Double heterostructure

A double heterostructure is formed when two semiconductor materials, one with an energy gap, less than the other are joined together. When there is just one boundary between the semiconductor material this is known as simply a heterostructure. It becomes a double-heterostructure when there are two boundaries. The semiconductor with the larger energy gap is used for the cladding while the smaller energy gap semiconductor forms the 'filling' in this semiconductor sandwich. This is a very useful structure in optoelectronic devices as has interesting electronic properties. If one of the cladding layers is p-doped and the other cladding layer is n-doped then the smaller energy gap semiconductor material is undoped, we form a p-i-n structure. When a current is applied to the ends of the pin structure, electrons and holes are injected into the heterostructure. The smaller energy gap material forms energy discontinuites at the boundaries, confining the electrons and holes to the smaller energy gap semiconductor. The electrons and holes recombine in the intrinsic semiconductor emitting photons. If the width of the intrinsic region is reduced to the order of the de Broglie wavelength, the energies in the intrinsic region no longer become continuous but become discreet. (Actually, they are not continuous but the energy levels are very close together so we think of them as being continuous) In this situation the double heterostructure becomes a quantum well.