1. A semiconductor structure comprising: a gate dielectric located on a semiconductor layer and comprising a first gate dielectric portion and a second gate dielectric abutting said first gate dielectric portion, wherein said first gate dielectric portion has a first horizontal top surface and a first horizontal bottom surface separated by a constant distance of a first thickness, said second gate dielectric portion has a second horizontal top surface and a second horizontal bottom surface separated by another constant distance of a second thickness, and said first thickness is greater than said second thickness; and a gate electrode vertically abutting said first horizontal top surface of said first gate dielectric portion and said second horizontal top surface of said second gate dielectric portion.