Semiconductor device having buried gate structure and method for fabricating the same

A semiconductor device, comprising:

• a gate trench extended into a semiconductor substrate and including a bottom channel and a side channel;
• a first work function layer covering a lowest portion of the gate trench to overlap with the bottom channel;
• a second work function layer that continues from the first work function layer and overlaps with the side channel;
• a gate conductive layer covering the first work function layer and the second work function layer and filling the gate trench;
• and
• doped regions isolated from each other by the gate trench and contacting with an upper portion of the side channel,
• wherein the second work function layer has a work function lower than the first work function layer.