= Modulation doping
<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>.
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