A heterojunction is an interface between two different materials, typically semiconductor materials, that have different band gaps and/or electrical properties. Heterojunctions are important in various electronic devices and are used to control the flow of electrons and holes, as well as to create special electronic properties such as rectification and amplification. They are often used in optoelectronic devices like light-emitting diodes (LEDs) and solar cells. Heterojunctions can be formed by different methods such as alloying, epitaxial growth, or layering different materials together.