There are two types of FETs. First we will look at
Metal-Oxide-Semiconductor FETs (MOSFETs). These are true "field effect"
transistors because electrostatic fields are used to attract or repel
charge carries to control current flow. They are traditionally made of
three layers of material: first a layer of aluminum (metal); then an
insulating layer of silicon dioxide (oxide); and finally the main part
of the transistor is made of silicon
(semiconductor)—metal-oxide-semiconductor—MOS. Then we will
look at Junction FETs (J-FETs). They control current flow by
manipulating the size of the depletion region around a reverse-biased
PN
junction.
MOSFETs
First let's look at an N-channel enhancement mode
Metal-Oxide-Semiconductor Field Effect Transistor (a mouthful,
isn't it). It is made from two areas of N-type
silicon embedded in a block of P-type silicon (review
Semiconductors).
An N-channel enhancement mode
MOSFET.
The terminal connected to one N-type region is called the drain. This
is where conventional current will enter the transistor. The terminal
connected to the other N-type region is called the source. This is
where where current will exit the transistor; conventional current goes
into the drain and comes out of the source. A layer of metal (usually
aluminum, blue in the
diagram) is deposited onto the N-type regions to make electrical
contact with them.
The gate terminal connects to a large area of metal that is deposited
on a thin layer of insulating material (traditionally silicon oxide).
There is no electrical connection between the gate and the conducting
channel. MOS transistors are often called "insulated gate" transistors
because of this feature. Now let's put a battery across the transistor
and see what happens.
When
connected to a battery in the proper configuration the PN junction
between the drain region and the P-type material is reverse biased.
The positive
terminal is connected to the drain and the negative terminal is
connected to the source. There is no connection to the gate, but we
have a battery there ready to connect when we need it. In this
configuration the PN junction between the N-type drain region and the
P-type body is reverse biased. Like any reverse-biased junction
(such as a reverse-biased diode) no current can flow across this
junction so current
cannot flow between the drain and source.
Now let's take a look at the distribution of free electrons in the
transistor.
The
P-type material has a few stray electrons (represented by minus
symbols). They don't do much now, but will come into play later.
The N-type regions have an abundance of free electrons. Recall from
Semiconductors
that P-type material has a few stray free electrons called minority
carriers. These stray electrons are scattered
throughout the P-type material.
Their effect is significant but tiny, so we will assume that they no
effect at this point.
Now let's put some voltage on the gate of the transistor.
When
a positive voltage is put on the gate stray electrons
are attracted to the gate region creating a conducting channel.
The positive charge at the gate attracts the stray electrons into the
gate region. This concentration of
electrons overwhelms the characteristics of the P-type material making
it act like N-type material. We now essentially have a region of N-type
material extending from the drain to the source. There are no-longer
any PN
junctions, just a block of conductive N-type material. This creates a
conductive channel and current will flow from the drain to the source
through the transistor. This is called an
enhancement mode MOSFET because the nonconductive channel is
enhanced with electrons to make it conductive. It is called an
N-channel MOSFET because free electrons in the conducting channel,
although in P-type material, make it act like N-type material.
Complementary to the N-channel enhancement mode MOSFET there is a
P-channel version. This works the same as the N-channel version except
the voltages polarities are reversed. In normal operation the
drain is connected to the negative terminal of the battery and the
source is connected to the positive terminal. The gate is made negative
to attract stray holes scattered around the N-type material into the
conducting
channel. Conventional current flows into the source and out of the
drain.
Now let's look at an N-channel depletion mode MOSFET.
The N-channel depletion mode
MOSFET has a small channel of N-type material between the drain and the
source.
This is constructed like the N-channel enhancement mode MOSFET
except there is a channel of N-type material between the drain and
source. This channel is already conductive when there is no voltage on
the gate.
When there is no voltage on the
gate the transistor conducts current through the N-type material in the
conductive channel.
Now let's put a negative voltage on the gate and see what happens.
A negative voltage on the gate
drives electrons out of the conducting channel.
The negative voltage on the gate drives electrons out of the conducting
channel. This
stops the current flow through the transistor. This is opposite to an
enhancement mode N-channel MOSFET. There, a positive voltage at the
gate turns the transistor "on". Here, a
negative voltage at the gate turns the transistor "off". There is also
an P-channel version
of the depletion mode MOSFET that works with opposite polarity.
An useful feature of depletion mode MOSFETs is that they can also act
much like enhancement mode MOSFETs. Look at what happens if we put a
positive voltage on the gate of the N-channel depletion mode MOSFET.
A positive voltage on the gate
of an N-channel depletion mode MOSFET pulls more electrons into the
conducting channel.
The positive charge on the gate pulls even more electrons into
the conduction channel, making the depletion mode MOSFET act like an
enhancement mode MOSFET.
Static Electricity Precautions
MOS transistors are very sensitive to static electricity. Even low
voltages of static electricity can damage the insulation between the
gate and the conductive channel. Modern MOS transistors have protection
in the form of zener diodes between the gate and the other terminals.[1]
Nevertheless, antistatic precautions should be taken
when working with MOS devices.
MOSFET Schematic Symbols
There are more schematic symbols for MOSFETs than there are types of
MOSFETs. Let's take a look at them.
Enhancement mode MOSFET symbols
N-channel
P-channel
Traditional
Simplified
Digital
Enhancement
mode MOSFET symbols.
Enhancement mode MOSFET symbols have a solid line representing the gate
and a dashed line representing the channel. The dashed line reminds us
that the channel is nonconductive until a voltage is placed on the
gate. The arrow indicates the orientation of the drain-to-gate PN
junction. Like the zener diode, this junction is reverse biased in
normal operation so the arrow points against conventional current flow.
The arrow of an N-channel MOSFET points into the transistor, so
remember
that "N-channel points N". Sometimes the bulk or body connection,
that's the connection with the arrow, connects to a separate fourth
terminal
on the MOSFET package. This is usually just tied to the source
terminal. Any other use of this terminal is beyond the scope of this
discussion.
Many people don't think things are complicated enough so they have
created additional "simplified" symbols for MOSFETs. The FET mode is
indicated by the
thickness of the line between the drain and the sorurce. The direction
of The arrow indicates the orientation of the gate-to-source PN
junction. It arrow points in the opposite direction to the traditional
symbol. It also points in the direction of conventional current flow.
I actually prefer the simplified symbols because they are similar to
the symbols for bi-polar junction transistors.
If this isn't confusing enough there are symbols without arrows popular
with digital circuits. The channel type is
indicated by the presence or absence of a circle on the gate. The
absence of the circle indicates that the transistor is turned "on" by a
positive voltage. The presence of the circle indicates that the
transistor is turned "on" by a negative voltage.
Depletion Mode MOSFET Symbols
N-channel
P-channel
Traditional
Simplified
Digital
Depletion
mode MOSFET symbols.
Depletion mode MOSFET symbols have a solid line representing the gate
and a solid line representing the channel. The solid line for the
channel reminds us
that the channel is conductive until a voltage is placed on the
gate. The simplified and digital symbols use a thick line to represent
the existence of a conductive channel with no gate voltage.
Interchangeability of the Drain and Source with MOSFETs
You may have noticed that the drain and source regions look alike. Are
they interchangeable? The short answer is no. The actual construction
of the transistor has differences between the drain region and the
source region. Some MOSFETs have an internal diode between the drain
and source that is reverse biased in normal operation. If you try to
reverse the drain and source connections you will forward-bias this
diode, essentially creating a short circuit between the drain and
source. Also, as you can see in the traditional schematic symbol, the
body connection (the one with the arrow) is connected to the source
internally. This also complicates any potential interchangeability.
Junction Field Effect Transistors
A junction Field Effect Transistor or J-FET is essentially a diode with
two connections on one side of the junction. The following example is
an N-channel
J-FET.
An N-channel
J-FET
With the N-channel J-FET the gate terminal is connected to P-type
material and the drain and
source terminals are connected to a single block of N-type material.
Let's connect a battery across the drain and source and see what
happens.
An N-channel J-FET with no gate
voltage.
The depletion region extends only part way into the N-type material.
This leaves a contiguous block of N-type material where
current will
readily flow. With no voltage on the gate, current will flow from the
drain to the source through the transistor. Now let's put some voltage
on the gate.
With
a negative voltage applied to the gate the depletion region grows. If
the gate voltage is high enough the depletion can completely block
current flow.
The gate voltage is now lower than the voltages at either the drain
or the source. This reverse biases the PN junction making the depletion
region larger (review
Semiconductors)
blocking the flow of current through the transistor (the depletion
region is larger toward the drain because the voltage difference
between the drain and the gate is greater than the voltage difference
between the source and the gate). The gate voltage must be lower (more
negative) than the source voltage. Otherwise the junction will become
forward biased. Since the PN junction is reverse biased very little
current flows through the gate. Therefore, very
little current is required to operate a J-FET. The J-FET has a lower
gate impedance than a MOSFET, but it still has a very high impedance.
A P-channel J-FET works the same as an N-channel J-FET except the
voltage polarities are reversed.
J-FET symbols
N-channel
P-channel
Drain
Drain
Gate
Gate
Source
Source
Drain
Drain
Gate
Gate
Source
Source
J-FET Symbols
There are only two symbols for J-FETs. The N-channel symbol has the
arrow on the gate terminal pointing inward and the P-channel has the
arrow
pointing outward. Sometimes the gate terminal is drawn in line with the
source terminal. Some sources say that when the gate is in the center
of the symbol it indicates an FET with drain and source terminals that
can be swapped. However, you cannot
rely on this. Always check the datasheet for device specifications.
Interchangeability of the Drain and Source with J-FETs
The drain and source terminals of a J-FET may be swappable. Refer to
the transistor datasheet for an individual transistor to see if they
can be swapped.
