An intrinsic semiconductor is basically a pure semiconductor, though some might argue that a small amount of doping can still yield an intrinsic semiconductor. In the crystal structure of this material, there are very few electrons crossing the band gap into the conduction band, and this stuff doesn't want to conduct much current. But as temperature increases, more electron-hole pairs will appear as electrons jump that band gap and take up places in the conduction band. And if you guessed that increasing temperature will permit the intrinsic semiconductor to conduct current flow a bit better, you'd be right. The intrinsic semiconductor has a positive temperature coefficient. More heat, more conduction under the same conditions.