Electrical transport properties of CaB$_6$*

We report results from a systematic electron-transport study in a broad temperature range on twelve CaB$_6$ single crystals. None of the crystals were intentionally doped. The different carrier densities observed presumably arise from slight variations in the Ca:B stoichiometry. In these crystals, the variation of the electrical resistivity and of the Hall effect with temperature can be consistently explained by a variable charge state of intrinsic defects, most likely B-antisites (B atom replacing Ca atom). Our model is also consistent with the presence of a narrow, defect related, impurity band close to the Fermi level. Thus it may indicate the validity of defect-driven intrinsic ferromagnetism in alkaline-earth hexaborides. The magnetotransport measurements reveal that most of the samples we have studied are close to a metal-insulator transition at low temperatures. The magnetoresistance changes smoothly from negative---for weakly metallic samples---to positive values---for samples in a localized regime.