Methods for fabricating transistor structures are provided, the methods including: forming a fin structure with an upper fin portion and a lower fin portion, the upper fin portion including a sacrificial material; forming a gate structure over the fin; selectively removing the upper fin portion to form a tunnel between the gate structure and lower fin portion; and providing a channel material in the tunnel to define the channel region of the gate structure. The sacrificial material may be a material that can be selectively etched without etching the material of the lower fin portion. The channel material may further be provided to form source and drain regions of the transistor structure, which may result in a junctionless FinFET structure.