  We propose a semi-suspended device structure and construct nanogapped,
hysteresis-free field-effect transistors (FETs), based on the van der Waals
stacking technique. The structure, which features a semi-suspended channel
above a submicron-long wedge-like nanogap, is fulfilled by transferring
ultraclean BN-supported MoS$_2$ channels directly onto dielectric-spaced
vertical source/drain stacks. Electronic characterization and analyses reveal a
high overall device quality, including ultraclean channel interfaces,
negligible electrical scanning hysteresis, and Ohmic contacts in the
structures. The unique hollow FET structure holds the potential for exploiting
reliable electronics, as well as nanofluid and pressure sensors.
