  We show that an insulated electrostatic gate can be used to strongly suppress
ubiquitous background charge noise in Schottky-gated GaAs/AlGaAs devices. Via a
2-D self-consistent simulation of the conduction band profile we show that this
observation can be explained by reduced leakage of electrons from the Schottky
gates into the semiconductor through the Schottky barrier, consistent with the
effect of "bias cooling". Upon noise reduction, the noise power spectrum
generally changes from Lorentzian to $1/f$ type. By comparing wafers with
different Al content, we exclude that DX centers play a dominant role in the
charge noise.
