Silicon – The Internet’s Favorite Element: Crash Course Chemistry #35


In this episode, we talk about Silicon Valley’s namesake and how network solids are at the heart of it all. Hank also discusses Solid-State Semiconductors, N-Type and P-Type Semiconductors, Diodes, Transistors, Computer Chips, and Binary Code. All from the same thing that makes up sand!

Transcript Provided by YouTube:

00:00
If they know it at all, most people know silicon from the valley in California that bares its name.
00:04
The birthplace of modern computing where technological dreams became reality.
00:08
Where brilliant twenty-somethings became millionaires.
00:10
But we chemists know silicon first and foremost as the most abundant element in the earth’s crust.
00:15
Oxides of silicon, known as silicates, make up sand,
00:18
which covers not only every desert and shoreline on the earth, but also most of the ocean floor.
00:24
Silicates also include quartz and most types of clay and they’re found in living things,
00:29
like little spikes and nettles.
00:31
And as a polishing agent in toothpaste. It’s everywhere!
00:34
And, of course, because of their uniquely wonderful chemical properties,
00:37
crystals of elemental silicon form the basis of semiconductors.
00:41
These crystals can be combined to make diodes and transistors acting essentially as
00:44
chemical on/off switches that make binary code, and, therefore, computers possible.
00:49
Fascinatingly enough, only a few minor differences in atomic arrangement allow an element
00:52
found on the bottom of the ocean to make the computer that you’re watching me on possible.
00:57
As with carbon, everything comes down to the network these atoms form.
01:00
Once you learned the chemistry of these networks, you’ll come to understand why glass is glass and clay is clay.
01:06
You’ll understand what chemists mean when they talk about “doping.”
01:09
And you’ll finally get why a valley in California is named for arguably one of the most intriguing element in the universe.
01:15
[Theme Music]
01:25
This sand, and this glass, and this quartz are basically the same thing,
01:30
just with the atoms arranged a little differently.
01:32
The chemical name of this substance is silica made up of silicon and oxygen in a 1:2 ratio.
01:38
For that reason we say that the chemical formula of silica is SiO2.
01:42
Even though it doesn’t actually exist as separate individual molecules.
01:45
Like graphite and diamond, the two forms of pure carbon, silicons and it’s oxides are network solids.
01:51
And also like graphite and diamond, the different forms that they take are all about bonding differences.
01:56
The form of the silica that makes sand, is exactly the same as the form that comprises quartz.
02:00
In fact the silica in sand is basically just tiny bits of quartz.
02:04
Silica structure is based on a tetrahedral arrangement of a silica atom bonded to oxygen atoms.
02:08
That’s right — 4, not 2.
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The trick is that each of the oxygen can form two bonds,
02:14
so they also bond to another silicon atom on the other side.
02:17
So each silicate is bonded to 4 oxygens, while each oxygen is bonded to 2 silicons.
02:21
Si2O4 simplifies to SiO2, and that’s where we get the formula.
02:26
As the molecule continues to build on itself, it can make various crystal structures
02:30
depending on the orientation of the little tetrahedrons.
02:33
These are different forms of quartz, clays, and other minerals.
02:37
They can make two dimensional forms as sheets, or three dimensional crystals.
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And each form has its own properties and behaviors.
02:43
Ceramics, and other clays, get their strength from the two dimensional types.
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When the clay is wet, silica sheets move around freely in the clay
02:49
and as it dries they move closer to each other and bond to together forming a rigid framework.
02:54
This resulting composite is extremely hard and heat resistant.
02:57
Both of these properties make ceramics useful in tons of ways
03:00
that you’ve probably been exploring since you were a toddler and got your first box of modeling clay.
03:03
Three dimensional silicates can take lots of shapes, too.
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Although not all of them are true crystals.
03:09
Glass is an enormously important example of a three dimensional silica based amorphous solid.
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It’s worth mentioning here, even though its not a crystal
03:15
because it’s molecular structure is extremely similar to the structure of quartz, just not as orderly.
03:20
The arrangement of silicon and oxygen in quartz is very regular and rigid.
03:24
The molecular structure of glass, on the other hand,
03:26
looks like Mother Nature tried to build a quartz crystal while she was a little bit drunk.
03:29
The atoms are attached in random numbers and shapes, creating a gradual structure with no definite order.
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This is why glass can be formed into almost infinite shapes when it’s heated,
03:38
while quartz keeps it’s characteristic crystal shape.
03:40
Because it’s an amorphous solid with bonds of varying strength, glass doesn’t have an exact melting point.
03:45
Instead, it softens as its heated and becomes honey-like, except extraordinarily hot and not sweet at all.
03:51
Quartz doesn’t do that.
03:52
It’s melting point is much higher but it melts at an exact temperature and has no pliable in-betweeny state.
03:58
Like all silica crystals, quartz, glass and ceramics are electrical insulators,
04:03
because they have no free electrons to transfer charges over a distance.
04:06
The atoms have 4 valence electrons and they form 4 bonds so all the electrons are perfectly well used.
04:12
For this reason, these materials are used widely as insulators in electrical applications.
04:16
Ceramics, for example, are used to hold live wires and power lines and to make capacitors,
04:21
which are basically layers of electrical conductors separated by layers of insulators.
04:24
But all of these silica based network solids I talked about so far contain both silicon and oxygen atoms.
04:29
Crystals made up of silicon are insulators too, in their peer state,
04:32
but they can be made to transfer electricity by a process called doping.
04:36
And no, we’re not gonna make a Lance Armstrong joke.
04:39
We might make an analogy though —
04:40
it’s like, much like a perfectly fit athlete injecting himself with all kinds of weird foreign stuff.
04:45
Silicon doping involves incorporating impurities into a crystal that mess up its electron balance.
04:50
It can do that in one of two ways.
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If the impurity is an element with more electrons than silicon has, like Phosphorus or Arsenic,
04:57
the resulting crystal will contain excess electrons.
04:59
But, if the impurity is an element with fewer electrons, like Aluminum or Boron,
05:03
it leaves holes where electrons are missing from the normal structure.
05:07
Doped silicon crystals are known as solid-state semiconductors.
05:10
Those doped with elements that add electrons are called N-type,
05:13
or negative semiconductors, because of the resulting charge from the additional electrons.
05:17
With N-type semiconductors, the charges carried by the excess electrons
05:20
and because there’s no room for them to integrate into the structure, they move freely about the crystals.
05:25
Similarly, crystals dope with elements that create an electron deficiency are called P-type,
05:29
or positive semiconductors, because they have a more positive charge than pure silicon would have on its own.
05:34
P-type semiconductors work a little differently.
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Because they have empty spaces available here and there,
05:39
the electrons that are normally part of specific silicon atoms are able to jump around from one atom to another.
05:45
Each jump fills a hole and creates a new one, allowing another electron to move in the same way.
05:50
So if both N-type and P-type crystals are both electrical conductors, why do we call them semiconductors?
05:56
Well, thank you for asking.
05:57
The reason semiconductors are so valuable is that they have the amazing ability to conduct
06:01
electricity freely under some conditions,
06:04
but completely refuse to conduct it under other conditions.
06:07
So yes, they are partial, or semi conductors.
06:10
So these properties become most useful when a P-type and N-type semiconductor are placed
06:15
next to each other to form a diode.
06:17
Because their charges are opposite,
06:18
the free electrons in the N-type are attracted to the P-type and tend to migrate to its holes.
06:23
And when an electrical field is applied to this arrangement,
06:25
so that the N-type is attached to the positive terminal, also called the anode,
06:28
and the P-type is attached to the negative terminal, or the cathode,
06:32
all of the free electrons are pulled completely into the N-type.
06:36
These free electrons are attracted away from the holes, not into them,
06:39
so any further motion is blocked and no more current passes through the diode.
06:44
This type of arrangement is called reverse bias.
06:46
However, just by reversing the polarity of the electron flow, if you will, we can create a very different situation.
06:52
If the N-type side is attached to the negative terminal, and the P-type side attached to the positive terminal,
06:56
then the electrons are pulled in to the P-type making this part an electrical conductor again.
07:01
This process called forward bias is self perpetuating, it allows the current to readily flow through the diode.
07:07
The fact that this conduction only works in one direction is extremely useful!
07:11
You know AC and DC right? Back in Black for those about to rock?
07:14
AC is alternating current, where the flow of electrons alternates in one direction to the other.
07:20
In DC, or direct current, is exactly what it sounds like, the electrons go directly
07:24
where they’re headed without backtracking.
07:26
And simple arrangements of diodes can be used to convert AC to DC like in this power supply for my laptop.
07:32
P and N-type semiconductors can also be combined in sets of three.
07:35
These are called transistors and they’ve completely revolutionized the field of electronics,
07:39
also the entire world since they were invented in 1947.
07:42
One of the main functions of a transistor is to switch an electrical signal on and off.
07:46
Basically when it allows a current to flow in the forward bias direction, the switch is turned on.
07:50
When the current drops below the minimum the switch is turned off.
07:53
This switching is the basis of binary code.
07:56
Ones and zeroes representing the on and off states of the transistor.
08:00
And that code is how we store and process information using computer chips,
08:04
which are just collections of transistors working together.
08:07
Such a tiny simple device made mostly from one single element, and yet revolutionizing our entire world.
08:15
That my friends is how Silicon Valley got its name
08:17
and how network solids of silicon made this conversation we are having today possible.
08:22
Thanks for taking advantage of all your tiny transistors today by watching this episode of Crash Course Chemistry.
08:26
If you paid attention you learned that sand, and glass, and ceramics, and computer chips,
08:29
and must more are all really just different types of network solids that can be formed by silicon and its oxides.
08:37
You learned the differences in their arrangements and bonding of atoms,
08:40
and you learned how those differences result in wildly varying properties among the solids.
08:45
You also learned what a solid-state semiconductor is, both P-type and N-type,
08:49
and you learned how they can be combined to create diodes and transistors.
08:53
Finally you learned how transistors are combined to make computer chips,
08:56
and how their switching mechanism tells computers what to do.
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I don’t know about you, but it blows my mind!
09:01
This episode was written by Edi González and edited by Blake de Pastino.
09:05
The chemistry consultant was Dr. Heiko Langner.
09:06
And the script supervisor was Michael Aranda, who was also our sound designer.
09:10
It was filmed, edited, and directed by Nicholas Jenkins.
09:13
And our graphics team, as always, is Thought Café.


This post was previously published on YouTube.

Photo credit: Screenshot from video