Hello folks.
In this video I want to talk about this very important layer that is your
transistor back plane or more commonly just referred to as back plane.
So shown here are three different types of display
that you will find in your current electronic devices.
One is this standard LCD that you will find in your you know,
your laptops and your monitors and this has, you know, a
display, a maximum resolution of, you know, less than 200 pixel per inch.
And then you have these displays that you
find in your smartphone, so either they are.
These high resolution LCD which have displays exceeding
300 pixel per inch and also these OLED display which
have resolution nearing around 250 pixel per inch.
So, these display they defer from each other, some of these LCD based
display they use this liquid, their enabling module is this liquid crystal.
And they use these color filter.
in contrast to that the OLED displays are much more simpler.
They use these light emitting diodes and
theirs is encapsulated with this cover glass.
Which protects them from moisture.
Also they don't use any back light which you
have to use if you have a liquid crystal-based display.
But one common layer that all of these
displays use is this thin film transistor back.
plane.
And this is essentially they layer which consist of these
thin film transistors, which control each of these individual pixels.
So they are responsible for either applying
these in the case of, liquid crystal display.
Applying the voltage which turns the individual pixel on or off, or in the, in
the case of organic light emitting diodes,
since these displays are based, are based on
a diode which needs to be driven by a current, the thin film transistor in
this case It's responsible for providing this
current, which will drive your light emmitting diodes.
So, so far the working house of the industry has been amorphous
silicon, so most of these displays so far have used these amorphous silicon.
As the thin-film transistor material and that carried the industry for a long
time, you know, all the way from 1980s till you know, the later part of 2000s
This was the
only material available, this was a material used
for making these thin film transistors, for your back plane.
But the problem with amorphous silicon is that since it's
amorphous material, it does not have a very high mobility.
So the maximum electron mobility that you get in amorphous silicon based
display is around one or two centimeter squared per volt-second.
And this was fine when you know, you had a low resolution display or if you are not
refreshing your display at a very high rate but if you put, if you have
this kind of mobility, the maximum refresh rate
that you can get with this mobility is around 240 or 250
hertz and many of our, games
that we use, play like Quake or other games, they have, a refresh
rate of you are approaching 150 Hz, and if you are especially using
a 3D television or a 3D display, the way this 3D display work is that it has two
of these images and you each of your eye has a different polarizer placed in
the glass that you, that you wear, so it has to be refreshed twice the frequency.
These 3D TVs, they require even higher
refresh rate, or twice the refresh rate of what you'll get in a normal HDTV.
So clearly we see that amorphous silicon is running out of steam.
So what people have been exploring in the literature, what people have been actively
deploying and exploring to apply are these newer, materials.
That is, there are two candidates, which are,
currently, under, you know, both are under, implementation.
One is this low-temperature polysilicon, also known as LTPS.
The other one is this metal oxide based thin-film transistor. Before we delve into
each of these individual thin film transistor
and the material inflections that are happening
in the material of this transistor.
Let's understand more how does this back plane look like.
So, if you look at this back plane, it's consists of these orthogonal set of lines.
These green lines over here are scan lines, and these
perpendicular lines, which are drawn in yellow, are these data lines.
A good analogy if you want to derive from our
study on memories is that these are similar to these bit-lines and word-lines
that you see on your flash or your DRAM array and what they allow you to
do is essentially you can you can select, using this scan line which particular
row or column your address, and then using this data line, you can inject that data
into that particular cell. And at the intersection of these
Scan line and data line is located this control circuitry.
So over here is located this control circuitry.
And this consists of anywhere between one to six of these thin filmed transistors.
And it can consist of a few of these capacitors as well.
So it consists
of you know a few of these transistors. And a few of these capacitors.
And what they do is that they take the input from these data line and scan line.
And what they generate as a, as an output is either the voltage or the current.
And that voltage or the current is applied from this
control circuitry.
To either this, liquid crystal, or this, OLED, which is, placed overhead.
So, important thing then in the, the backplane, and this
term called as, fill factor, and that refers to the percentage of
the area which is occupied by these data lines and this control circuitry.
And that
is important because especially if you have a LCD
based display, so, from beneath this display, there would
be light coming in, so there'd be a backlight
below, and there'll be light coming in from below.
And the amount of area which is occupied by this thin
film transistor, that will essentially block the light to pass through.
So the light will essentially be blocked in this much area.
So it's, that important term to minimize the amount, or
the percentage of area which is occupied by this control circuitry.
So as I mentioned this control circuitry is
consist of multiple of these thin-film transistor and capacitors.
And there's a bifurcation in the requirement for this control circuitry.
if you have a liquid crystal based display, then
the important thing that you need is you need to apply a voltage.
So, for a Liquid Crystal Displays an important output parameter is the voltage.
For the OLED based display the important output parameter is the current.
So that essentially leads in a bifurcation of how this circuitry
looks for a liquid crystal based display.
So for a liquid crystal-based display, you'll have this data line.
And you have this scan line.
And this is connected to this thin-film transistor.
And this thin-film transistor essentially, depending upon what is the data and if its
scan line turns this thin-film transistor on, it passes that voltage over here.
And this charges up this capacitor which
maintains this potential on your liquid crystal.
Similarly, if you're, or you reduce this
potential to zero, you discharge this capacitor.
And you have zero voltage on your liquid crystal cell.
In comparison, in a OLED based display, you need to drive this light
emitting diode with a current, so the control
circuitry for your OLED based display is slightly different.
You have this extra power line, and you
have this extra transistor which is the drive TFT.
So what this data line and this This scan line do is that if you turn on the
scan line and you have a high voltage on this data line, what they do is they store
a charge on or they apply the resultant potential on this storage
capacitor, which in turn puts high voltage on this capacitor in
turn turns ON this drive TFT, and this can now this,
using this power line, it can apply this current on this, OLED.
So, this drive TFT for OLED based display, it needs
to be, you know, capable of, supplying a high enough, current, and also maintaining
a stable, stable current at low voltage levels.
So these are, you know, the two different
requirements that you see for a TFT based display.