A semiconductor device includes a stack of layers defining a sidewall surface and comprising source and drain layers. A channel structure extends through the stack of layers, is oriented in a vertical direction perpendicular to a main surface of the stack of layers, and is configured to have a current flow path in the vertical direction. The channel structure includes a two-dimensional (2D) semiconductor material. A core structure is positioned inside and surrounded by the channel structure, and a gate structure surrounds at least part of the channel structure.