Editing High-electron-mobility transistor (section)

=== By electrical behaviour: eHEMT and dHEMT ===
HEMTs made of semiconductor hetero-interfaces lacking interfacial net polarization charge, such as AlGaAs/GaAs, require positive gate voltage or appropriate donor-doping in the AlGaAs barrier to attract the electrons towards the gate, which forms the 2D electron gas and enables conduction of electron currents. This behaviour is similar to that of commonly used field-effect transistors in the enhancement mode, and such a device is called enhancement HEMT, or '''eHEMT'''.

When a HEMT is built from [[AlGaN]]/[[GaN]], higher power density and breakdown voltage can be achieved. Nitrides also have different crystal structure with lower symmetry, namely the [[wurtzite crystal structure|wurtzite]] one, which has built-in electrical polarisation. Since this polarization differs between the [[GaN]] ''channel'' layer and [[AlGaN]] ''barrier'' layer, a sheet of uncompensated charge in the order of 0.01-0.03 C/m<math>^2</math> is formed. Due to the crystal orientation typically used for epitaxial growth ("gallium-faced") and the device geometry favorable for fabrication (gate on top), this charge sheet is positive, causing the 2D electron gas to be formed even if there is no doping. Such a transistor is normally on, and will turn off only if the gate is negatively biased - thus this kind of HEMT is known as ''depletion HEMT'', or '''dHEMT'''. By sufficient doping of the barrier with acceptors (e.g. [[magnesium|Mg]]), the built-in charge can be compensated to restore the more customary '''eHEMT''' operation, however high-density p-doping of nitrides is technologically challenging due to dopant diffusion into the channel.