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BABATUNDE OGUNLADE:
Hello, everyone.

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Today we're going to be
working through Goodie Bag

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5, which is on electronic
materials, namely

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semiconductors.

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In order to follow
along, you'll need

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a multimeter, a pair
of alligator clips,

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some large LEDs--

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I'll be using red,
white, blue, and green--

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and some regular sized LEDs.

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I'll be using red,
blue, and purple.

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Our main objectives today
are to identify differently

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colored LEDs by
irradiating them with light

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across the visible
light spectrum.

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And in the process, to
explore the relationship

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between the bandgap
of a semiconductor

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and its critical
absorption wavelength.

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As we work through
the Goodie Bag,

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I'd like you to think about
how the size of the bandgap

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of material may influence
which wavelengths of light

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you can absorb.

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Let's get started.

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First, connect your
black or negative lead

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to the 10 amp port
on the multimeter,

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and then your red
or positive lead

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to the voltage
ohm milliamp port.

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Then connect the probe end
of each respective lead

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to one end of an alligator clip.

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After you do this,
you should still

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have two free ends of
the alligator clips.

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Grab one of your small LEDs
and connect the free end

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of the alligator clip
connected to the red lead probe

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to the positive end of the LED.

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This is the longer
leg of the LED.

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And then connect
the other free end

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that's connected to
the black lead probe

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to the negative end of the LED.

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That's the short leg of the LED.

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Then turn on your
multimeter and set the dial

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to 2,000 milli DC volts.

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This is the order of
magnitude of voltage

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we'll measure across
our small LEDs

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when we irradiate them with
light from the larger LEDs.

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Finally, make sure to
keep track of which small

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LED you're testing
so that which LED

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to refer to when
we start comparing

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data between small LEDs.

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So now we have our small LED
hooked up to our multimeter.

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And now we're going to shine
each one of our large LEDs

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onto the small LED and
record whether or not we have

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some non-zero voltage reading.

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If the large LED
has enough energy

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to excite electrons across
the bandgap of the small LED,

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we should see some non-zero
voltage across the small LED.

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So first I'm going to try
and shine this blue light

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onto my small LED, and I'm
going to look at the multimeter

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and try to see if there's
some voltage drop.

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As you can see, our
volt meter is now

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measuring some
non-zero value, which

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means that this blue light
has enough energy to excite

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electrons across its bandgap.

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After you've done this, repeat
this for the other large LEDs,

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and then repeat the entire
experiment for the other two

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small LEDs.

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This should generate a
set of 12 data points.

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And using our knowledge
of bandgaps and wavelength

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absorption, as well as the
data that you've acquired,

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we're going to determine
which small LED is what color.

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So after running through
your experiments,

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you should have some chart
that looks like mine, where

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I have my small LEDs
going down the side here

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and the large LEDs going across.

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So each square
represents a time when

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a small LED he was
irradiated by a large LED.

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And a check represents when a
non-zero voltage was measured

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across the multimeter,
and an x represents

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when there was a
zero or very low

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voltage reading across the LED.

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So in order to dig
into what this means

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it's important to
understand the balance

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of the visible light spectrum in
terms of energy and wavelength.

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So if you remember the
visible light spectrum,

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known wonderfully by the
acronym ROYGBIV, where

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you have red on one end and you
have violet on the other end,

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we know that red
corresponds to around 700

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nanometers in wavelength.

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And we know that violet
or purple corresponds

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to around 40 nanometers
of wavelength.

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And if we think back to the
relationship between the energy

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of light and its wavelength,
we know that the formula

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is E equals hc over lambda.

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So as the wavelength of light
decreases, the energy of light

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increases.

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So going across, as we go from
red, green, blue, and white,

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we have increasing
energy because we

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have decreasing wavelength.

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And just as a reminder,
white is a composite

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of red, blue, and green.

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So it has all three of
these combined in it.

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So now let's start to identify
which LED is what color.

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So if you look at
small LED 1, it

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was irradiated by red,
green, blue, and white light.

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And there was a voltage drop
across the LED for all of them.

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That means that
at the very least,

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red had enough energy
to excite electrons

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across the bandgap
of that small LED.

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Because that's the
case, and we know

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that we're dealing
with LEDs which

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emit like in the
visible light spectrum,

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we know that this small
LED then must be red.

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So if we look at
small LED 2, we can

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see that both red and
green light did not

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have enough energy
to excite electrons

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across the bandgap
of the second LED.

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But when we shined blue
light onto the LED,

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we got a non-zero voltage drop.

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And the same thing when we
shined white light, which

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also has blue light in it.

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Because, like I said
at the beginning,

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we knew that our small these
were red, blue, and purple,

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and we've already
found our red LED,

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that means that this
LED must be the blue.

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So finally, if we look
at small LED number 3,

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we can see that red, green,
blue, and white light

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did not have sufficient energy
to cause some voltage drop

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across the LED.

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If we look back
to ROYGBIV, and we

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think about the
energy of red to blue,

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that means that they
didn't have enough energy

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to excite anything after it.

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And that means that our final
LED must be violent or purple.

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So by shining light across
the visible light spectrum

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onto our small LEDs,
we're able to identify

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which one of our small
LEDs are what color.

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So right here I have small LED
1 hooked up to the multimeter.

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And we determined it was red.

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And so we can check that
by just turning on the LED.

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So as you can see,
this LED is shining

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red, which means
it's the red LED,

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and we're able to confirm that
by using our data as well.

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You can check that
the other two LEDs

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are the colors
that we determined

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by doing the same thing.

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So today we looked at a direct
application of semiconductors

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in LEDs.

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We looked at the relationship
between the bandgap of an LED,

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of a semiconductor, and
the range of light that

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can absorb across its bandgap.

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We found that for
semiconductors, there

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are some critical
absorption wavelengths,

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some maximum wavelength, below
which the LED will absorb

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all other wavelengths of light.