- [Instructor] The simplest semiconductor
component is called a diode.
Which consists of a piece of N-Type
and a piece of P-Type material joined together.
The junction between those two different types
of materials behaves in a way
that allows electrons to easily pass
from the N-Type region to the P-Type region
with just a small amount of voltage to push them
but if the direction of that voltage is reversed,
the diode makes it very difficult
for electrons to flow in the other direction, from P to N.
The electrons need a lot of voltage
to push them the other way.
To use a diode, you don't need to understand
why the PN junction behaves that way
and the low-level physics that are going on here.
So for this course,
I'm just going to focus on how to use diodes
for practical applications.
I like to think of a diode like a turnstile
that you might encounter at the entrance
to a sports stadium or a subway.
People pass through the turnstile in the same way
that electrons pass through a diode.
When somebody enters through a turnstile
in the correct direction,
they simply push on the arm
and it moves to allow them through.
But if they try to go through the turnstile
in the wrong direction,
that arm locks up
and blocks them.
That one directional behavior makes diodes useful
for controlling how current flows
through parts of a circuit in the same way
that turnstiles are used to control how people enter
and exit a building.
The schematic symbol for a basic diode
look like a triangle
with a bar attached to the tip.
The side at the base of the triangle
corresponds to the P-Type section of the diode
and is called the anode.
And the side with the bar is called the cathode
and corresponds to the N-Type material.
The triangle acts like an arrow
that points in the direction that current can flow
through the diode from anode to cathode.
When building a diode circuit,
it's important to pay close attention
to the polarity
or direction of the diode
to make sure it's inserted in the correct orientation.
Diodes often come in small axial packages
with two metal leads like this one.
There's usually a small marking on the package
to indicate which side of the diode is the cathode.
Now, I will admit, I tend to get the terms cathode
and anode mixed up in my head all the time.
And I can never seem to remember
which one is the positive one
and which one is negative.
So instead of remembering
that the marking on a diode
corresponds to the cathode,
I think of that marker line
as representing the bar on the diode symbol.
Because I can easily remember
which way current flows through the diode.
A diode has two basic modes
of operation called forward biased
and reverse biased.
Which basically mean that the diode is on
or off.
Whether a diode is forward or reverse biased
will depend on the voltage across it
and the current through it.
It's common to plot the relationship
between a diode's voltage and current
to characterize its performance on what's known
as an I-V curve.
On a I-V curve, the voltage across the diode
is represented on the horizontal axis
and the amount of current passing
through it is represented on the vertical axis.
A diode will become forward biased
or turn on when a positive voltage is applied
to push current through the diode
in its forward direction.
In an ideal world, a forward biased diode
should act like a short circuit
and allow current to flow through it,
completely unrestricted.
Since an ideal diode does not put up any resistance
to that forward current,
there won't be any voltage across it.
It's just acting as a pass through
for the current.
So, on the I-V curve
the forward-biased region is represented
by a vertical line when the voltage is equal to zero.
No matter how much forward current flows
through that diode,
the voltage across it will never be
greater than zero volts.
If I flip the diode around
to apply a negative voltage across it,
which tries to push current
through the diode in the other direction,
then the diode will become reverse biased.
It basically turns off
and acts like an open circuit
to prevent any reverse current
from passing through it.
On the I-V curve for an ideal diode
that reverse-biased region is represented
by this horizontal line.
No matter how much negative voltage
I apply to the diode,
it will not allow any current to pass through it.
This plot represents the ideal behavior for a diode
but in reality, the relationship between voltage
and current through a diode looks more like this.
On the I-V curve for an actual diode
the forward and reverse regions of operation
look similar to the ideal diode regions,
they're just slightly off.
But there's also a new third distinct region
of operation to the left
called the breakdown region.
Focusing in on the forward region
which represents a diode's behavior
when a positive voltage is applied across it.
Unlike the ideal diode model,
which allows forward current
to flow completely unrestricted,
an actual diode requires a small amount of voltage
to be applied to it before it really opens up
and allows current to flow through.
The forward voltage, labeled as Vf,
is the voltage at which the diode
will become forward biased
and turn on.
And for basic silicon diodes,
Vf is usually around 0.7 volts.
You can relate the forward voltage of a diode
to how hard you need to push on a turnstile
to get through.
The arm on a turnstile is spring-loaded,
so it puts up a small amount of resistance.
So you have to give it a little push
or voltage to move it aside
so you can pass through in the forward direction.
When voltage is applied in the other direction,
the diode becomes reverse biased
where it turns off
and prevents current from passing through.
That's similar to when you try
to go the wrong way through a turnstile.
The arm locks up.
Even if you push really hard on it,
it won't budge.
The turnstile is reverse biased.
But if you push really, really, really
hard on the turnstile,
eventually you will break the arm
and then you can pass through the turnstile
in the wrong direction.
And that corresponds to the breakdown region
of the I-V curve.
If a large enough voltage is applied
to the diode in the reverse direction,
the diode will eventually turn on
and allow reverse current to flow through it.
This breakdown voltage, abbreviated as VBR,
can be as low as negative a hundred volts
or more for standard silicon diodes.
Now, unlike the turnstile arm example,
when a diode enters the breakdown region
it's not necessarily broken.
Operating in the breakdown region itself
does not damage the diode.
However, under breakdown conditions
the diode won't be limiting the amount
of current through it.
That means unless there's something else in the circuit
to limit current through the diode,
it can easily overheat
and burn up.
The same thing can happen to the diode
when it's operating in the forward biased mode.
For that reason, it's important
to design your circuits in a way
that will limit the current through the diodes
operating in the forward biased
or breakdown modes
to prevent them from overheating.