HEMT" redirects here. For the military truck, see Heavy Expanded Mobility Tactical Truck.
HEMT stands for High Electron Mobility Transistor, and is also called heterostructure FET
(HFET) or modulation-doped FET (MODFET). A HEMT is a field effect transistor incorporatin
g a junction between two materials with different band gaps (i.e., a heterojunction) as the channel i
nstead of a doped region, as is generally the case for MOSFETs. A commonly used material combi
nation is GaAs with AlGaAs, though there is wide variation, dependent on the application of the d
evice. Devices incorporating more indium generally show better high-frequency performance, whi
le in recent years, gallium nitride HEMTs have seen a massive increase in research effort, due to th
eir high-power performance.
Cross section of a GaAs/AlGaAs/InGaAs pHEMT
In general, to allow conduction, semiconductors need to be doped with impurities to generate mob
ile electrons in the layer. However, this causes electrons to slow down because they end up collidi
ng with the impurities which were used to generate them in the first place. HEMT, however, is a s
mart device to resolve this seemingly inherent unsolvable contradiction.
HEMT accomplishes this by 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 genera
ted in the n-type AlGaAs thin layer drop completely into the GaAs layer to form a depleted AlGaA
s 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 w
ithout colliding with any impurities because the GaAs layer is undoped, and from which they cann
ot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons wit
h very high concentration, giving the channel very low resistivity (or to put it another way, "high e
lectron 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 constan
t (spacing between the atoms). As an analogy, imagine pushing together two plastic combs with a s
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