When a piece of pure silicon is doped with aluminium, the doped silicon piece will become p-type. Because Silicon is having 4 electrons in its outermost orbit, so it can make four bonds but aluminium is trivalent. By adding a trivalent material to the crystal structure, holes are introduced and, thus, provide a mechanism for conduction. Because trivalent materials can accept an additional electron, they are called acceptor atoms. A silicon crystal doped with the trivalent material is called ptype material.