Effects of time-accumulated ultraviolet (UV) irradiation and surface treatment on thermally oxidized p-type silicon wafers were investigated by using the surface photo voltage (SPV) technique via the direct measurement of the total surface charge, Q(SC). The rise and fall times of Q(SC) curves, as a function of accumulated UV irradiation, depended on the thermal oxide thickness. A simple model was proposed to explain the time-varying characteristics of Q(SC) based on the UV-induced bond breaking of SiOH and SiH, and photoemission of bulk electrons to wafer surface where O-2 charges were formed. While these mechanisms resulted in charge variations and hence in Q(SC), these could be removed by rinsing the silicon wafers in de-ionized water followed by spin-dry or blow-dry by an ionizer fan. Empirical parameters were used in the model simulations and curve-fitting of QSC. The simulated results suggested that initial changes in the characteristic behaviour of Q(SC) were mainly due to the net changes in the positive and negative charges, but subsequently were dominated by the accumulation of O-2 during the UV irradiation. (C) 2009 Elsevier B.V. All rights reserved.