Forming a semiconductor device includes forming a first conductive line on a substrate, forming a memory cell including a switching device and a data storage element on the first conductive line, and forming a second conductive line on the memory cell. Forming the switching device includes forming a first semiconductor layer, forming a first doped region by injecting a n-type impurity into the first semiconductor layer, forming a second semiconductor layer thicker than the first semiconductor layer, on the first semiconductor layer having the first doped region, forming a second doped region by injecting a p-type impurity into an upper region of the second semiconductor layer, and forming a P-N diode by performing a heat treatment process to diffuse the n-type impurity and the p-type impurity in the first doped region and the second doped region to form a P-N junction of the P-N diode in the second semiconductor layer.