Present disclosure provides a FinFET structure, including a substrate, a fin protruding from the substrate, including a first portion and a second portion below the first portion, wherein the first portion includes a first dopant concentration of a dopant, and the second portion includes a second dopant concentration of the dopant, the second dopant concentration is greater than the first dopant concentration, a gate over the fin, wherein the second portion of the fin is below a bottom surface of the gate, and an insulating layer over the substrate and proximal to the second portion of the fin, wherein at least a first portion of the insulating layer includes a third dopant concentration of the dopant, the third dopant concentration is greater than the first dopant concentration.