Editing High-electron-mobility transistor (section)

=== Electrostatic mechanism ===
{{main|Heterojunction}}

Since GaAs has higher [[electron affinity]], free electrons in the AlGaAs layer are transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 100 [[ångström]] (10 [[nanometre|nm]]) of the interface. The n-type AlGaAs layer of the HEMT is depleted completely through two depletion mechanisms:
* Trapping of free electrons by surface states causes the surface depletion.
* Transfer of electrons into the undoped GaAs layer brings about the interface depletion.

The [[Fermi level]] of the gate metal is matched to the pinning point, which is 1.2 [[Electron volt|eV]] below the conduction band. With the reduced AlGaAs layer thickness, the electrons supplied by donors in the AlGaAs layer are insufficient to pin the layer. As a result, band bending is moving upward and the two-dimensional electrons gas does not appear. When a positive voltage greater than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.