HEMT stands for High Electron Mobility Transistor. A HEMT is a Field effect transistor with a junction between two materials with different band gaps (i.e. a heterojunction) as the channel instead of an n-doped region. A commonly used combination is GaAs with AlGaAs. The effect of this junction is to create a very thin layer where the Fermi energy is above the conduction band, giving the channel very low resistance (or to put it another way, "high electron mobility"). This layer is sometimes called a two-dimensional electron gas . As with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.
Ordinarily, the two different materials used for a heterojunction must have the same lattice constant (spacing between the atoms). 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 are a kind of "trap", and greatly reduce device performance.
A HEMT where this rule is violated is called a PHEMT or pseudomorphic HEMT. This feat is achieved by using an extremely thin layer of one of the materials - so thin that it simply stretches to fit the other material. This technique allows the construction of transistors with bigger bandgap differences than otherwise possible. This gives them better performance.
To the best of the author's knowledge, PHEMTs and related devices are the fastest transistors available. They can be used to make amplifiers which work at over 200 GHz. Applications are similar to MESFETs - microwave and millimetre wave communications, radar, and radio astronomy.
Cross section of an InGaAs PHEMT
