本揭露提供一种鳍式场效应管组件结构(FinFET device structure)，包括：一鳍式结构，形成于一基板之上；以及一闸极结构，横跨于该鳍式结构之上，其中该闸极结构包括一闸极电极层，其中该闸极电极层包括一上部份高于该鳍式结构与一下部份低于该鳍式结构，该上部份具有一顶表面，该下部份具有一底表面，该顶表面具有一第一宽度，该底表面具有一第二宽度，且该第一宽度大于该第二宽度。

A FinFET device structure and method for forming the same are provided. The fin field effect transistor (FinFET) device structure includes a fin structure formed over a substrate and a gate structure traversing over the fin structure. The gate structure includes a gate electrode layer which includes an upper portion above the fin structure and a lower portion below the fin structure. The upper portion has a top surface with a first width, the lower portion has a bottom surface with a second width, and the first width is greater than the second width.