page_title: Quantum well
section_title: Description and overview
hierarchical_section_title: Quantum well / Description and overview
caption_attribution_description: English: The band structure in a quantum well of a thin layer of GaAs in between layers of AlGaAs.
context_page_description: A quantum well is a potential well with only discrete energy values.
  The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers, leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy values.
  A wide variety of electronic quantum well devices have been developed based on the theory of quantum well systems. These devices have found applications in lasers, photodetectors, modulators, and switches for example. Compared to conventional devices, quantum well devices are much faster and operate much more economically and are a point of incredible importance to the technological and telecommunication industries. These quantum well devices are currently replacing many, if not all, conventional electrical components in many electronic devices.
context_section_description: One of the simplest quantum well systems can be constructed by inserting a thin layer one type of semiconductor material between two layers of another with a different band-gap. Consider, as an example, two layers of AlGaAs with a large bandgap surrounding a thin layer of GaAs with a smaller band-gap. Let’s assume that the change in material occurs along the z-direction and therefore the potential well is along the z-direction (no confinement in the x-y plane.). Since the bandgap of the contained material is lower than the surrounding AlGaAs, a quantum well (Potential well) is created in the GaAs region. This change in band energy across the structure can be seen as the change in the potential that a carrier would feel, therefore low energy carriers can be trapped in these wells.  
  Within the quantum well, there are discrete energy eigenstates that carriers can have. For example, an electron in the conduction band can have lower energy within the well then it could have in the AlGaAs region of this structure. Consequently, an electron in the conduction band with low energy can be trapped within the quantum well. Similarly, holes in the valence band can also be trapped in the top of potential wells created in the valence band. The states that confined carriers can be in are particle-in-a-box like states.
caption: The band structure diagram in a quantum well of GaAs in between AlGaAs. An electron in the conduction band or a hole in the valence band can be confined in the potential well created in the structure. The available states in the wells are sketched in the figure. These are 'particle-in-a-box like states.
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