An organic/inorganic composite is provided. The organic/inorganic composite comprises a silicon (Si) substrate formed with nanorods or nanoholes and three-dimensional networks of carbon nanotubes (CNTs) grown horizontally in parallel and suspended between the adjacent nanorods or inside the nanoholes. In the organic/inorganic composite, metal catalysts can be uniformly formed on the nanorods or inside the nanoholes, irrespective of the height of the nanorods or the depth of the nanoholes and the shape and aspect ratio of the nanorods or nanoholes. In addition, the carbon nanotubes grow in a three-dimensional network structure directly over the entire surface of the nanorods or the whole inner surface of the nanoholes and are directly connected to the base electrodes. With this configuration, the three-dimensional carbon nanotube networks are highly dense per unit volume, and the organic/inorganic composite is highly electrically conductive and has a large surface area. Therefore, the use of the organic/inorganic composite enables the fabrication of an electronic device with greatly improved efficiency. Further provided are a method for preparing the organic/inorganic composite and an electronic device fabricated using the organic/inorganic composite.