Intrinsic and Extrinsic Semiconductors

Intrinsic (pure) silicon has equal numbers of free electrons and holes, $n = p = n_i \approx 10^{10}$ cm⁻³ at room temperature — very few.

Extrinsic silicon is doped:

• n-type — add a group-V element (phosphorus, arsenic). Each dopant gives a spare electron: electrons are the majority carriers.
• p-type — add a group-III element (boron). Each dopant creates a hole: holes are the majority carriers.

At equilibrium the mass-action law holds: $n \cdot p = n_i^2$. Doping by just one part per million increases conductivity by many orders of magnitude.