The electronic properties of Ag/GaAs(100) contacts, were modified using interlayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). The influence of the organic films on the electronic transport properties was investigated using current-voltage, (C-V) and capacitance-voltage (C-V) measurements. The in situ I-V curves reveal a strong dependence on the PTCDA interlayer thickness d(PTCDA). The effective barrier height varies between 0.81 and 0.64 eV by changing dPTCDA between 0 and 60 nm. For a layer thickness above 5 nm,space-charge limited currents in the organic layer strongly influence the carrier transport in the diodes. The C-V characteristics do hardly vary upon introducing an organic interlayer with dPTCDA below 30 nm. The capacitance of the organic film is larger than that of the depletion layer within the GaAs substrates, the latter one being unaffected by the PTCDA modification and dominating the C-V characteristics. Therefore, the change in the effective barrier height can be explained by an increasing image-force lowering in the presence of the organic interlayer. Exposure of Ag/PTCDA/GaAs(100) contacts with dPTCDA 30 nm to oxygen results in an increase in effective barrier height to a value comparable to the one of a bare Ag/GaAs(100) Schottky contact. This sensitivity to air is attributed to oxygen induced deep traps. This results in a shift of the Fermi level from its original position of 2 eV above the highest occupied molecular orbital towards the middle of the band gap of PTCDA. (C) 2003 American Vacuum Society. [DOI: 10.1116/1.11562636].