Аннотация:The wave-number- and frequency-dependent dielectric function of a semiconductor is derived and calculated in terms of a model consisting of an electron gas with an energy gap. From it are deduced, as a function of the gap width, ($i$) the screening of a point defect, ($\mathrm{ii}$) the annihilation rate of positrons, and ($\mathrm{iii}$) the stopping power for swift charged particles. A partition rule holds between the contributions of single-particle excitations ${L}_{s}$ and collective resonance excitations ${L}_{r}$ to the stopping number $L={L}_{s}+{L}_{r}$ in the sense that ${L}_{s}=C+{L}_{r}$; the constant $C$ grows with the gap width.
