= Solution
Substitutional <arsenic> has one more valence <electron> than <germanium>, so it acts as a <donor impurity>. Thermal ionization of its shallow bound state supplies mobile conduction-band <electrons> and leaves fixed positive donor charge. The resulting <n-type semiconductor> can have $n_e\gg n_i$ even when only a small fraction of host atoms are replaced. Its <electrical conductivity> then rises because the mobile carrier count increases; the opposite donor charge preserves macroscopic <charge neutrality>.
This argument assumes that the donors are substantially ionized at the measurement <temperature> and that the added impurity scattering does not overwhelm the carrier increase. The screened bound-donor picture and its departures from a simple hydrogenic model are described in institutional <semiconductor> spectroscopy notes.<Optical Properties of Solids by Mildred S. Dresselhaus>[Chapter 7, sections 7.2–7.3, printed pp. 76–79: hydrogenic impurities and limitations]{ref}
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