A heterojunction is created when two different layers of crystalline semiconductors are placed in conjunction or layered together with alternating or dissimilar band gaps. Mostly utilized in solid state electrical devices, heterojunctions can also be formed between two semiconductors with different properties, such as one that is crystalline while the other is metallic. When the function of an electrical device or device application depends on more than one heterojunction, they are placed in formation to create what is called a heterostructure. These heterostructures are used to increase the energy produced by different electrical devices, such as solar cells and lasers.
There are three different types of heterojunctions. When these interfaces between semiconductors are created, they can form what’s called a straddling gap, a staggered gap, or a broken gap. These different types of heterojunctions depend on the energy gap that is created as a result of the specific semiconductor materials.
The amount of energy a material can produce is directly relevant to the size of the energy gap created by the heterojunction. The type of energy gap is also important. This energy gap is made up of the difference that lies between the valence band, which is produced by one semiconductor, and the conduction band, which is produced by the other.
Heterojunctions are standard in every laser manufactured since the science of heterojunctions became the standard across the industry. Heterojunction allows for the production of lasers that are able to function at a normal room temperature. This science was first introduced in 1963 by Herbert Kroemer, though it did not become the standard science in the laser manufacturing industry until years later, when the actual material science caught up with the principle technology.
Today, heterojunctions are a vital element to every laser, from cutting lasers in CNC machines to the lasers that read DVD movies and compact audio discs. Heterojunctions are also used in high-speed electronic devices that operate at very high frequencies. An example is a high electron mobility transistor, which operates much of its functions at over 500GHz.
The manufacturing of many of the heterojunctions today is done through a precise process referred to as CVD, or chemical vapor deposition. MBE, which stands for molecular beam epitaxy, is another process used to manufacturer heterojunctions. Both of these processes are extremely precise in nature and very expensive to conduct, especially when compared to the mostly outdated process of silicon fabrication of semiconductor devices, though silicon fabrication is still widely popular in other applications.