A semiconductor device includes: a first semiconductor region disposed over a second semiconductor region, wherein the first and second semiconductor regions have a first doping type and a second doping type, respectively; a first source/drain contact region and a second source/drain contact region having the second doping type and laterally spaced; and a gate electrode disposed laterally between the first and second source/drain contact regions, wherein the gate electrode comprises a first sidewall relatively closer to the first source/drain region and a second sidewall relatively closer to the second source/drain region, and wherein respective cross-sectional areas of the first and second sidewalls of the gate electrode are different from each other.