of the semiconductor substrate, gate oxide, and other materials used in
the MOSFET construction can impact its overall performance.
4. Capacitance (Cgs): In a MOSFET, the gate-source Cgs represents the capacitance
between the gate terminal and the source terminal. It is a result of the oxide layer
between the gate electrode and the semiconductor substrate and is influenced by
factors like the gate oxide thickness and the overlap between the gate and the
source/drain regions. A higher gate-source Cgs can influence the small-signal
behavior of the MOSFET and affect transconductance.
5. Temperature: MOSFETs are sensitive to temperature variations. Any change in
the temperature can affect the transconductance. Higher temperatures can result in
increased electron scattering, leading to a reduction in mobility and, consequently, a
decrease in transconductance.

Parameters Considered when Purchasing MOSFETs

• Breakdown Voltage
• Forward Trans-conductance
• Drain source on resistance
• Switching characteristics
 • Zero gate voltage drain current
• Input capacitance

Electrical characteristics of MOSFETs (Static Characteristics Vth)

Gate threshold voltage (Vth)
Vthstands for "threshold voltage." Vth is the gate voltage that appears when the
specified current flows between source and drain.
Vth measurement:

MOSFET Characteristics
In general, any MOSFET is seen to exhibit three operating regions viz.,
1. Cut-Off Region
Cut-off region is a region in which the MOSFET will be OFF as there will be
no current flow through it. In this region, MOSFET behaves like an open
switch and is thus used when they are required to function as electronic
switches.
2. Ohmic or Linear Region
Ohmic or linear region is a region where in the current I DS increases with an
increase in the value of VDS. When MOSFETs are made to operate in this
region, they can be used as amplifiers.
3. Saturation Region
In saturation region, the MOSFETs have their IDS constant inspite of an
increase in VDS and occurs once VDS exceeds the value of pinch-off voltage
VP. Under this condition, the device will act like a closed switch through which
a saturated value of IDS flows. As a result, this operating region is chosen
whenever MOSFETs are required to perform switching operations.