Isolation features for semiconductor devices and methods of fabricating the same

A method, comprising:

• receiving a workpiece comprising a vertical stack of alternating first semiconductor layers and second semiconductor layers over a substrate;
• patterning the vertical stack and a portion of the substrate to form a first fin-shaped structure and a second fin-shaped structure, the first fin-shaped structure comprising a first portion of the vertical stack and a first mesa structure directly under the first portion of the vertical stack, the second fin-shaped structure comprising a second portion of the vertical stack and a second mesa structure directly under the second portion of the vertical stack;
• depositing a dielectric layer over the workpiece to fill a trench between the first fin-shaped structure and the second fin-shaped structure;
• recessing a first portion of the dielectric layer to form a first isolation feature surrounding a bottom portion of the first fin-shaped structure;
• and
• recessing a second portion of the dielectric layer to form a second isolation feature surrounding a bottom portion of the second fin-shaped structure,
• wherein a height of the second isolation feature is greater than a height of the first isolation feature.

A method, comprising:

• forming a first active region including a first base region protruded from a substrate and a first plurality of channel layers over the first base region;
• forming a second active region including a second base region protruded from the substrate and a second plurality of channel layers over the second base region;
• and
• forming an isolation feature disposed between the first base region and the second base region, wherein the isolation feature comprises a first portion interfacing a lower portion of a sidewall of the first base region and a second portion interfacing a sidewall of the second base region, wherein a top surface of the first portion of the isolation feature is below a top surface of the second portion of the isolation feature;
• and
• forming a gate structure over the first active region and the second active region and extending on the isolation feature, wherein the gate structure further extends along an upper portion of the sidewall of the first base region.