an n-type semiconductor

An n-type semiconductor is one doped so that its current is carried mainly by negative electrons — the 'n' stands for negative. You make it by adding donor atoms that each hand a spare electron to the conduction band. The result is a crystal awash in mobile electrons, far more than the pure material would ever have on its own, ready to conduct.

The recipe, in a picture: silicon has 4 valence electrons and each atom shares them in four bonds. Drop in a phosphorus atom (group V, 5 valence electrons) in a silicon site — four of its electrons complete the bonds, and the fifth is barely held. At room temperature that fifth electron is easily set free into the conduction band, so each phosphorus donates one mobile electron. Because these donor electrons vastly outnumber the electron-hole pairs from thermal excitation, electrons are the majority carriers and holes are the minority carriers. The Fermi level shifts UP, toward the conduction band, reflecting the surplus of electrons.

n-type material is one half of every semiconductor device — a p-n junction, a transistor, a solar cell all need n-type joined to p-type. A subtle but important honesty: n-type silicon is electrically neutral overall. It is not negatively charged; the extra mobile electrons are exactly balanced by the fixed positive phosphorus ions left behind. The 'n' refers only to the SIGN of the mobile carriers doing the conducting, not to any net charge on the block. Common phosphorus, arsenic, and antimony are the usual donors for silicon.