A semiconductor structure includes a HV NMOS structure. The HV NMOS structure includes a source region, a drain region, a channel region, a gate dielectric, and a gate electrode. The source region and the drain region are separated from each other. The channel region is disposed between the source region and the drain region. The channel region has a channel direction from the source region toward the drain region. The gate dielectric is disposed on the channel region and on portions of the source region and the drain region. The gate electrode is disposed on the gate dielectric. The gate electrode includes a first portion of n-type doping and two second portions of p-type doping. The two second portions are disposed at two sides of the first portion. The two second portions have an extending direction perpendicular to the channel direction.