  Background charge fluctuators are studied individually by means of a modified
single-electron pump. Operation of the device in a feedback mode allows
electrometric sensing of the charged background and its behavior upon electric
potential variations due to geometrically different gates. Pulse height spectra
and hysteresis of charge trapping transitions are discussed as a specific
signature of distinct fluctuators. The location of individual traps is
determined from the experimental data and based on electrostatic calculations.
