<human>:  I have a question about a semiconductor device called a Heterojunction. What is it?
<bot>: A heterojunction is a device that is made by combining two different semiconductors. The two semiconductors are usually different types of materials, such as silicon and germanium. The heterojunction is used to create a new material that has properties that are different from the two materials that are combined. For example, a heterojunction can be used to create a material that is a better conductor of electricity than either of the two materials that are combined.