Physical Sciences 3
Wed 1-2 and Wed 2-3 sections
TF: Widagdo Setiawan
Warning: You do NOT need to know the stuff that I wrote here for this class. But it might help you
understand different kind of materials (insulator, conductor, semiconductor)
There are hundreds of materials about semiconductor on the internet, but I decided to write another
one anyway. It can’t hurt to see the same concept from multiple point of views.
Insulator
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Contains a lot of positive charges (example: copper nuclei)
Contains a lot of negative charges (electrons that pair up with the nuclei)
Overall NEUTRAL. Unless if you charge it. Even if you charge it, there is only a very small fraction
(
of imbalance between the positive charges (nuclei) and negative charges (electrons)
All electrons are bound to the nuclei. Electrons cannot jump from one nucleus to another
nucleus. This is exactly why current cannot flow. The electrons are really really bound to the
nuclei.
Electron
Nucleus
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Nucleus
Nucleus
Nucleus
Nucleus
The picture above describes how insulator works. Each nucleus contains a lot of positive charge
(remember, atomic nucleus is made out of protons and neutrons, so it can only have positive
charge). But for each proton, there is always an electron can balance the charge, so overall, each
atom is neutral.
The key about insulator is that every single electron is tightly bound to a nucleus. Therefore,
none of the electrons can move, unless if you put a massive amount of electric field.
Conductor
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Contains a lot of positive charges (example: copper nuclei)
Contains a lot of negative charges (electrons that pair up with the nuclei)
Overall NEUTRAL. Unless if you charge it.
Contains a LOT of free electrons. And this time, I mean a lot. Could be 5% of all electrons are
free. Electrons that can hop around easily from nucleus to nucleus. This is the exact reason why
conductor can conduct current with the tiniest electric field. There are so many free electrons.
Electron
Nucleus
Nucleus
Nucleus
Nucleus
Nucleus
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The picture above shows how a conductor works. The difference with the insulator case is that
now there are two kinds of electrons (well, electron is still an electron, but they just behave
differently). The first kind is the same as in the insulator case (the blue electrons). They are
super tightly bound to the nucleus, and cannot move. The second kind is the green electrons.
These green electrons can move freely from nucleus to nucleus. In most conductors, there is
exactly one of this green electron for each nucleus. There rest of the electrons are tightly bound.
Remember that each copper atom has 29 electrons, so 28 bound electrons and 1 free electrons
for each atom
Semiconductor
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Contains a lot of positive charges (Si nuclei) and negative charges (electrons)
Undoped semiconductor contains no free charge. Pretty much an insulator
You can dope it such that you have free negative charge carriers, or free positive charge carriers.
The number of free charge carriers is far less than in real conductor. Could be a billion time less.
Even if you have free charge carriers, the semiconductor is still NEUTRAL. This is the most
important part to understand. The semiconductor acts pretty much like a conductor now.
Semiconductor with negative charge carrier (called N-type) is easy to understand. The charge
carrier is just electrons, exactly like a normal conductor. The difference is that there is only
around 1 of the green electron for every 1 billion silicon atom. In the picture bellow, the green
electron can move freely across multiple atoms
N-Type Semiconductor
Electron
Nucleus
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Nucleus
Nucleus
Nucleus
Nucleus
Semiconductor with positive charge carrier (called P type) is a lot harder to understand. The
positive charge carrier is NOT the Si nuclei, since nuclei can never move. In fact, the charge
carrier is a hole, which is an absence of electron. You can think P-type semiconductor as an
insulator with some of the electrons removed (1 in 1 billion electrons removed), while keeping
the material itself neutral.
P-Type Semiconductor
Hole
Electron
Nucleus
Nucleus
Nucleus
Nucleus
Nucleus
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The red circle on the picture above represents an absence of electron, which is called a hole. A
good analogy of hole is a bubble in water. Imagine you have a bottle of water and you put an air
bubble at the bottom of the bottle. This air bubble will rise up. You can think about this in two
ways:
o The water on top of the air bubble wants to go down because of gravity. Well, the water
on top of the bubble will fall down, and it will fill the bubble. But in its place, another air
bubble will start forming. So the water is falling down slowly in this bottle, around this
bubble.
o Think the bubble as an entity. Well, the bubble just rises up.
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Same thing with the hole. As the picture above shows, an electron jumps from right to left. The
electron “jumps” because the electron cannot actually move freely, it has to jump across a
barrier. This is very different from a conductor case above where the electron can really move
freely. You can think of this process as electron jumping from right to left, creating a new hole
on the middle site, and destroying a hole on the left site. Or, you can think about this as the hole
moving to the right.
Remember, both N-Type and P-Type semiconductor are electrically NEUTRAL. There is no net
charge.
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Diode
A diode is just a P-type semiconductor attached to an N-type semiconductor (each is originally
electrically neutral).
On the picture above, a P-Type semiconductor (left) is attached to an N-Type semiconductor (right). At
this stage, they are both electrically neutral. However, this condition is not stable. The electrons from
the right side can move to the left and fall into the holes. Within a fraction of a second, a new
equilibrium will be reached.
Some of the green electrons rolls to the left and fill the hole. Note that not all of the green electrons will
fill the hole. As more and more electrons fill the holes, an electric field will start forming. Remember
that originally, the semiconductors are neutral. Since the electrons move to the left, now the left side
has a net negative charge, and the right side has a net positive charge. The resulting electric field
prevents more electrons from filling the remaining holes. The shaded purple region above is called the
depletion region, which is a region where there is no free electron and no holes either. This depletion
region now acts like an insulator, while the region outside the depletion region can still conduct
electricity.
Now let us apply electricity on this diode
1. Negative voltage on the left, positive voltage on the right
The negative voltage on the left provides more electrons on the left side. More and more holes
will be filled. Similar thing happens on the right side. The positive voltage will attract free
electrons from the N-Type. Now there is even less free electron. The combined effect is to make
the depletion region larger. But remember that depletion region is an insulator, so no current
can flow through the diode.
2. Positive voltage on the left, negative voltage on the right
The positive voltage on the left side will attract electron from the p-type. More and more holes
will be created. On the right side, the negative side will push electrons on the n-type. More and
more free electrons will be available. The depletion region shrinks as this happens. If enough
voltage is applied (around 0.6 V), the depletion region will disappear, and current will flow from
left to right.
Transistors
There are many types of transistors.
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Field effect transistor (FET)
Bipolar Junction Transistor (BJT)
Isolated Gate Bipolar Junction Transistor (IGBT)
The lecture slide claims that they are explaining FET. But I does not look like a FET to me. It’s not BJT or
IGBT either. So I have no clue what it is. So I will try to explain how it might look like if it is a FET. My
discussion follows my favorite reference source. http://en.wikipedia.org/wiki/MOSFET
N
P
N
Like most transistors, there are 3 terminals, Drain, Gate, and Source (ignore the substrate source pin).
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The black areas at the gate, source, and drain are conductors (metal).
The shaded area labeled silicon dioxide is an insulator.
The drain is connected to the n-type patch on the top
The source is connected to the p-type patch on the bottom.
The gate is not connected to anything!! This is the reason why the current through the gate is
always zero in FET.
Imagine that the Gate voltage is zero. Now imagine if you try to apply a current from drain to source.
Current cannot flow. There is basically two diodes back to back in the diagram formed by the two N
patches and the P block. This arrangement prevents current from flowing in any direction!!! This is why
when the gate voltage is zero, current cannot flow through the FET.
Now imagine that you apply a voltage on the Gate. The gate + insulator + the substrate look like a
capacitor. In fact, it is a capacitor. Positive charge will start building up at the gate, and negative charge
will start building up at the opposite side of the gate on the p type. Well, negative charge is just electron,
which makes it an n-channel semiconductor! The capacitor’s electric field turns a p-type semiconductor
into an n-type semiconductor! Now the two N-type patches are connected by another n-type patch. The
back to back diodes disappear. Current can now flow either from drain to source or from source to drain
without any problem. This is why a positive voltage at the gate turns the mosfet into the conducting
state.