Substances such as silicon in a pure state are usually good:

In its pure, intrinsic state, a silicon atom shares all four of its valence electrons with neighboring atoms in a tight crystalline lattice. Because there are practically no free charge carriers (electrons or holes) available to move, pure silicon acts as a very good insulator. To make pure silicon useful for electronic components, it must be altered. It only gains its useful semiconductor properties—the ability to conduct electricity under specific, controlled conditions—when its pure state is modified through a process called doping.