A semiconductor device having a semiconductor substrate including a first region and a second region is provided. The semiconductor device further includes a gate electrode on the first region and having a first sidewall and a second sidewall, a first source region in the first region proximate to the first sidewall, a first drain region in the first region proximate to the second sidewall, an upper electrode on the second region and having a first sidewall and a second sidewall, a second source region in the second region proximate to the first sidewall of the upper electrode, and a second drain region in the second region proximate to the second sidewall of the upper electrode, wherein an impurity doping concentration of the first source region and the first drain region is greater than an impurity doping concentration of the second source region and the second drain region.