Are you interested
in understanding the functionality and structure of an electronic device? Why
not decode the MOSFET symbol and equip yourself with the right knowledge?
Whether it
is a microchip or a power device, MOSFETs are present in all electrical
appliances. In this blog, we explain everything you need to know about the
MOSFET symbol, complete with diagrams.
Read on to
find out more.
What is a MOSFET?
A Metal-Oxide-Semiconductor
Field-Effect Transistor (MOSFET) switches or amplifies electronic signals. It
is an essential part of modern electronics, usually in power supplies,
converters, inverters, microprocessors, and many other applications.
MOSFETs are of two
types:
●
P-Channel MOSFET
●
N-Channel MOSFET
The
P-channel MOSFET conducts current when a negative voltage is applied to the
Gate, whereas the N-channel MOSFET conducts when a positive voltage is applied
to the Gate.
Basic MOSFET Symbol
Explained
N-channel MOSFET symbol
An arrow
pointing towards the channel indicates that electrons are the charge carriers.
It turns on when a positive voltage (higher than the threshold voltage) is
applied to the Gate. However, when the Gate has no voltage, it remains off.
Here are a
few features:
●
The arrow indicates a direction towards the
channel.
●
Electrons are the charge carriers.
●
The device turns ON when Gate maintains a higher voltage level
than Source.
●
The device functions in switching and
amplification operations.
P-channel MOSFET symbol
The symbol
is an arrow pointing away from the channel (holes as charge carriers). It turns
on when the negative voltage is applied to the Gate. Otherwise, it stays off by
default.
●
The arrow directs itself in the opposing
direction from the channel.
●
Holes are the charge carriers.
●
When the Gate maintains a negative potential
compared to the Source, then the device activates.
●
MOSFETs with P-channel configuration find
utility in particular applications even though N-channel MOSFETs have broader
usage.
The three
MOSFET terminals are Gate(G), Drain (D), and Source (S).
Gate
The Gate is
one of the MOSFET pins that Controls the current flow between Drain and Source.
Basically, it functions like a switch which allows or blocks the flow of
current based on the applied gate-to-source voltage (V_GS).
Drain
The Drain
is the output
terminal from where the current exits. For an N-channel MOSFET, conventional
current flows from Drain to Source when ON. For a P-channel MOSFET, it flows
from Source to Drain.
Source
The Source
is the terminal where the current enters. It is typically connected to the
ground (N-channel) or a positive voltage supply (P-channel).
Moreover,
it also has a substrate body connected to the source.
Classification of MOSFETs
MOSFETs are
classified based on two criteria: channel type (N-channel or P-channel) and
operating mode (enhancement or depletion mode). The channel type could be
N-channel (negative charge carriers) or P-channel (positive charge carriers);
however, the operating modes are classified into depletion and enhancement.
Let’s have
a detailed breakdown to make the classification of MOSFET understandable for
you.
1. N-Channel Enhancement Mode MOSFET
This
channel is based on 2 heavily doped n-regions: i) Source and ii) Drain. Both
are embedded in a P-type substrate. The Gate is insulated from a substrate
using a thin silicon dioxide (SiO2) layer. No current flows when no voltage is
applied to the Gate, as there is no conductive channel.
The
N-channel enhancement works when a positive voltage is applied to the Gate. It
attracts electrons close to the silicon dioxide layer. At a particular voltage
(VTH or Threshold), a conductive n-channel is formed after enough electrons
accumulate between the n-regions, leading to current flow.
Increasing
the gate-to-source voltage (VGS) enhances the channel and increases
conductivity. The MOSFET enters saturation mode when VDS ≥ VGS - VTH, and it
operates effectively as an amplifier in this region.
2. P-Channel Enhancement Mode MOSFET
(PMOS)
Holes are
the majority of charge carriers in PMOS MOSFET. Contrary to the n-channel,
electrons dominate in this device. Unlike the n-channel, it is built on an
n-type substrate. A thin layer of silicon dioxide is used for insulation
purposes.
In
P-channel enhancement, a negative voltage activates the MOSFET at the Gate (G),
allowing current to flow from Source to Drain.
With the
increase in negative gate-to-source voltage (VGS), the conductive channel
becomes wider, resulting in increased current flow.
The
P-channel enhancement mode MOSFET acts as a voltage-controlled switch and is
ideal for high-side switching applications. It ensures smooth regulation of
current between Source and Drain.
3. N-Channel Depletion Mode MOSFET
2 N-regions
(source and Drain) are embedded in the p-type substrate in this channel type.
The channel region lies between these two n-regions. Moreover, the channel is
pre-defined during the manufacturing process. A Silicon Dioxide layer is used
for insulation.
By default,
the current flows in n-channel depletion even when VGS=0V. In depletion mode, the current reduces when
the negative VGS is applied, repelling electrons. It acts as a
voltage-controlled switch in many electronic applications.
4. P-Channel Depletion Mode MOSFET
The
P-channel depletion mode is opposite to the N-channel depletion mode. It is
pre-built using the p-type channel. When a negative voltage is applied, it
attracts electrons and affects the conductivity. The Drain is reverse-biased in
p-channel depletion, allowing the current to flow.
The
increase in negative drain voltage forms a depletion region and reduces the
current flow.
Adjusting
the voltage controls the conductivity in P-channel depletion. The Gate and
Drain remain at negative polarity, whereas the Source stays at zero voltage.
Reading MOSFET Symbols in Schematics
Switches
and amplifiers are among the top circuits powered by MOSFET.
MOSFET as a Switch
MOSFET
switches are used in motor drivers, digital circuits, and power control. They
have it as a standard on/off switch based on gate voltage. The N-channel MOSFET
turns on with a positive gate voltage, whereas the P-channel MOSFET turns on
with a negative gate voltage.
MOSFET as an Amplifier
They are
used in signal processing, audio amplifiers, and RF circuits. MOSFET amplifiers
typically operate in the saturation (or active) region, where a small change in
gate voltage controls a larger drain current.
Quick tips on recognizing N-channel vs
P-channel MOSFETs in a circuit
With a
clear knowledge of MOSFET types, you can better decode it. The most common
difference between N-channel and P-channel MOSFETs in a circuit is:
●
The N-channel MOSFET is typically connected to
the lower side of a load or ground, while the P-channel MOSFET is connected to
the high side, i.e., the power supply.
●
Secondly, an N-channel MOSFET turns on once
the gate voltage exceeds the source voltage. On the contrary, the P-channel
MOSFET turns on when the gate voltage is lower than the source voltage.
●
In an N-channel MOSFET, current flows from the
Drain to the source, while in a P-channel MOSFET, it flows from the source to
the Drain.
●
Lastly, the body diode in an N-channel MOSFET
conducts from the source to the Drain, while in a P-channel MOSFET, it conducts
from the Drain to the source.
Common Mistakes When Interpreting MOSFET
Symbols
It's common
to make mistakes when reading MOSFET symbols.
1. The most
common mistake when interpreting the MOSFET is confusing the symbols, which can
lead to incorrect circuit behavior. In an N-channel MOSFET symbol, the arrow
points from the source to the channel. In a P-channel MOSFET symbol, the arrow
points from the channel to the source, indicating the direction of conventional
current (hole flow).
2. The second
common mistake is misinterpreting the gate voltage requirements, which can
prevent the MOSFET from switching on.
3. Sometimes,
many hobbyists and even professionals
misinterpret terminals. This is because they ignore their labels. As a result,
they consider all terminals to be the same. So, it is essential to check the
details of symbols.
4. Ignoring
the body connection can cause unexpected circuit reactions.
5. While
switching between depletion and enhancement mode symbols, you might assume
MOSFETs are off. So, don't overlook the instructions of the symbols.
6. Many people
fail to notice the optional Body or Substrate symbol during operation.
Understanding the function of this terminal is crucial even when it is absent
from a separate display. The terminal plays a role in modifying the MOSFET
operational characteristics for specific circuit applications. Neglecting this terminal could cause
incorrect interpretation in power applications.
7. Schematics
can vary. Different sources can present symbols which appear different from one
another. Verify component specifications through reference to the official
datasheet.
Conclusion
Understanding
and correctly interpreting MOSFET symbols is essential to creating an efficient
electronic circuit. While choosing MOSFETs for specific applications, especially
in high-power applications, you need to consider their power ratings, such as
the maximum power dissipation in watts (W) or kilowatts (kW). This helps ensure
the transistor can handle the desired power level efficiently and without
failure.
This guide
includes all the necessary information to understand the workings of MOSFETs
and recognize the PMOS vs. NMOS symbol so engineers can create efficient
electronic circuit designs.
If you
design electronics as a hobby, this guide will help you understand the basic
mechanism of MOSFETs and their functionality.
