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

·  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 (10-10) 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.

·  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

·  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.

·  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

·  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

·  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.

·  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.

·  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.

### 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.

1. 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.

·  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

Like most transistors, there are 3 terminals, Drain, Gate, and Source (ignore the substrate source pin).

·  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.