Planar diffusion process is the process used for production of ICs or discrete devices. Since all the fabrication steps are performed at the surface of Silicon crystal in a single phase, this is so called Planar Diffusion Process.
In the planar diffusion fabrication process, diffusion takes place in an electronic furnance whose atmosphere can be very accurately controlled as required, such a furnance is called a diffusion furnance.
Although Germanium (Ge) and Gallium-arsenide (GaAs) are also used to make semiconductor devices, Si is still the most popular material.
Silicon (Si) is much more suitable for fabricating active devices because of its good electrical characteristics and physical properties. Plus Si is used to form excellent insulation layer (ie SiO2 glass)- eventually to construct JFETs, MOSFETs and CCDs.