We report on the characteristics of metal-semiconductor-metal (MSM) photodiodes based on GaN films grown on silicon substrates by electron cyclotron resonance (ECR) plasma-assisted metalorganic chemical vapor deposition (PA-MOCVD) at growth temperature of 200 and 600 degrees C. Structural analysis of the GaN samples used for the photodiodes fabrication were performed by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX) to analyze the crystalline quality of the samples. The analysis has revealed that the GaN samples grown at 200 and 600 degrees C were in amorphous and microcrystalline phase, respectively. Electrical characterization of the MSM photodiodes were carried out by using current-voltage (I-V) measurements. At 10 V, the photodiodes based on amorphous GaN has a dark current of 0.18 mu A while the microcrystalline GaN based photodiode has a dark current of 18 mu A. (c) 2004 Elsevier B.V. All rights reserved.