The present invention provides a semiconductor device having a plurality of MOS transistors with controllable threshold values in the same face and easy to manufacture, a manufacturing method thereof and a display device. The invention is a semiconductor device having a plurality of MOS transistors in the same face each having a structure formed by stacking a semiconductor active layer, a gate insulator, and a gate electrode, wherein the semiconductor device includes: an insulating layer stacked on a side opposite to a gate electrode side of the semiconductor active layer; and a conductive electrode stacked on a side opposite to a semiconductor active layer side of the insulating layer and extending over at least two of the plurality of MOS transistors.