A gas sensor includes a first substrate, at least one first electrode, a sensing structure, at least one second electrode, and a second substrate. The at least one first electrode is located on the first substrate. The sensing structure is located on the at least one first electrode and the first substrate, and the sensing structure includes a first semiconductor layer and a second semiconductor layer. The first semiconductor layer having a first conductive type covers the first substrate and the at least one first electrode; the second semiconductor layer having a second conductive type is located on the first semiconductor layer. The at least one second electrode covers the sensing structure. The second substrate covers the at least one second electrode and the sensing structure.