A 3D IC based mobile system including: a first semiconductor layer including first mono-crystallized transistors, where the first mono-crystallized transistors are interconnected by at least one metal layer including aluminum or copper; a second layer including second mono-crystallized transistors and overlaying the at least one metal layer, where the at least one metal layer is in-between the first semiconductor layer and the second layer; a plurality of thermal paths between the second mono-crystallized transistors and a heat removal apparatus, where at least one of the plurality of thermal paths includes a thermal contact adapted to conduct heat and not conduct electricity; and a heat spreader layer between the second layer and the at least one metal layer.