Problem 5 - Entrance Test

Silicon (Si) belongs to Group 14 of the periodic table, having 4 valence electrons. It forms 4 covalent bonds in its crystal lattice. Arsenic (As) belongs to Group 15 of the periodic table, having 5 valence electrons. When silicon is doped with arsenic, an arsenic atom replaces a silicon atom in the crystal lattice. Four of arsenic's valence electrons form covalent bonds with the four surrounding silicon atoms. The fifth valence electron of arsenic is not involved in bonding and becomes a delocalized electron, which is free to move through the crystal lattice. This introduction of extra free electrons increases the electrical conductivity of silicon. Since the majority charge carriers are negatively charged electrons, this type of semiconductor is called an n-type semiconductor ('n' for negative). Therefore, the correct description is n-type, with electrons as the majority charge carriers.