Editing High-electron-mobility transistor (section)

=== By growth technology: pHEMT and mHEMT ===
Ideally, the two different materials used for a heterojunction would have the same [[lattice constant]] (spacing between the atoms). In practice, the lattice constants are typically slightly different (e.g. AlGaAs on GaAs), resulting in crystal defects. As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form [[deep-level trap]]s and greatly reduce device performance.

A HEMT where this rule is violated is called a '''pHEMT''' or '''pseudomorphic''' HEMT. This is achieved by using an extremely thin layer of one of the materials – so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger [[bandgap]] differences than otherwise possible, giving them better performance.<ref name=Cooke2006>{{cite web|url=http://www.semiconductor-today.com/features/Semiconductor%20Today%20-%20Transcending%20frequency%20and%20integration%20limits.pdf|title=Indium Phosphide: Transcending frequency and integration limits. Semiconductor TODAY Compounds&AdvancedSilicon • Vol. 1 • Issue 3 • September 2006}}</ref>

Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the '''mHEMT''' or '''metamorphic''' HEMT, an advancement of the pHEMT. The buffer layer is made of [[Aluminium indium arsenide|AlInAs]], with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the [[Indium gallium arsenide|GaInAs]] channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low [[Noise (electronic)|noise]]; high indium concentration gives high [[Gain (electronics)|gain]]).{{cn|date=January 2016}}