Video narration
How can adding a tiny impurity make a semiconductor conduct so much better? Salt in water does this by supplying mobile ions, but a semiconductor changes in a different way. An intrinsic semiconductor is nearly pure, with no intentional doping. Its electrons and holes-absences of electrons that act like positive carriers-are created by heat, in equal numbers. There are carriers, but few. Now the material is deliberately modified. This is an extrinsic semiconductor. A chosen dopant creates many more useful carriers. Here's the key: it is not useful because it is randomly dirty. It is useful because the dopant increases one kind of carrier. Add a dopant that contributes electrons, and electrons dominate: that is n-type. Use one that creates holes, and holes dominate: that is p-type. Engineers tune conductivity by choosing the dopant. So why can a tiny impurity matter? It adds charge carriers-electrons in n-type, or holes in p-type-making conductivity controllable.