We report on high-resolution electronic measurements of doped organic thin-film transistors using Kelvin probe force microscopy. Measurements conducted on field effect transistors made of N,N-I-diphenyl- N, N-I-bis(1-naphthyl)-1,1(I)-biphenyl-4,4(I)-diamine p-doped with tetrafluoro-tetracyanoquinodimethane have allowed us to determine the rich structure of the doping-induced density of states. In addition, the doping process changes only slightly the Fermi energy position with respect to the highest occupied molecular orbital level center. The moderate change is explained by two counter-acting effects on the Fermi energy position: the doping-induced additional charge and the broadening of the density of states.