So looking further, you know, each of these individual TFT, it looks something
like this. So what you have is equivalent of a
channel channel last or a gate first process which is used over here.
So the first layer you deposit is this gate layer.
And then you have this, gate insulator,
so this would be your, gate, oxide. Then you deposit this, channel
layer, which, has been, amorphous silicon primarily.
And then you deposit these source and drain, region.
So it's It's made very differently, as compared to normal transistors.
So that you first deposit your gate electrode, and then deposit
your gate oxide on the top, and then deposit your channel.
As compared to, in a normal logic transistor, where you, you'll have
your channel and your substrate, and then you'll deposit your gate oxide.
And then finally your gate.
So it's a reverse process flow that the, it is used for making these thin film
transistors and shown here is one of these thin film transistor in more details.
So you can see there's this gate line which was deposited first.
And then you have this gate insulator.
The material over here which is used is this metal oxide and the most
promising metal oxide for use in displays is indium gallium based zinc
oxide, also known as IGZO. So these indium and gallium are dopants
into this zinc oxide, so it's a high-conductivity, amorphous metal oxide.
And that's one of the materials which is used
to replace amorphous silicon, and then you have these
source and drain, which are deposited together and then
etched separated from each other using this etch pattern.
So, the difference the key difference why people
are trying to replace these amorphous silicon with
either this IGZO, which is a metal oxide, or
this low temperature polysilicon Is because of this higher mobility.
So IGZO has you know, has much higher mobility than amorphous silicon,
and then Low Temperature Polysilicon has higher mobility
,or, even higher mobility as compared to IGZO.
So, amorphous silicon mobility maxes out at
one or two centimeter square per volt-second.
IGZO gives you mobility around five and maximum of ten.
And Low Temperature Polysilicon
gives you average mobility of around 60 but it ranges
between 50-100 centimeter squares per second.
So another important thing about this as
we talked about LCD versus OLED, another very important
requirement for OLED-based display is that you
want this TFT should be capable of supplying very stable current.
So this current should be stable both when it's high and when it's low.
So for OLED based display the only material that can
work is this low temperature polysilicon. So this low temperature polysilicon is the
only material of choice for OLED based display.
The reason is that both these amorphous silicon
or these metal oxides, they are amorphous materials.
So both of these are amorphous materials, and they have trouble
maintaining, supplying the stable current at low current levels.
So the lot of Vt
unstability and there's a random capture of electrons which happens in the amorphous.
materials, especially when your, current is low, it,
fluctuates, quite a lot in these amorphous material, so
these are ruled out, for, OLED-based, display, and OLED-based
display's only option is the use of this, low-temperature,
Polysilicon.
For the LCD based display, people are trying to replace this amorphous silicon
with either IGZO which can, you know, mean the requirement in the near term.
And, you know, finally, they might migrate to lower
temperature polysilicon also because it has a much higher mobility.
So finally a thing I want to mention is how this low-temperature polysilicon
material is made. So, this is the only material, as we saw,
which is suitable enough to enable this OLED-based display.
And the reason why it's called Low Temperature is because this
silicon which is used to make this polysilicon material, it's deposited
at a lower temperature, and then what you do is you convert that
amorphous material into a polysilicon material using
this process known as sequential lateral solidification.
What it does, it is, it's essentially a laser based process and
what you have is you have your amorphous silicon to start with.
And, then you fire it up with a laser and it converts
into these polysilicon material with this small grain size.
And, then you fire it up with a laser again.
And these, some of these grain merge and you get a larger grain, and then
you fire it up with a laser again and these grains start to become even bigger.
And each time you make the grain bigger,
that improves your mobility in this polysilicon material.
The downside of that is that
currently they're not powerful enough laser available.
And the reason this process requires this laser limits the maximum
size of the display that that, that maximum size of the panel that you
can use. So far.
Normal amorphous silicone are these LCD-based displays.
People are Generation 10 or 11, which are, you know, glass
panels which are more than, 15 feet by 15 feet, in size.
But, for low-temperature, polysilicone, because a few of this
laser-based process people are still operating at Generation 3.
There are talks of, you know, people moving to generation
Five and Six.
And, each, each generation, what do I mean by each generation?
Is that, with each generation, you move to a larger panel of glass that is used
to make these displays, and on each of these panels you have a
multiple of the, you cut-out a multiple of the display, and the larger size
of glass you use, you get economy of scale, and you drive your price down.
So this is one of the reasons
why OLED based displays so far are only
used, prevalent only in smart phones, and if
you try to buy an OLED based TV
it's so expensive, because of low temperature poly silicon.