In the last lesson we saw that heat lifts electrons across the small band gap of a semiconductor, leaving holes behind. A pure semiconductor, such as silicon with no impurities, is called intrinsic. It makes electrons and holes in equal numbers, but there aren’t many at room temperature, so it conducts poorly and its conductivity changes a lot with temperature.
Engineers need much better control than that, and the answer is doping. Adding a tiny number of atoms of a different element, as little as one in a million, adds extra electrons or creates extra holes. This raises the conductivity enormously and fixes it at a level we choose. Depending on the impurity, we get n-type or p-type semiconductors, the building blocks of diodes and transistors.