In today’s tutorial, we will be learning about the simplest type of field effect transistor or FET.
This simple FET is called the JFET, the Junction Field Effect Transistor.
JFETs, like other FETs, are semiconductor devices and if you’re not familiar with
semiconductor technology, the terms n-type or p-type semiconductors and how PN junctions work,
I highly recommend you start at the beginning of our Basics of Semiconductors tutorial series.
Let’s start by identifying the different parts of a field effect transistor. In all FETs,
the path of flow of the charge carriers in the device, either electrons or holes,
is known as the channel. If the channel is made up of an n-type semiconductor material, it's known as
an n-channel FET. Similarly, a p-channel FET’s channel material is of p-type semiconductor.
If we look at a generic symbol for FETS, you can see the current flow happens between two terminals
on the device – the source and the drain. The current flow is controlled by applying
a voltage at the third terminal – the gate. This voltage creates an electric field across
the device that affects and controls the current flow – hence the name “field effect transistors”.
There are different types of FETs out there and each type has a different structure,
performance, and type of voltage that turns a device on or off.
A JFET, being the simplest FET, starts with an n-type semiconductor wafer. This will form the
base semiconductor wafer, or the substrate, as more commonly known in semiconductor technology.
Specific regions on either side of this n type substrate are now converted to
p-type by doping p-type impurities to the specific area of the substrate.
Before understanding why this is done in a JFET, it's beneficial to first review what happens when
a p-type and n-type semiconductor meet each other side by side, at a PN junction. Both n-
and p-type semiconductors have mobile charge carriers – electrons and holes respectively.
You can see that when a p-type semiconductor meets an n-type semiconductor, the electrons
from the n side move to the p side and combine with the holes there. As they do so, they leave
behind positive fixed ion cores on the n side. Negative fixed ion cores are similarly formed on
the p side when the hole and the electron combine. This side-by-side region of immobile positive
and negative ions is known as the depletion region – a region depleted of mobile charge
carriers. The width of the depletion region can be increased by reverse biasing the pn junction.
"For an n-channel JFET, as p-wells are formed in the n substrate, a pn depletion junction
like this is formed along the boundary where the p well meets the n substrate.
The n-channel is flanked by the two depletion regions on either side, creating a channel between
the two p-wells. This is the physical, internal construction of a JFET. Metallic contacts are
provided at the drain and the source and on either side of the substrate where the two p wells are.
The contacts of the p wells are tied together into the third terminal – the gate.
The JFET is now ready for use. To operate the device, a positive voltage is applied between the
drain and the source. Electrons will begin to flow from the source to the drain, even when there’s no
voltage applied at the gate. We’ll discuss the VI characteristics of the JFET in another video.
The conventional current here is from drain to source, so electron movement is from source to
drain. Other than understanding how a device works at the semiconductor level, conventional current
is much more common and, if you have to remember one or the other, remember that conventional
current flows from the drain to the source. Besides current flow, there is one other important
thing happening inside the JFET. As the channel is not perfectly conductive, you can consider it as a
spread-out resistor. And as current passes through the channel, there is a voltage drop across the
channel. You can see in this example, VDD has been taken as 5V and VGS as 0V. The positive
voltage of VDD in the n-channel and 0 volts at the p-wells will reverse bias the PN junction.
As the voltage at the top of the channel is more than that at the bottom of the channel,
the depletion region width is not uniform and makes a chokepoint.
If we keep increasing that drain to source voltage,
the channel current continues to increase and the width of the depletion region also,
especially at the top near to the drain, as it gets more and more reverse biased. Soon
depletion regions from either side of the channel will have grown so much that they
nearly touch each other and there is almost no channel left between. This is known as the
regions as long as there is current flowing. When we talk about the source and drain terminals
in a JFET they may seem interchangeable as they look symmetrical from the images,
but there are small differences between the two terminals at the semiconductor level
which are part of the fabrication process. While the source is always kept at ground,
the drain is kept at a positive potential for an n-channel JFET and kept at negative
potential for p-channel JFET. So, a JFET is the simplest type
of FETs. The gate impedance is extremely high, even without the oxide layer found with MOSFETs.
However, MOSFETs have become much more popular than JFETs as the voltages applied both to
the gate and drain of the MOSFET are the same polarity- either both positive or both negative,
making them a bit easier to work with. Despite this, JFETs are less prone to noise and the
absence of an oxide layer makes them simpler than MOSFETs, meaning they’re still in use today.
We will do another video where we discuss the VI characteristics and the different
operating regions of the JFET but for now, we have learned what a JFET is,
what its basic symbol looks like, the basics of how it works, and what the internal structure
looks like both when there is no current flowing and also when it is conducting.
We hope you found this tutorial helpful and that you’ll join us when we discuss the VI
characteristics and operating regions. If you did like this video, give it a like,
subscribe to our channel, all that good stuff, and we will catch you in the next one. Take care!
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