Heterostructure devices using three different organic materials, namely PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), DiMe-PTCDI (dimethyl-3,4,9,10-perylene tetracarboxylic diimide) and CuPc (copper phthalocyanine) evaporated on GaAs substrate have been prepared. To accomplish the diode structure a silver metalisation on top of the organic layer (OL) and an ohmic contact to GaAs were fabricated. The lateral geometry of the samples is convenient to contact with a microwave probe in ground-signal-ground configuration for characterization directly on the wafer. Four groups of diodes with different active area diameters 10, 20, 40 and 80 mum have been prepared, respectively. Their characteristics in dependence on the device geometry have been determined and compared in a large frequency range (up to 2 GHz). Reflection parameters have been evaluated in dependence on the bias current. The data have been used for non-linear modeling of the diode equivalent circuit. We have measured frequency conversion ability of the devices in a single diode mixer scheme and discuss the suitability of the devices for microwave applications. The active role, played by the organic material and the organic-inorganic interface is considered in the explanation of the experimental results. (C) 2004 Published by Elsevier B.V.