Impurity effects on Schottky barriers have been investigated using a concentrated Ti-W alloy for barrier-height measurements on both n-type and p-type Si(100). The measurements were performed in the temperature range 175 295 K using forward current-voltage characteristics. Silicide-formation reaction was monitored by Rutherford backscattering spectroscopy, Auger-electron spectroscopy, and glancing-angle x-ray diffraction. The results showed that Ti dominates the barrier height, and that W does not play any role in barrier formation. The interdiffusion of Ti and Si dominated the reaction between the alloy and Si at the interface. For both the metal and the reacted silicide phase the n- and p-type barrier heights were found to decrease with increasing temperature with the sum equal, within the experimental accuracy, to the indirect energy gap of Si in the range of temperature measured. These results indicate that the temperature dependence of the barrier heights is mainly due to that of the indirect energy gap in Si.