Silicon is the base material in a great deal of computing equipment. It has been used extensively for decades and is a material that Engineers and Scientists understand well and can easily manipulate. Advances in this manipulation has led to both increased speed and reduced size of complex computing equipment. In this article, I'll explain how silicon is used in computers and in the next couple of articles I'll talk about some potential replacements for silicon and the benefits and drawbacks of each of them.
In computer chips and transistors silicon is known as a semi-conductor. But silicon by itself is not a semi-conductor; in fact it's an insulator. This is due to the chemical structure of the element Silicon. Silicon has 4 valence electrons (outer electrons that can participate in the forming of bonds with other atoms), this allows silicon atoms to form strong covalent bonds with other silicon atoms with no free electrons as a result of the bond. This means that when electricity is applied to silicon there is no way for it to travel through the material, because there are no free electrons.
A covalent bond is a special chemical bond between atoms formed when the atoms share one or more outer electrons.
So how can silicon be used as a conductor? Silicon can become a semi-conductor through a process known as doping. There are two kinds of doping used. The first kind is referred to as N-type. In this type of doping either phosphorous or arsenic is added in very small quantities to the silicon. Both phosphorous and arsenic have 5 outer electrons so when they form covalent bonds with silicon atoms there becomes a free electron. Even a small amount of phosphorous or arsenic can produce enough free electrons for silicon to become a semi-conductor. These free electrons will give the doped silicon a negative charge; that's why this type of doping is called N-type.
Another type of doping is called P-Type. In this type of doping either boron or gallium is used to bond with silicon. The difference with this type of doping is that boron and gallium each have three outer electrons. So, when the covalent bonds are formed with silicon atoms there is a 'hole' that is formed. This absence of an electron gives the effect of a positive charge (hence the 'P-type' name) which is really the opposite of the N-type doped silicon.
By themselves these doped silicon semiconductors are not that special. However, when we put them together interesting things can happen. In figure 1, there is a P-type silicon block next to an N-type silicon block. At first glance this might look a little weird. We have what looks very much like positive and negative charges next to each other - wouldn't the electrons travel to the positive side to balance out the charges?
Figure 1: P-type and N-type silicon forming a diode
No. The electrons of the N-type silicon will not travel to the P-type silicon to balance out the charges. This is because of the band gap. By itself the amount of charge is not high enough to encourage mobility of the electrons. This band gap allows us to do some amazing things with the doped silicon.
If we put N-type silicon next to P-type silicone and combine them with a power source we can make a diode. A diode is a basic electronic device that allows electricity to flow in only one direction - the direction that supplies energy greater than the band gap of the doped silicon. Figure 2 shows the P-type and N-type silicon together in a circuit with a power source. When the power source is in the right direction electricity will flow through the diode, when it is in the wrong direction electricity will not flow.
Figure 2: a diode connected to a power source
It's worth noting here that if the power source is large enough, then the diode will fail and electricity will flow in either direction. This is because there is also a band gap in the opposite direction, while it requires a much greater amount of energy to surpass the band gap, it is not infinite.
Diodes are a very simple, yet highly valuable and often used electronic component. However, one of the most important electronic components made with silicon is the transistor. To make a transistor with doped silicon we can combine the doped silicon into a sandwich of sorts. These types of transistors are called "Junction Transistors", and there are two kinds of these junction transistors. There is an NPN kind which has P-type silicon sandwiched between two N-type silicon pieces. There is also the PNP type of junction transistor which has N-type silicon sandwiched between two P-type silicon pieces. These two types of junction transistors are basically the same except that they operate with the reverse polarity of the other.
So to consider how this works, let's just examine the NPN type junction transistor. If you remember when I was explaining the diodes you might think that this looks like two diodes back to back which would stop electricity from flowing in either direction - you'd be right. However, if we apply a small electrical current to the middle P-type silicon (often referred to as the 'base') we can allow current to flow from one N-type silicon (often referred to as the 'collector') to the other N-type silicon (often referred to as the 'emitter'). Likewise if we remove the electrical current from the base the current from collector to emitter will stop. This type of action allows us to use this junction transistor as a simple switch. It is simple switches like this that we can combine together to form more complex logical gates.
Figure 3: a diagram of an NPN junction transistor
Another type of transistor we can make with doped silicon is called a Field Effect Transistor or a FET. There are a couple of subtypes of FET transistors, but they each work basically the same way. In a FET transistor only two types of doped silicon are used, and N-type and a P-type. This type of transistor takes advantage of the magnetic field created along with any current. Basically a FET transistor will allow electricity to flow through one type of silicon which is used as the channel. When electricity is applied to the other type of silicon a magnetic field is produced which interferes with the current flowing through the channel thus significantly reducing it. By utilizing this magnetic field effect we can use the FET as a switch in much the same way as I explained we could use the junction transistor as a switch.
So that's a simplified explanation of how silicone is used in electronic components, including computer chips and processors of all sorts. You can see how improving the electrical performance characteristics and decreasing the size of these components can have dramatic effects on the performance and size of the finished computer parts. However, as you reduce the size of silicone enough the physical properties start to change, making it more difficult to achieve the desired results. In my next article I'll discuss this along with some alternatives to silicon that are currently being explored.
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