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So then the thing we want to talk about next, is talk about,
we've talked about light, and you know, what are the
required.
But now let's talk about LED technology, and, I want to touch upon four things.
One is how does it actually work? How do we get light out of LED?
How do we package it?
And it's also followed by the diode and that requires a lot goes
into actually how you control the bias and how you drive this LED.
So I
want to talk about that a little bit.
So typically, you know, the LEDs you get in in these small packages, that
you might have used in an E-lab or so, those typically look like this.
You have a cathode and an anode, and your LED is actually that piece
of gallium nitride that's mounted on a very small dish, which is again metal.
And it more, it is used to act as a heat sink also.
And that package, the whole purpose is to get maximum light
out of that chip into your into
your into your atmosphere. And that typically has a refractive index
which is an intermediary between the air and your substrate material.
So, anti-reflective coating, again, which we talked about solar, it, it works
both ways.
So if you have a light coming in, it helps that light to get in.
But you have light coming in from inside it,
if you have intermediate if you have material in between
which has an intermediate refractive index between your semiconductor and
the atmosphere, it also helps that light to get out.
So, it works both ways.
If, if anything works in trapping the light,
it also helps the light to escape as well. So, the material set which is needed
for RGB is essentially determined by increasing amount of band gap.
So most of the R is typically done by gallium arsenide or
[COUGH]
aluminum gallium arsenide based material. And then for for yellow and green, people
use gallium phosphide or a mixture of gallium phosphide with aluminum or indium.
And these have been around for a while.
People knew how to make LEDs on gallium phosphide and gallium arsenide
in the 70s and 80s. What was missing was the blue part of of
it, which is needed to complete this gamut.
And that happened in the late 90s with the invention of this person uh,
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Shuji Nakamura who, he did actually two, two main things.
He perfected the MOCVD process of gallium nitrite.
So as we heard in Monday as well that
gallium nitride does not a have a crystal no matter
there's no easy way that you can do the Czochralski growth
or zone refining process to make a crystal out of gallium nitride.
It's it's just not stable; it's very brittle.
So people typically try to grow it on sapphire, which is aluminum oxide.
And you get defect densities of when he started to
work on it, the defect densities were around 10 to
power 11, or 10 to power 10.
And he brought it down by two orders of magnitude, using his MOCVD process.
And that's still used by Nichia, and they, they're still a number
one player in LEDs because of this MOCVD process.
And the other thing which was missing was, for
LED you need a p-type and a n-type material.
So people knew how to make
n-type gallium nitrite but it was very difficult to make p-type gallium nitrite.
So he figured out that you know, it was due to hydrogen that that material was
not turning p-type, and a very simple bake
which removes the hydrogen could actually make it p-type.
And because of these two main things, he was able to invent the blue LED.
This was one of the first chips that he made.
And, again, it's a very simple chip.
It's grown on this aluminum oxide sapphire substrate.
And then you have a thick buffer of gallium nitride.
And then you have, it's an indium gallium nitride, which is
a smaller band gap, and then p gallium nitride on the top.
And since sapphire is essentially aluminum
oxide it's it's semi-insulating, it's an insulator.
You cannot have a bottom contact, so you need to make your both top and bottom con,
both your cathode and anode on your on your substrate, it says.
So you have to etch it out and make another contact.
And both of them have to be top contacts, essentially.
And this is from his original paper in 1995 where
he showed the blue LED And so you know,
this shows the, the progress map of LEDs, both red, blue and green.
So the blue really arrived in 1995 and since then their efficiency
have been increasing, but it kind of saturated again around 2000.
And now there has been some recent progress, again made
by the same guy, where he again invented a few things.
He got rid of polar planes and we heard more about it on Monday.
So now that roadmap again has started going up, it's comparable
here with respect to some of the incumbents so incandescent
bulb as you can see has a maximum efficacy of around 10 or 20.
Fluorescent bulb goes above, about 80.
the best HID lamp goes to 140. So the LEDs currently are above
fluorescent, but slightly lower than your discharge based lamps
So this is your very simple device physics, so people
are used to diode by this equation, but this is essentially
your forward bias current, this doesn't give you any light.
The part which gives you light is this recombination emission
current, and if you have a simple p-n diode you actually get
a lot of this forward bias current, and way less of this
recombination emission current.
So what people do is essentially you play fine engineering tricks
to increase your recombination emission so what is typically down in
the quantum well under a layer which has a small band
gap with respect to your pnn layer and this does two purposes.
It actually makes your recombination
emission more efficient because because you have this local source
with electron hole available, and the other thing it does if you have
a simple pl junction if you aim at a photon, there's a large
probability that, you know it will get reabsorbed either in your
But if you have this quantum well, you now aim at a photon which is,
which has an energy which is less than the band gap of your pnn
layer, so it's not reabsorbed back in your LED so it achieves
both these purpose if you have a quantum well in between.
And people typically have more than one quantum
well because they again want to minimize that forward bias current which
just goes from your p2n without recombining.
So what you want to ensure is, give your carrier, both your
electron and holes, multiple chances that they can recombine before they escape out.
So you have these multiple quantum wells in your, in
your stack and typically you have around three to five.
If you're a laser you have much more, of course.
But three to five is what is gives you the maximum.
If you're going above that, it's really a workout.
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And, The wave length also to some extent can
be fueled by both the diametrically of this material.
That you're making the quantum well from and also the width of this quantum well.
Especially if you make it small you can
again make it more slightly higher in band gap.
So the actual band-diagrams, especially
for your nitride based LED. The quantum width,
you know, they don't look like these nice isometric quantum wells shown over here.
In reality, they look something like this, so the
quantum wells have a lot of spikes in between.
So, this is an actual band-diagram, and we can see
that your, your quantum wells don't, aren't straight like that.
They have spikes, like this.
So, does anybody know why, why you have spikes, especially if you have.
Gallium nitride based LEDs.
So, gallium nitride is a polar material, right, and indium gallium nitride is
also a polar material, so when two
of these polar materials form a heterojunction,
you get a local electric field as the interface.
And because of that, you have, you know, whenever you find a
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you get very strong electric field at that interface.
And you get these spikes in your band
gap, instead of getting those nice quantum wells.
And
this one of the reasons which actually limits the efficiency also.
And more recently, Nakamura has been able to get rid
of these polar planes and get, use non-polar planes and that
gives further improvement If you have more structures
in those spikes in your quantum wells that further increase your efficiency because
these spikes and electric fields, they tend to force those guys
out there maybe these quantum wells are not efficient for trapping light.
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So, again, the way we generate.
The most common way of generating, white from,
from blue is to strike it on phosphorous.
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So you get two peaks essentially.
You get one peak which is from the LED and then
you get this broad peak, which looks like a black body generalization.
And that's from, phosphorous.
The downside of this is you get you get better color response, but
the down side is you do get a lot of Stoke's Loss errors.
You're essentially down converting high energy photon in to a low energy photon.
In to black body duration and off a low energy photon.
So all that, the down conversion loss is what is called a Stoke's loss.
You know, that is one of the disadvantage of doing this phosphorus 14 base approach.
But again you know the nature of, the way our human vision is
in terms of color, this does appear to be a good way to achieve color
[INAUDIBLE].
So the other thing which is very important and
actually limits the efficiency of you aura system is
the total internal reflection, so these LED materials, galium
nitrite for example, has that refractive index of 2.4.
Gidium Phosphate has even higher, it has a refractive index of
3.5, so what it means is that, your light from if
it's a point source, if it tries to come out.
It will be only able to come out in this area of this cone.
Wherever is outside this area of the
cone will essentially be above your critical angle.
And it will not be able to escape the, escape out of the semiconductor.
And all those photons will again be reflected back in.
Many of them will actually get out, some of them will get
reabsorbed or they wont be able to get out.
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So this essentially limits how much of your light you can get out.
So this is for a point source, for an actual LED where your.
Recombination is happening in all over the place.
This is even more complex, and you have
multiple of these cones and, order from each
of these source, only that light in that
particular cone can get out of your system.
So people play a lot
of tricks to
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to overcome this problem.
But we need to understand that, you know, where light is emitted, either it's
it's it's not able to come out because of total internal reflection.
A lot of that light, if it's not able to come out,
and it goes back into those quantum wells it can get re-absorbed.
And then you have these two legs which
are your contacts coming from, especially the contact coming
from the top.
So whatever that thickness of that wire that
is again blocking the light from coming out.
And each of these limits are your efficiencies at which you can extract your
[UNKNOWN].
And those numbers typically at best, people are
able to get 80% of the light out.
And there, also they have to play a lot of tricks.
So the tricks people play as you know, first
of all they put this in a high-index plastic.
So your package is typically made of a refractive
index which is somewhere in between 2.4 and 1.
If it has to be ideal,
it has to be
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of 2.4 if it's a gillium nitrate based LED.
And the other thing to do, is the substraight itself, they make it take
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And for that you have multiple surfaces available so your light can come all of
these surfaces and that can improve the total light which comes out
of how many number of surfaces you have at maximum you can have six.
So, the scheme that people use are again
very similar to what we discussed in solar.
So, one of the most
common scheme where it uses to
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make this gallium nitrite layer thin.
At the same time, make that top surface very rough.
What it does it again when your light is generated, it does not have
too much volume of gillium nitrite to get reabsorb it.
At the same time if you made that surface rough,
that light, when it hit, it can hit other angle.
which is, you know, more normal, and it has more probability of getting out.
But in principle, if you look from the processing point
of view, all these skin look very similar to solar.
But they're in fact achieving the reverse effect.
They're helping to get the light out instead of dropping the light.
And that is why, a lot of it has to do because light
is being generated inside the cell instead of light being coming out and
light be absolved.
So both, all of, a lot of these schemes and these techniques,
they work both with what's useful solar as again very useful for LEDS.
So, another trick which is used is to you, use this corrugated sapphire substrate.
Since you're having a plain sapphire substrate, you
have worked with these spikes, and again, that helps
to send out these lights in all these different
angles, so it can get out of your system.
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The other scheme which is used is is this flip-chip packaging, and this
is again very similar to what we discussed flip-chip
bonding using in our chips, and also that backside elimination.
So what did I get involves is that you are a minimize the loss.
Because of that wire on the top contact so you place your contacts,
to both PNN on the bottom instead of the top.
That top surface, again, kept wave pristine so that
light can get out without any shadowing from the.
From the wire on the top, and this
was invented by Philips and it's again used for
And
the way you can add and define that if you use a chip which has
[INAUDIBLE].
Non-flip chip, you see one which has wire coming in
versus one which has no wire coming in from the
top.
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