In a semiconductor the law of mass action states that the product of the electron concentration and the hole concentration is always equal to the square of the intrinsic carrier concentration (at a given temperature), i.e.: [tex] n p = n_i^2 [/tex] My book states that this law is valid for extrinsic semiconductors (with impurities) as well as for intrinsic semiconductors. I dont understand how it can be valid for extrinsic semiconductors. In an intrinsic semiconductor charge neutrality requires n=p. I understand that the law is valid for intrinsic semiconductors. But if I start out with an intrinsic semiconductor and put in some electron donors, only [tex]n[/tex] will increase, [tex]p[/tex] and [tex]n_i[/tex] will not change. So how can it be valid in the extrinsic case?