Semiconductors can be classified into two main categories: intrinsic semiconductors and extrinsic semiconductors.
Intrinsic semiconductors are pure, undoped semiconductors that have equal numbers of electrons and holes. These materials have a small bandgap, which means they have a small energy gap between their valence and conduction bands. Intrinsic semiconductors are not very conductive and require external energy to be stimulated into conducting electricity. Examples of intrinsic semiconductors include silicon and germanium.
Extrinsic semiconductors are doped semiconductors that have impurities added to their structure. These impurities, called dopants, can be either n-type or p-type, depending on the type of impurity added. N-type dopants, such as phosphorus and arsenic, donate extra electrons to the semiconductor material, making it more conductive. P-type dopants, such as boron and aluminum, create holes in the semiconductor material, also making it more conductive. Extrinsic semiconductors have a larger bandgap and are more conductive than intrinsic semiconductors. Examples of extrinsic semiconductors include silicon doped with phosphorus and germanium doped with boron.