MOSFETs are widely used to design various types of circuits from a basic
driver circuit to complex power rectifiers, inverters, etc. In general,
MOSFETs are known as the power electronic switching components (power
switches) and are very important devices among high power applications. But
the first question that comes to mind is- what is the full form of MOSFET?
In this article, we have briefly covered the most important topics about
MOSFET, such as the MOSFET full form, what is a MOSFET, its definition,
construction, symbols, types, various applications, and more.
What You Will Learn
What is the full form of MOSFET?
The full form of MOSFET is ‘Metal-Oxide Semiconductor Field-Effect Transistor’. Sometimes, it is also referred to as ‘Metal-Oxide Silicon Field-Effect
Transistor’. MOSFET is usually defined as a transistor used primarily to
switch electronic signals. They are widely used in the field of power
electronics and are best known for handling significant power levels.
The full form of MOSFET can be explained as:
| M | Metal |
| O | Oxide |
| S | Semiconductor |
| F | Field |
| E | Effect |
| T | Transistor |
Now, let us understand what a MOSFET is:
What is MOSFET?
MOSFET is defined as a particular type of field-effect transistor primarily
used to switch or amplify electronic signals. The MOSFET was mainly designed
to eliminate the disadvantages of the FETs, including moderate input
impedance, high drain resistance, and slower operation. Practically speaking,
MOSFETs are the advanced version of FETs.
In particular, the MOSFET is a device consisting of four terminals, such as a
source, drain, gate, and body terminal. However, the body is usually attached
to the source terminal, which makes MOSFET a three-terminal device similar to
other field-effect transistors. Since these two terminals are interconnected,
its electrical diagrams have three visible terminals.
MOSFETs operate by varying the width of a channel through which carriers, such
as holes or electrons, flow. A channel is formed by the application of a
voltage to the gate. The wider the channel or the greater the gate voltage,
the better the device conducts. Charge carriers enter the channel at the
source terminal and exit through the drain terminal.
Construction of a MOSFET
In terms of the construction, MOSFET has a bit similar structure as FET
(Field-Effect Transistor). However, MOSFET has an additional oxide layer
attached to the substrate, which is further connected with the gate terminal.
The oxide layer present in MOSFET functions as an insulator (silicon dioxide
insulates from the substrate), and therefore, MOSFET is also referred to as
IGFET (Insulated Gate Field Effect Transistor).
In MOSFET, a lightly doped substrate is diffused with a heavily doped region.
Based on the type of substrate used while constructing the MOSFET, they called
divided as the N-type and P-type MOSFETs.
The following image displays the structural view of the MOSFET:
The voltage applied to the gate is responsible for controlling the operation
of the MOSFET. In particular, both positive and negative voltages can be
applied because the gate is completely insulated from the channel. When there
is a positive gate bias voltage, the MOSFET acts as an enhancement mode.
Furthermore, with negative gate bias voltage, it acts as the depletion mode.
Classification of MOSFET
Based on the functioning and operations types, MOSFET is mainly classified
into the two types, such as the
Depletion mode MOSFET (D-MOSFET)
and
Enhancement mode MOSFET (E-MOSFET).
Let us briefly go through each type:
Depletion Type MOSFET
In the depletion type MOSFETs, the channel is already established and it
exhibits maximum conductivity even if there is zero voltage or no voltage
across the gate terminal. Besides, when the voltage, either positive or
negative, is applied across the gate terminal, the channel conductivity
decreases.
Depletion MOSFETs are further sub-divided into the following two types based
on the material used for doped construction (n-type or p-type):
• N-channel Depletion mode MOSFETs
• P-channel Depletion mode MOSFETs
Enhancement Type MOSFET
The enhancement type MOSFET is just opposite to the depletion type MOSFET. In
enhancement type MOSFETs, the device does not typically conduct in the absence
of gate voltage across the terminal. Furthermore, when the gate voltage is
applied, a channel is established. In this case, the device exhibits enhanced
conductivity with the maximum voltage across the gate terminal.
In enhancement type MOSFETs, the gate voltage is proportional to the current.
As the gate voltage increases, the current increases continuously with it, and
vice versa. It is also important to note here that the gate voltage must be
positive and exceed the source voltage.
Like depletion MOSFETs, the Enhancement MOSFETs are also sub-divided into two
different types based on the doped material. They are:
• N-channel Enhancement mode MOSFETs
• P-channel Enhancement mode MOSFETs
Symbols of MOSFET
As discussed above, there are four different types of MOSFET. Let us now
discuss the symbols of each type of MOSFET:
N-channel MOSFETs
N-channel MOSFETs are abbreviated as
NMOS. For N-channel MOSFETs, symbols in both modes, such as depletion and
enhancement, are as follows:
P-channel MOSFETs
P-channel MOSFETs are abbreviated as
PMOS. For P-channel MOSFETs, symbols in both modes, such as depletion and
enhancement, are as follows:
Among these four types of MOSFETs, the N-channel Enhancement MOSFET is the
widely used type of MOSFET. As per the construction of the MOSFET, the Gate
(G), Source (S), and Drain (D) pins are physically attached in a Depletion
mode. On the opposite side, the same pins are physically separated in the
Enhancement mode. That is the reason why the symbols appear broken in an
Enhancement mode for MOSFETs.
Applications of MOSFET
The following are the major applications of a MOSFET:
• Amplifiers manufactured using MOSFET are widely used in radio
frequency applications.
• MOSFETs function similarly to the passive elements such as an
inductor, resistor, and capacitor.
• With the help of power MOSFETs, DC (direct current) motors can be
easily regulated.
• Because of high-switching speed, MOSFETs are considered the best
choice for manufacturing chopper circuits.
• MOSFETs are extensively used as switches for various devices such as
LEDs, Arduino, AC loads, DC motors, microcontrollers, and more.
Advantages of MOSFET
The following are the advantages of a MOSFET:
• MOSFETs help achieve greater efficiency, even when they operate at
lower voltages.
• MOSFETs can operate at relatively low power without drawing much
current.
• When there is no gate current, MOSFETs typically achieve high input
impedance, resulting in high switching speeds.
Disadvantages of MOSFET
The following are the disadvantages of a MOSFET:
• In the presence of overload voltages, MOSFETs usually become
unstable.
• Since MOSFETs have a thin oxide layer, they are vulnerable and may
face permanent damage when evoked by electrostatic charges.
Summary
MOSFET (stands for Metal-Oxide Semiconductor Field-Effect Transistor) is a
special type of voltage-controlled field-effect transistor that mainly helps
in amplifying and switching electronic signals. It usually functions in two
ways, such as depletion mode and enhancement mode.
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