This is Sal here and I'm with Ben Eater, who I consider to be an expert at many things
What are we gonna talk about today.
Hey let's talk about semiconductor. And semiconductors is the computing revolution
in electronics, the phones in our pockets. they are all made of chips
that have semiconductors in them
Right right and they are made from silicon.
Silicon and that's why we have this periodic table here
we see silicon here with an atomic number of 14. Which means it has 14 protons
and a neutral silicon would also have 14 electrons
Right but what we care about is the electrons that are in the outermost shell the valence electrons
For any any viewers who aren't familiar we have a whole series on valence electrons and I encourage to watch them
if what ben and I are talking seems to be a bit confusing
But a neutral silicon atom will have 14 electrons but the 4 in the outermost shell
that are likely to react
and so when chemist
depict silicon or depict how it might react
they think primarily about this 4 electrons
but we just drew this 4 , these are 4 VALENCE electrons
there's another 10 for a neutral silicon atom that will be sitting closer to the nucleous
right but those dont react with other atoms as much so we dont really think about those
so in a semiconductor you've got basically a whole crystal of silicon atoms
se let's see if we can imagine that
so we have a cristal here
each of these circles represents a silicon atom
and we are only going to think about the valence electrons
so this silicon atom has 4 valence electrons
so it could be 1, 2, 3 and 4 valence electrons
so it could be bonded to a neighboring silicon
let me do this in a ifferent color so we can keep track of its valence electrons
to a neighboring silicon
whose valence electrons are 1, 2, 3 and 4
and when we see what happened here is that they formed a covalent bond
wich allows both of these silicons to share both of these electrons
yeah so he drew them in different colors but both of them feel like they own both electrons
right , and in chemistry , once again, the atoms get stable when they have 8 valence electrons
and so for silicon does this with 4 other silicons
it can pretend that it has 8 valence electrons
like this one right over here
it kinda feels like it partially owns 8 valence electrons
even though he only contributed 4
the other 4 comes from the neighboring silicons
yeah and all of the silicon atoms in this crystal structure feel that way so its very stable
and once again, what we are seeing is just a 2-dimensional slide
this thing would be 3-dimensional
yeah its a 2-dimentional depiction really cause the bonds are , you know , 3 dimensional
but yeah this is just a representation
so what we are interested in is- we are talking about computers and electronics and so we want to talk about current flow
and so the question is , can current flow through this , is this a conductor?
well what we just talked about , looks like everything would be very stable here it doesnt seems to be a lot of
strong incentive , even if you were to put a current through this , and let's do that
even if you were to put a current through that
so if i were to put a current across this
se there is a batery right over here
and this is the negative side, the negative terminal
this is the positive terminal
and we tipically talk about current going from positive to negative
but we know that is actually the electrons are flowing
yeah its a little confusing unfortunally i guess
benjamin franklin had 50% chance of getting it right and he got it wrong
he had it wrong well he did many other good things
so we'll excuse him this one
we'll give him a pass . but the electrons
the electrons want to , you know , if you have a complete circuit
the electrons would go the other way around
they would go to the positive terminal
but , all of these electrons in this silicone lattice
it looks like they are pretty happy , there is no reason for them to move
good conductors , when we think about the transition metals
things like copper they have a sea of electrons
lots of free electrons moving around around
in this metallic bonds
in fact this wire we have over here if it was made of , say , copper it would have just a lot of free electrons
the sea of electrons that could flow
but here the electrons look like they are pretty happy where they are
they dont really have a lot of incentive to move around
right so how is this usefull
when we are building something like a computer
yeah , so by itself its not
so what we do is , we add some impurites to the silicon
we call this process doping
we are just adding some impurities to the silicon that let's us conduct electricity
those are some interesting things
can there be like any impurity?
no , so there are actually very specific impurities
and so one impurity is phosforous
i see , so let's take a look to the periodic table
any time I hear the name of an element i need to refer to the periodic table
to get my bearings , ALRIGHT , so phosphorous
look is right next to silicon in the periodic table
it has an atomic number of 15
and so a neutral phosphorous would have 15 protons and 15 electrons
but if we think about its valence electrons
it would have one more
so it has 5
it would have 5 valence electrons
yeah and that 5th valence electron is like super important
so let's think about what's going on here
if i replace this silicon with a phosphorous (blue)
if i were to replace this silicon with phosphorous
it could have 1, 2, 3, 4 but then it has a 5th valence electron
that wouldnt be able to find a covalent bond
so he is kind of sitting there
and let me do that again maybe this one over here is a phosphorous
and so this is 1, 2 , 3, 4, and the it has a 5th valence electrons
so it has this extra electron
yeah so it has an extra electrons
and they are free to move around , they are not stuck in these covalent bonds
right , this covalent bond is kind of a stable configuraton
you have the 8 valence electrons
but here you have these extra ones , and yeah these are more , liberated i guess to move around
AND we have this battery hooked up here , you have this positive terminal
and electrons are gonna be atracted to that
yeah so this one , this one right ove here it might be able to be atracted to that one , that one can move there
and eventually get to the wire
like this guy can replace that guy
and eventually the electrons could start flowing
the way we are looking at this , with two atoms could kind of make that leap
right right , it could make the leap , it could go straight
so this stuff could start flowing
if this extra phosphorous electron were to move to the right
then this phosphorous would have a positive charge
and so maybe it grabs an eletron on from here and that grabs an electron from the wire a
and so you start having current flowing
fascinating
thats one way of doping silicon to get current flowing
but there is another way
and that is with something like , Boron
let's get another lattice here
alright so let me go back to the periodic table
you just said boron
so boron has an atomic number - let me go back and pick another color , purple for boron
so boron has an atomic number of 5
but it has 3 valence electrons
it has 3 electrons in his outermost shell
so boron's valence electrons may look like that depending on the convention you use
so let's dope this silicon lattice with a little bit of boron
so the ones that are not labeled , lets assume they are silicon
but i'll throw a couple of borons here
so this is a boron
the boron would have 3 valence electrons
let me get rid of this one 'cause that one would've only been there if it was a silicon
so let me change the color-im actually getting rid of this one
because boron only has 3 valence electrons its not able to fomr this fourth covalent bond
let me do a couple more borons
so maybe this is a boron right over here and it has 3 valence electrons
and so it wasnt able to form this fourth covalent bond
and so it almost feels like there is a hole here
yeah , and thats actually how its called
its a hole
that should go into "naming things"
very creative
sorry , yeah so this is called "holes"
those are called holes
they are places where an electron would want to be in a normal silicon crystal
but there isnt one there because we've introduced this boron impurity
i see, and i think i see were this is going becau-you know just having this extra valence electron
introduce a kind of inbalance
and allow electrons to flow
it made it a little bit less stable
eyup
this is also a bit of an unbalance
a little less stable , yeah
so let'ss hook a battery up to it
yeah let's put a battery on this
and we get-so this is a positive , this is the negative
so let's see , if this is a positive-this is a positive
so electrons would want to go here
electrons are going to be atracted to the positive terminal
but in this case maybe we dont have a lot of free electrons
we actually have more holes than electrons
we have more holes than electrons so what can happen
is that the electrons that are coming from the negative terminal
they could , you know , they could make their way
or , you know , they could make a chain
but eventually these electrons can fill this hole
yes , a nearby hole , and then maybe
, you know , the hole will kind of transition to the - you know , one way to think about it
is if this electron here
fills this hole
then you have a hole here
yeah , just like the electrons can wonder around when we had the phosphorous over at the left
the holes are kind of free to wonder around a bit
right , those kind of give a place for the electrons to jump to
exactly , yeah so electron coming out of the negative terminal of the battery
are going to find a nearby hole
and when they do that , then the boron
, you know , where that hole is filled is going to be a little more positivelly charged
a little bit more negativelly charged
so then , a hole is gonna want to free-up
so other electron can move
so what will eventually happen is that , just how we depicted
this electron could go there and then a hole would be here
and then , maybe that electron could get to the wire over here
and then the electrons would move to the right
the electrons would move to the right across the wire
and form a current
while the holes kind of move to the left
and we kind of see the holes as a positive thing
even though its a "non-thing"
yeah we like to think of the holes as moving even though its an absense of something
but yeah we think of the holes as moving
so , in both of these cases , in one case we have electrons moving and in the other case
we have holes moving
we have a term for this
they are called "charge carriers"
so either the electrons or the holes are both considered charge carriers
becuase they allow a charge to be carried through this material
right , they both create a kind of an unbalance
the phosphorous creates extra electrons
this creates holes that can be filled by other electrons
and then the holes can keep moving
so in the case of the left we call this N-tipe material
that makes sense , im guessing N is for negative carrier
yeah because the electrons are negative
and then the boron dope thing on the right we call P-type
and these are the two types of silicon that we use in semi-conductors
fascinating , i think that i can already see where this can go