The document discusses heterojunctions and p-n junctions. It defines a heterojunction as the interface between two dissimilar semiconductors with different band gaps. There are three types of heterojunctions based on band alignment: type I where bands straddle, type II where bands are staggered, and type III where there is a broken gap. A p-n heterojunction diode forms when a p-doped and n-doped semiconductor meet; electrons flow from the higher to lower Fermi level side and holes in the opposite direction.