A model for the slow, large signal SPV-transient is developed which accounts for carrier trapping to surface states. Both majority and minority carrier processes are included. SPV-transients are measured from various epiready n(+)-GaAs wafers using Kelvin probe. Typically the transients take place in 1-1000 s time scale. By fitting the model with experimental results trapping parameters, i.e. surface potential barrier height, energy and density of surface states and electron and hole capture cross-sections are determined. (c) 2008 Elsevier B. V. All rights reserved.