Unit 5 What is a Transistor?

Date: 2015-10-07; view: 504.

Transistors nowadays are made of silicon. So before we look at an actual transistor, what is silicon?

Well, after oxygen, silicon is the most common element on this planet. This is obviously important since it doesn't cost much to get the raw material. It's a solid and, as such, has the ability to conduct electricity. Let's look at this other table (Figure 2) which shows resistance of a number of different elements. One from the top is polythene — it's very resistant to electricity: it doesn't allow it to flow and therefore is used for insulating. You see this piece of copper wire is insulated with polythene. At the bottom of'the table are materials of very low resistance such as iron and copper — these are very good conductors — they allow electricity to flow and are therefore used for making electric cable. Now, silicon is right in the middle, in other words neither a good conductor nor a good insulator — that's why it, and other solids such as germanium, are known as semiconductors.

As a semiconductor, silicon is very sensitive to impurities — these are called dopants. If you add as little as 0.0001% of a dopont to silicon, you can increase conductivity by 1000 times. So, now let's look at this schematic diagram of a transistor. As you can see it consists of two types of silicon. One type has been doped with boron which gives it a positive charge — this is termed p-type silicon. The other type has been doped with phosphorus which gives it a negative charge — this is known as n-type silicon. By doping a silicon crystal with p and n type dopants a p-n junction is formed. So, here in this diagram you've got a substrate of p-silicon and then two pockets of n-silicon. There are three contact leads — the one on the right is connected to the metal source. When a voltage is applied to this contact, a current flows through the p-n junction and out through the metal drain — this contact on the left. In the middle you have another electrode — the gate which can be used to regulate the current flow. If we look at ... (Fade)