Q. I am studying transistors, and the lesson says that the emitter is more heavily doped than the collector, and that the base is lightly doped. What does this mean?
A. Recall that in order to make a pure semiconductor crystal into P or N type material we add impurities called dopants. When a material is "heavily" doped, we add a relatively large amount of dopant to it. When it is "lightly" doped, a relatively small amount of dopant is added.
Sometimes you will see + and - signs used to indicate the amount of doping. The + sign just means that the material is heavily doped and the - sign means that it is lightly doped.
So, for example, material marked p+ means that P-type dopant is added in a relatively large amount. If the material is marked p-, then it means that the amount of P-type dopant is relatively small.