To analyse electrostatic disorder in doped sub-100 nm Si-MOSFET channels we investigate the sub-threshold regime by transport spectroscopy at very low temperature. We measure resonances in the differential drain current and analyse their distribution in gate voltage, which is found to be gaussian. The mean value is interpreted as the electrostatic charging energy associated to a local well in the surface potential, whose dimensions is comparable to the channel length. The channel dopants concentration does not affect significantly this result. (C) 2004 Elsevier Ltd. All rights reserved.