A semiconductor device includes a semiconductor layer including first and second regions and a third region therebetween, a gate insulating layer between the third region and a gate electrode, first and second electrodes connected to the first and second regions in a first direction, a first conductive layer between the first region and the first electrode and/or between the second region and the second electrode. The first conductive layer includes a metal element, aluminum, and nitrogen, and has first and second portions. An atomic concentration of the metal element is higher than that of aluminum in the first portion. An atomic concentration of aluminum is higher than that of the metal element in the second portion. The device further includes a second conductive layer between the oxide semiconductor layer and the first conductive layer. The second conductive layer includes oxygen and at least one of indium, zinc, tin, and cadmium.