AC Electrics - Semiconductors — Page 259, Lesson 270

I want to walk you through the next part of our semiconductor lesson. We've already looked at pure silicon and germanium crystals. Now we're going to see how we deliberately change their electrical properties by adding tiny amounts of other elements — a process called doping. Let's start with N-type material. To make N-type material, we take our pure silicon or germanium and dope it with atoms that have five valence electrons in their outer shell — elements like arsenic or antimony. The ratio is incredibly small: for every one impurity atom, there are about one hundred million original atoms — that's a doping ratio of roughly 1 to 10⁸. Here's what happens inside the crystal lattice. Four of those five valence electrons from each impurity atom form normal covalent bonds with the surrounding silicon or germanium atoms. But that fifth electron has no partner to bond with — it has no such ties. So it becomes a free electron, able to move through the material. Because we now have extra free electrons available to carry current, the conductivity of the material increases. We call this N-type because of the surfeit — that means an excess — of free electrons, and electrons are, of course, negatively charged. That's where the 'N' comes from. But here's a really important point: the material as a whole remains electrically neutral. Why? Because for every free electron that's roaming around, there is a fixed positive ion left behind in the lattice. The positive charge of that ion balances the negative charge of the free electron. So overall, no net electrical charge. Now let's move to P-type material. This time we dope the silicon or germanium with impurities that have only three valence electrons in their outer shell — elements like aluminium or indium. We use the same tiny doping ratio, again about 1 to 10⁸. In the lattice, those three electrons try to form covalent bonds with the surrounding atoms, but one bond is missing — there's a hole in the valence structure. That hole behaves like a missing electron. Now, electrons from adjacent atoms tend to move into these holes. When an electron jumps into a hole, it leaves a new hole behind at the atom it came from. That new hole then 'steals' an electron from its neighbour, and the hole moves on further. So we get this apparent movement of holes through the material, and this movement increases the conductivity — just like the movement of free electrons does in N-type, but here the charge carriers are holes. Because there's a shortage of electrons, we classify this material as P-type — 'P' for positive, since a hole behaves as if it carries a positive charge. Again, the material possesses no net electrical charge. There is an equal number of holes and fixed negative ions within the material, so overall neutrality is maintained. So to summarise: N-type has an excess of free electrons from five-valence impurities; P-type has a shortage of electrons — holes — from three-valence impurities. Both are electrically neutral overall, and both have increased conductivity compared to pure semiconductor material.