MESFET

MESFET stands for Metal Semiconductor Field Effect Transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky (metal-semiconductor) junction is used. MESFETs are usually constructed in compound semiconductor technologies lacking high quality surface passivation such as GaAs, InP, or SiC, and are faster but more expensive than silicon-based JFETs or MOSFETs. Production MESFETs are operated up to approximately 30 GHz, and are commonly used for microwave frequency communications and radar. From a digital circuit design perspective, it is increasingly difficult to use MESFETs as the basis for digital integrated circuits as the scale of integration goes up, compared to CMOS silicon based fabrication
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The MESFET differs from the common insulated gate FET in that there is no insulator under the gate over the active switching region. This implies that the MESFET gate should, in transistor mode, be biased such that one does not have a forward conducting metal semiconductor diode instead of a reversed biased depletion zone controlling the underlying channel. While this restriction inhibits certain circuit possibilities, MESFET analog and digital devices work reasonably well if kept within the confines of design limits. The most critical aspect of the design is the gate metal extent over the switching region. Generally the narrower the gate modulated carrier channel the better the frequency handling abilities, overall. Spacing of the source and drain with respect to the gate, and the lateral extent of the gate are important though somewhat less critical design parameters. MESFET current handling ability improves as the gate is elongated laterally, keeping the active region constant, however is limited by phase shift along the gate due to the transmission line effect. As a result most production MESFETs use a built up top layer of low resistance metal on the gate, often producing a mushroom-like profile in cross section.


High Electron Mobility Transistor

"HEMT" redirects here. For the military truck, see Heavy Expanded Mobility Tactical Truck.

















Cross section of a GaAs/AlGaAs/InGaAs pHEMT
High Electron Mobility Transistors, also known as heterostructure FETs (HFETs) or modulation-doped FETs (MODFETs), are field effect transistors incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFETs. A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have seen a massive increase in research effort, due to their high-power performance.























Band structure in GaAs/AlGaAs heterojunction based HEMT
In general, to allow conduction, semiconductors are doped with impurities to generate mobile electrons. However, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. HEMTs avoid this through the use of high mobility electrons generated using the heterojunction of a highly-doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case).
The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, "high electron mobility"). This layer is 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). 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 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.
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 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 those of MESFETs - microwave and millimeter wave communications, imaging, radar, and radio astronomy - any application where high gain and low noise at high frequencies are required. HEMTs have shown current gain to frequencies greater than 600GHz and power gain to frequencies greater than 1THz. (Heterojunction bipolar transistors were demonstrated at current gain frequencies over 600 GHz in April 2005.) Numerous companies worldwide develop and manufacture HEMT-based devices. These can be discrete transistors but are more usually in the form of a 'monolithic microwave integrated circuit' (MMIC). HEMTs are found in many types of equipment ranging from cellphones and DBS receivers to electronic warfare systems such as radar and for radio astronomy.
The invention of the HEMT is usually attributed to Takashi Mimura (三村 高志) (Fujitsu, Japan). However, Ray Dingle and his co-workers in Bell Laboratories also played an important role in the invention of the HEMT.