A vertically protruding structure is formed. The vertically protruding structure includes a substrate, a first semiconductor layer disposed over the substrate, a channel layer disposed over the first semiconductor layer, and a second semiconductor layer disposed over the channel layer. The first semiconductor layer and the second semiconductor layer each contain a first type of semiconductive material. The channel layer contains a second type of semiconductive material different from the first type. First recesses are formed in the first semiconductor layer and the second semiconductor layer. Each of the first recesses protrudes laterally inward. The first recesses are filled with dielectric spacers. The channel layer and the substrate are laterally trimmed. The remaining portions of the channel layer and the dielectric spacers define second recesses that protrude laterally inward. Gate structures are formed in the second recesses.