A semiconductor device, disposed on a substrate, includes a first channel layer, a patterned doped layer, a gate insulating layer, a conducting gate electrode, a second channel layer, a first electrode and a second electrode, and a third electrode and a fourth electrode. The first channel layer is disposed on the substrate and in a first region. The patterned doped layer includes a doped gate electrode disposed in a second region, and two contact electrodes electrically connected to two sides of the first channel layer, respectively. The conducting gate electrode is disposed on the gate insulating layer in the first region. The second channel layer is disposed on the gate insulating layer in the second region. The first electrode and the second electrode are electrically connected to the contact electrodes, respectively. The third electrode and the fourth electrode are electrically connected to two sides of the second channel layer, respectively.