Question Video: Determining If a Semiconductor Diode Can Be Made of Pure Silicon Physics

Video Transcript

Can a semiconductor diode be made of pure silicon?

So the question is, if we have a chunk of pure silicon, can this block of silicon serve as a semiconductor diode? A diode is a circuit element that allows charge flow in one direction while preventing it in the other direction. The circuit symbol for a diode helps us see this. A diode allows charge to flow in this direction through it while blocking charge that would flow in the opposite direction.

We can figure out whether pure silicon can serve as a diode by connecting both ends to the terminals of a cell oriented in a certain direction. Because the positive terminal of the cell is on the right and the negative terminal is on the left, we know that conventional current will travel in a counterclockwise direction around this circuit. That means that the actual flow of electrons will travel in the opposite direction, clockwise. We know pure silicon to be a semiconductor. That is, it’s neither a very good conductor nor a very good insulator. This means that for a block of pure silicon at room temperature, there will be some charge flow across this material.

To a limited extent, electrons will be able to move from left to right across the silicon and complete the circuit. Since there is some flow of charge through the circuit, we could say that this is the allowed current direction. But then let’s see what happens if we reverse the polarity of our cell so that the positive and negative terminals are reversed. Oriented this way, we expect electrons, negative charge, to flow in a counterclockwise direction. And once again, when electrons reach our sample of silicon, they are able, in limited numbers, to cross that material. For a pure sample of silicon, charge will flow equally well across that sample in any direction. Electrons are just as able, we could say, to flow left to right as they are right to left through the silicon.

Just like before then, some charge is able to flow through the silicon sample, and that tells us an important fact. The current through a sample of silicon does not depend on the direction or the polarity of our cell. That is, it doesn’t depend on the direction of electron flow through the silicon. Therefore, this sample does not function as a diode, which allows current in one direction and blocks it in the other. Our answer then is that no, a semiconductor diode cannot be made of pure silicon.