Modulation doping places ionized dopants away from the region in which their supplied charge carriers move. In a semiconductor heterostructure, band alignment can transfer electrons from a doped layer into a nearby lower-energy channel.
This separation reduces scattering from nearby charged impurities while maintaining a substantial mobile-carrier population. It supports a two-dimensional electron gas and the channel of a high-electron-mobility transistor. Separation is not complete decoupling: the donors still set electrostatics, and remote impurities, interfaces and other scattering mechanisms can still limit charge carrier mobility.
A two-dimensional electron gas supplied by remote donors in aluminium gallium arsenide and confined on the gallium arsenide side of a heterojunction. An undoped spacer separates ionized donors from mobile carriers, reducing impurity scattering. A conduction-band band offset and self-consistent band bending create the confinement well.
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