NOVEL FIN STRUCTURE OF FINFET

A fin structure disposed over a substrate and a method of forming a fin structure are disclosed. The fin structure includes a mesa, a channel disposed over the mesa, and a convex-shaped feature disposed between the channel and the mesa. The mesa has a first semiconductor material, and the channel has a second semiconductor material different from the first semiconductor material. The convex-shaped feature is stepped-shaped, stair-shaped, or ladder-shaped. The convex-shaped feature includes a first isolation feature disposed between the channel and the mesa, and a second isolation feature disposed between the channel and the first isolation feature. The first isolation feature is U-shaped, and the second isolation feature is rectangular-shaped. A portion of the second isolation feature is surrounded by the channel and another portion of the second isolation feature is surrounded by the first isolation feature.