We analyze experimental data of Spinelli at al for conductivity for the set of n-type doped samples of SrTiO_3 within broad electron concentration N range 4 10^15- 4 10^20 cm^-3, at low temperatures. We obtain reasonably good fit of the data for N > 3 10^16 cm^-3  assuming that samples are strongly compensated. At smaller concentration N < 3 10^{16} cm^-3 Drude theory fails and conductivity collapses. We argue that this is the metal-insulator transition (MIT). We use Anderson-Ioffe-Regel and Efros-Shklovskii theories of MIT in the case of strongly compensated semiconductors and find that they predict N_c within factors 4 and 2 correspondingly, while Mott theory for uncompensated samples is 10^5 times off.  We study temperature dependence of lowest concentration sample with N=3.8 10^15 cm^-3 till temperature 0.4K and find that temperature dependence is weaker than expected for variable range hopping.  This may be caused by shunting effect of dislocations decorated by oxygen vacancies or the surface.

Y. Ayino, C. Leighton, B. I. Shklovskii, University of Minnesota