Extrinsic Semiconductors

Doping silicon, which is tetravalent, with a pentavalent atom such as arsenic introduces an atom that forms four covalent bonds with neighbouring silicon atoms while its fifth valence electron remains very loosely bound. At room temperature this extra electron is easily excited into the conduction band, so each arsenic atom donates one free electron; the impurity is therefore called a donor and the material is n-type. As a result, electrons become the majority carriers while holes, still produced only by thermal pair generation, remain the minority carriers. The option naming holes is wrong because holes would dominate only with trivalent acceptor doping. The option of equal carriers describes an intrinsic crystal, but doping deliberately breaks that balance. The option naming arsenic ions is wrong because those donor ions are locked into the lattice and cannot drift, so they carry no current even though they are positively charged. Checking the logic by charge neutrality, the immobile positive donor ions are balanced by the large mobile electron population, consistent with electrons being the dominant conduction carriers in an n-type semiconductor.