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It was recently found that ReSi 1.75 based semiconductor single crystals can be of either p or n type with a fixed composition, just depending on their different crystal orientations. To investigate the mechanism of this interesting phenomenon, we grow ReGe x Si 1.75−x ͑x = 0.02 and 0.04͒ single crystals with a floating zone method with radiation heating. The Seebeck coefficient and electric resistivity of these samples are measured along ͓100͔ and ͓001͔, respectively. The conduction mechanism is of p type along ͓100͔ and of n type along ͓001͔, like binary ReSi 1.75 , in the temperature range 50 to 800°C. The mobility ratio between electron and hole is calculated from the Seebeck coefficient data and it is highly anisotropic along two different orientations ͑about 0.4 to 0.6 along ͓100͔ while 4 to 5 along ͓001͔ direction͒, giving rise to the orientation-dependent conduction sign reversal phenomenon observed in ReSi 1.75 .

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