How a MOSFET Works

How a MOSFET Works
David Zych
Introduction to the MOSFET
MOSFET stands for metal-oxide-semiconductor field-effect transistor and is a four
terminal device that uses very little current to turn on and supplies a large amount of current to
a load. The four terminals are the source, drain, gate, and body. There are two types of
MOSFETs being NMOS and PMOS. The main differences between the two devices are the
charge carriers that produce current and the material that makes up the body. NMOS uses
electrons as carriers and PMOS uses holes, holes being the absence of electrons and therefore
representing positive charge, while each body is made of p-type and n-type material
respectively. Referring to the body, or substrate, as p-type or n-type means that the material
used was made to either have a higher concentration of electrons or holes in it. For the sake of
keeping things simple this document will discuss how the NMOS device works.
Figure 1: Cross section of NMOS
The Four Terminals
The Gate
First we will discuss the gate of the NMOS. The gate consists of a metal contact that is
separated from the body by a high-k dielectric. It is important that the gate is kept insulated
from all other device regions or else the voltage applied to the gate can also reach the source
and drain causing the device to not work. A high positive voltage is applied to the gate which
induces an electric field through the dielectric. This induced electric field pushes any holes away
from the interface between the dielectric and the substrate while also attracting any free
electrons into the channel that separates the source and drain regions. A certain threshold
voltage, VT, needs to be overcome by the gate-to-source voltage, VGS, for the device to operate.
Depending on VGS the device will operate in three different modes cut-off, triode, and
saturation. In cut-off the VGS is less than VT causing the NMOS to be turned off and there is
ideally no current following between the source and drain. In triode the VGS has exceeded VT
and there is current flowing between the source and drain, however since the drain-to-source
voltage, VDS, is less than the difference between VGS and VT the current is small and increase
exponentially with VDS. Finally once VDS becomes larger than the difference between VGS and VT
the NMOS enters saturation. In this region the current has reached its maximum and no longer
depends on VDS, but instead is now controlled by VGS.
Figure 2: Graph showing the different regions that an NMOS operates in.
Source and Drain
The next topic for our discussion is the source and drain of the NMOS. These two
separate regions are where the charge carriers come from and are separated by a distance L
called the channel length. The channel length is an important part of the MOSFET since it is one
of the factors that determines how fast it can operate. The source and drain in an NMOS are
implanted with an element that has excess electrons in it such as phosphorus. These extra
electrons are what make up the current the flows through the channel. When a voltage is
applied to the drain, called the drain voltage VD, and the source, called the source voltage VS, a
strong electric field pushes the electrons from the drain to the source. For this to happen V D
must be at a higher voltage than VS which is usually achieved by grounding the source terminal,
although a negative voltage could be applied to the source terminal as long as it is not larger
than VD.
Figure 3: NMOS with electron channel formed.
The Body
Finally the last terminal of the NMOS is the body, which is usually composed of pure single
crystalline silicon, but other semiconducting elements like germanium are used. Pure single crystalline
silicon is used because any defects in the crystal structure of the silicon atoms could cause electrons that
are moving between the source and drain to become trapped therefore cause the current to decrease.
Having to many of these defects can greatly hinder the performance of the device such that the NMOS
will not operate at all. Another important feature of the body is that it must be made a different type
than the source and drain. In the case of the NMOS the body must be p-type since the source and drain
are n-type. The body also has an effect on the threshold voltage. If the body does not have the same
voltage as the source, then the resulting difference between the two voltages will cause the threshold
voltage to change. This is known as the body effect and it plays a vital role in the operation of the
device. Since it causes VT to change VGS must be adjusted so that the device will turn on.
Summary of Terminals
Here is a quick summary of each terminal.
1. Gate: Controls the concentration of electrons in the channel and determines if the
device is on or off.
2. Source and Drain: These two terminals are the sources of the electrons in the channel.
Applying a voltage at the drain that is higher than the source causes the electrons to
move through the channel. Made to be n-type material.
3. Body: The main bulk of the NMOS which is made to be p-type material. Positive charges
are pushed out of the interface between the gate dielectric and the body so that
electrons can accumulate and flow through the channel.
The MOSFET is a very powerful device that is used in a wide variety of electronic devices such as
smart phones, computers, and amplifiers. Due to its size a large number of them can be fit onto
a chip. Since the device can operate under low voltages and currents the amount of power that
dissipates from it is also low making the MOSFET a very efficient device. When one gains the
necessary knowledge in mathematics, physics, and engineering they will be able to understand
how MOSFETs work without a problem.
Works Cited
electrical4u. MOSFET | Working Principle of p-channel n-channel MOSFET. 2015. 27
October 2015.
National University of Singapore. 26 October 2015. 27 October 2015.
wikipedia. MOSFET. 21 October 2015. 27 October 2015.