SEMICONDUCTOR DEVICE

A semiconductor device includes a substrate, a bit line that extends in a first horizontal direction on the substrate, a first mold layer on the bit line, wherein the first mold layer defines a mold opening that exposes a portion of an upper surface of the bit line and extends in a second horizontal direction that intersects the first horizontal direction, a channel layer on the bit line, one or more word lines on sidewalls of the channel layer and that extend in the second horizontal direction, and a gate insulating layer between the word line and the channel layer, where the channel layer includes a first oxide semiconductor layer, a second oxide semiconductor layer, and an auxiliary channel layer between the first oxide semiconductor layer and the second oxide semiconductor layer.