= Solution
\b[Sulfur substituting for phosphorus acts as a <donor impurity>, giving <n-type semiconductor> behaviour and usually increasing <electrical conductivity>.] A P site normally supplies five <valence electrons>; S supplies six. After forming the surrounding bonds, the extra <electron> can occupy a donor level below the <conduction band> and be thermally ionized into that band. The <donor impurity> then carries positive charge.
This assignment is supported experimentally: infrared transmission measurements on S-doped <gallium phosphide> resolved bound-donor transitions and determined a sulfur donor optical ionization energy of about $0.104\,\mathrm{eV}$, far smaller than the host <band gap>.<Optical absorption due to excitation of electrons bound to Si and S in GaP by A. A. Onton>[1969, abstract: sulfur donor transitions and optical ionization energy]{ref}
The effect depends on substitution on P sites, donor ionization, compensation by acceptors and <charge carrier mobility>. A dilute active donor population increases the electron term in $\sigma=e(n\mu_n+p\mu_p)$; not every dopant must donate a mobile electron at room temperature. Sulfur's larger <Allen electronegativity> does not overturn this electron-counting argument or make a P-site sulfur atom an acceptor.
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