A semiconductor device may include a semiconductor substrate, first and second source/drain regions on a surface of the semiconductor substrate, and a channel region on the surface of the semiconductor substrate with the channel region between the first and second source/drain regions. An insulating layer pattern may be on the channel region, a first conductive layer pattern may be on the insulating layer, and a second conductive layer pattern may be on the first conductive layer pattern. The insulating layer pattern may be between the first conductive layer pattern and the channel region, and the first conductive layer pattern may include boron doped polysilicon with a surface portion having an accumulation of silicon boronide. The first conductive layer pattern may be between the second conductive layer pattern and the insulating layer pattern, and the second conductive layer pattern may include tungsten. Related methods are also discussed.