The high - electron -mobility transistor (HEMT, dt " high electron mobility transistor " ) is a special design of the field effect transistor for very high frequencies and is so constructed that a special design of a JFET.
Other names for this Transitortyp are modulation - doped field - effect transistor ( MODFET ), two- dimensional electron gas field - effect transistor ( TEGFET ), Selectively - doped heterojunction transistor ( SDHT ) and heterojunction field - effect transistor ( HFET ).
Design and operation
It consists of layers of different semiconductor materials having different band gaps large (see heterostructure ). Frequently, the aluminum gallium arsenide materials system / gallium arsenide ( AlGaAs / GaAs ) is used, the high n- doped AlGaAs and GaAs is undoped. As the band gap of the AlGaAs is greater than that of GaAs, is formed at the interface of these two materials on the side of the GaAs, a two dimensional electron gas (2DEG ) of which can serve as the conductive channel. The electron mobility is very high.
They can be further improved by modulation doping of the AlGaAs as it was first proposed, among others, by Horst Stormer. Thereby, the scattering of the electrons in the 2DEG drops charged impurities and thus leads to a further increase of charge carrier mobility which condition the discovery of the fractional quantum Hall effect was ( Nobel Prize in physics in 1998 ).
The HEMT principle is also applicable to other material systems such as InGaAs / InP / AlInAs, AlGaN / GaN, AlInN / GaN, and Si / SiGe.
Subject of current research are combinations of materials of gallium nitride (GaN ) and aluminum gallium nitride ( AlGaN ) or Aluminiumindiumnitrid ( AlInN ), which allows a higher operating voltage due to its relatively high band gap, before it comes to the field breakdown. Particularly for the production of power transistors, this combination of materials is found to be advantageous, since the output impedance increases at the same power and thus the coupling of the power is simplified ( adjustment). On silicon carbide (SiC ) was deposited, it additionally has a lower thermal resistance than GaAs material combinations with positive effects on the maximum power dissipation and durability and reliability.
The HEMT is suitable because of the high carrier mobility for high frequency applications. The control of the device, similar to the metal oxide semiconductor field effect transistor, a metal - gate is connected to the n-AlGaAs layer.