A double heterostructure, sometimes called double heterojunction, is formed when two semiconductor materials are grown into a "sandwich". One material is used for the outer layers, and another of smaller band gap is used for the inner layer. In this example, there are two AlGaAs-GaAs junctions, one at each side of the inner layer. There must be two boundaries for the device to be a double heterostructure. If there was only one side of cladding material, the device would be a simple, or single, heterostructure.