I have read this paragraph: [...] "When an electrical field is applied to a metal, a potential drop associated with that field must take place outside the metal, usually at the metal contacts: any change in potential inside the metal is prevented by the mobility and number density of the carriers. This can be understood from Poisson equation" (Hamnett and Christensen) I have some question which need your help: 1. Why potential drop must "take place outside" the metal ? 2. Why "mobility and density of carriers" prevent such thing to happen? 3. Poisson equation is the second derivative of the potential in the x direction = -p (epsilon*const) where p = density of charge carrier and epsilon = dielectric function of the semiconductor. What are the relation between this equation the above paragraph? Thank you very much!