If we add aluminium atoms to silicon wafer, the resulting substance will be _.

Aluminium is a trivalent impurity (3 valence electrons) while silicon is tetravalent (4 valence electrons). When aluminium atoms are added to silicon, they create electron deficiencies called 'holes' in the crystal structure, making it a p-type semiconductor where holes are the majority charge carriers. Aluminium acts as an acceptor impurity.