A photo sensor includes a first substrate, a switching element and a second substrate. The switching element is disposed at the first substrate and defined by a control electrode, and first and second current electrodes. The switching element includes a channel disposed between the first and second current electrodes. The channel has a first length to receive an incident external light. The second substrate includes a light receiving unit that is disposed corresponding to the channel. The light receiving unit has a second length longer than the first length and shorter than a third length of the control electrode.