In electronic devices we mainly use 3 types of materials for manufacturing purposes.
Conducting materials are those materials in which electrons of the outermost shell are weakly bonded with nucleus. Hence as force of attraction between nucleus and the outermost shell electrons is weak, the outermost electrons become free and roam in the substrate of the material. These are mostly those materials which have 1 or 2 or 3 electrons in the outer shell. E.g. Al, Mg, Cu
Insulating materials are those materials whose outermost electrons are tightly bonded with the nucleus and hence at room temperature they don’t get free. In this type of material, we usually have more than 4 electrons in the outer most shell.
The bond strength between the nucleus and the outermost shell electrons increases with increase in number of electrons in the outermost shell. Hence bond strength between nucleus and outermost electron of atom with 1-outermost electron less than atom with 2-outermost electron and so on.
The order of bond strength between nucleus and outmost electron of an atom with number of electrons in the outer shell vary as shown:
Semiconductor materials are those which lie in between conductors and insulators. These types of materials have usually 4 electrons in the outer shell. Hence semiconducting materials are called tetravalent. E.g. Si, Ge etc.
Although we have such type of materials also which act are semiconductor materials but are not tetravalent. These materials are made by the combination of trivalent and pentavalent materials. Some of materials of the above type are GaAs, GaAsP etc.