I am struggling to understand this. In the intrinsic semiconductor magnesium, the Hall coefficient is positive (Table 3.1 in the textbook). However, we know that for every hole in the valence band, there is a free electron in the conduction band. Won't the current from these electrons in an applied electric field be the same as in metals? Is the current from holes in the opposite direction and bigger? I feel like this is the subject of section 17.1.1 on Drude transport, but I don't quite understand it.