Ionization of donor impurities in semiconductors. A pentavalent impurity (called a donor) introduced in place of a tetravalent silicon atom in crystalline silicon acts like a hydrogen atom in free space, but with e2 /e playing the role of e2 and an effective mass m* playing the role of the electron mass m in the description of the ionization energy and radius of the ground state of the impurity atom, and also for the free electron. For silicon the dielectric constant € = 11.7 and, approximately, m* = 0.3 m. If there are 1017 donors per cm", estimate the concentration of conduction electrons at 100 K.