Abstract: Improved micro fuel cells suitable for portable electrical devices are provided, and processes for forming such cells. In one embodiment of the invention, silicon substrates are used both as the gas delivery structure for the fuel and the oxidant, and as the current collectors. Such use of silicon is advantageous in that it becomes possible both to utilize micromachining and lithographic techniques to form the desired structures, e.g., the gas delivery channels, and also to integrate the fuel cell with silicon-based control circuitry. Advantageously, the silicon substrates comprise both gas delivery tunnels and porous silicon gas diffusion regions formed over the tunnels in the surface of the substrate, i.e., the porous regions over the gas delivery tunnels are integral with the silicon substrate. In another embodiment of the invention, a monolithic structure is employed.