= Solution
<Arsenic> has five valence <electrons>, one more than <silicon>. As a substitutional <donor impurity>, it supplies a weakly bound extra <electron> which is largely thermally released into the <conduction band> at room <temperature>, leaving an ionized positive donor.
Consequently \b[the mobile <electron> density increases far above the intrinsic value], so the conductivity increases substantially even at ppm dopant levels. The crystal remains macroscopically neutral: fixed donor charge compensates the added mobile charge.
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