If a material is doped (introducing positive dopants to the lattice) there will be a build up of negative cahrge around them, screening. I have a couple of questions that seem obvious but I'm still a little unsure: 1) how would more screening effect resistivity? I would assume that a build up of negative charge would hinder conduction but the impression i'm getting from the notes is that it increases conductivity?? 2) (possibly unrelated to screening) if say the dopant has smaller atoms and introduced defects into the lattice please could someone tell me how that would effect the phonons? if that makes sense? how would a lattice deformation effect resistivity / conductivity. I have all the formulas for calculations but i'm trying to visualise these effects a little more. Any help would really be welcome.