n-type semiconductors are formed by doping pure Si or Ge with pentavalent elements like P, As, Sb, etc, Of the five valence electrons of the added elements, four of them form covalent bonds with Si (or Ge). The fifth electron, conducts the current.
In p-type semiconductor Si or Ge is doped with trivalent elements like Boron, aluminum or indium. The three valence electron of the added element form three covalent bonds with Si (or Ge) results as deficiency of one electron in the added element causes a positive hole. This type of extrinsic semiconductors are known as p-type semiconductors ( p-stands for positive hole).
