AC Electrics - Semiconductors — Page 259, Lesson 271

Let’s start with current flow in N-type material. If I apply an electromotive force — an EMF — across a piece of N-type semiconductor, the free electrons inside it will migrate toward the positive terminal. As electrons leave the material at the positive side, they are replaced by electrons entering at the negative terminal. That keeps the overall balance between free electrons and fixed positive ions stable. Now, in P-type material, the situation is a bit more complex. In general, electrons are attracted into the positive terminal, and that creates holes in that region. Those holes then ‘migrate’ toward the negative terminal, and they are ultimately filled by an electron entering at that point. So in P-type semiconductor material, we can think of current flow as the drift of holes in the conventional direction — that is, from the positive terminal to the negative terminal. Again, overall balance is maintained between electrons and fixed negative ions. Let’s move to the P-N junction. If I fuse two small pieces of N-type and P-type material together — by a process similar to welding — some free electrons from the N-type material migrate across the boundary into the P-type, and similarly, holes migrate the other way. This migration produces a charged region called the Depletion Layer, and it creates a Barrier Potential that restricts further electron and hole movement. We can represent this barrier potential as an imaginary battery, but remember: the regions of increased positive and negative charge exist only across the junction. The material as a whole possesses no net electrical charge. Now, Reverse Bias. If I connect an external EMF across the P-N material in one particular orientation — as shown in Figure 16.6 A — more electrons are drawn across the barrier into the P-type material, and more holes are drawn into the N-type. This deepens the depletion layer, and further electron/hole migration is prevented. Apart from a small leak current, in the order of microamps (µA), no significant current flows. The junction is said to be reverse biased. Forward Bias is the opposite. By applying the external EMF as shown in Figure 16.6 B, the direction of the electric field produces a drift of holes in the P-type material to the right, and of free electrons in the N-type to the left. In the junction region, free electrons and holes combine, so the barrier potential is overcome. Current can now flow. Finally, the Junction Diode. From Figure 16.7, you can see that current can only flow in one direction through a semiconductor formed from P-N type material. In other words, the material acts as a rectifier, and it has similar conduction characteristics to a thermionic diode — a valve. That’s why it’s called a Junction Diode.