Positron annihilation experiments which combine lifetime and doppler broadening measurements were performed using 10 MeV electron-irradiated Float-zone (Fz) and Czochralski silicon (Cz). In the case of irradiated float-zone Si, a lifetime of 305 ps is observed at 300 K decreasing from 290 ps at 30 K, and the positron trapping rate decreases strongly with increasing temperature. The Doppler measurements yield, when coupled with lifetime data, a S-value 6.7% larger than that for the bulk which is nearly twice the value hitherto claimed for divacancies. Isochronal annealing of the 1.8 $\mu$m infrared absorption band is accompanied by a significant change in the defect S-value to 3.8% larger than for the bulk. For the Cz-silicon, a set of rectangular wafers of n-type (P, Sb doped) and p-type (B doped) at various concentration levels and irradiated to a fluence of $\rm 1.2\times 10\sp{18}e\sp-/cm\sp2$ were investigated as a function of temperature and position dependence. The low dopant concentration samples of p-type or n-type present a dominance of negative divacancy defects, due to a lifetime of $\sim$300 ps, a strong temperature dependence of the trapping rate and a $\rm S\sb{D}/S\sb{B}$ value 1.07. For the middle concentration materials, we proposed that the formation of neutral $\rm PV\sb2,\ BV\sb2$ and SbV$\sb2$ type defects would explain the strong temperature dependence of the lifetime while maintaining constant trapping rate. In the lighly doped n- or p-type samples (both with $\rm 5\times 10\sp{18}/cm\sp3)$, the n-type (P-doped) shows a dominance of VP pairs, which are stable at room temperature. (Abstract shortened by UMI.)