We study single‐carrier traps in a GaAs/AlxGa1−xAs heterostructure by observing random telegraph signals (RTSs), which are caused by the traps having energy levels within a fewkBTof the Fermi level. RTSs are observed in a split gate device while a narrow channel is shifted by independently controlling the voltage applied to each part of the split gate. This measurement reveals the variations of the energy levels of traps with the channel position. From these variations the locations and the energy distributions of the traps are demonstrated. The strength of the confinement potential around the trap is also discussed. ©1995 American Institute of Physics.