Pure silicon at room temperature has an electron number density in the conduction band of about 5 x 10¹⁵ m^–3 and an equal density of holes in the valence band. Suppose that one of every 10⁷ silicon atoms is replaced by a phosphorus atom. 

(a) Which type will the doped semiconductor be, n or p?

(b) What charge carrier number density will the phosphorus add? 

(c) What is the ratio of the charge carrier number density (electrons in the conduction band and holes in the valence band) in the doped silicon to that in pure silicon?