HEMT

HEMT stands for High Electron Mobility Transistor, and is also called heterostructure FET (HFET). 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 of conducting electrons with rather high concentration, giving the channel very low resistivity (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.

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According to semiconductor theory, the semiconductor layer needs to be doped with n-type impurities (at Source and Drain) to generate electrons in the layer. However, this causes electrons to slow down because they end up colliding with the impurities residing in the same region, which were used to generate them in the first place. HEMT, however, is a smart device to resolve this seemingly inherent unsolvable contradiction.

HEMT accomplishes this by use of high mobility electrons generated using the hetero-junction of a highly-doped n-type AlGaAs thin layer and a non-doped GaAs layer (no impurities). The electrons generated in the n-type AlGaAs thin layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the hetro-junction created by different band-gap materials forms a steep canyon in the GaAs side where the electrons can move quickly without colliding with any impurities (because the GaAs layer is undoped).

Ordinarily, the two different materials used for a heterojunction must have the same lattice constant (spacing between the atoms). 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 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 the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible. This gives them better performance.

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 developed in recent years. In the buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the 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; high indium concentration gives high gain).

Applications are similar to MESFETs - microwave and millimeter wave communications, radar, and radio astronomy. (Heterojunction bipolar transistors were demonstrated at frequencies over 600 GHz in April 2005.) Numerous companies worldwide develop and manufacture HEMT-based devices. These can be discrete transistors but more usually in the form of an integrated circuit called a MMIC standing for 'monolithic microwave integrated circuit'. HEMT devices are found in many types of equipment ranging from cellphones and DBS receivers to electronic warfare systems such as radar and for radio astronomy.

See also

• Heterojunction bipolar transistor.