Most solar collectors are fixed in their array position mounting, but can have a higher performance if they track the path of the sun through the sky (however it is unusual for thermal collectors to be mounted in this way). Solar trackers, used to orient solar arrays may be driven by either passive or active technology, and can have a significant gain in energy yield over the course of a year when compared to a fixed array. Again passive solar tracking would rely on the inherent thermo-dynamic properties of the materials used in the system rather than an external power source to generate its tracking movement.