Well, the problem then is that you are changing the total number of electrons, structure of the crystal lattice, and overall electrical properties of the materi

Well, the problem then is that you are changing the total number of electrons, structure of the crystal lattice, and overall electrical properties of the material, to the point where it may not have any of the original useful properties. For example if you lightly dope silicon it modifies the semiconducting properties in useful ways, but if you go overboard with the doping it just becomes a regular conductor that isn't very special at all. All of these properties are inter-related, hence why it is so hard to find a recipe that balances them perfectly.