A semiconductor device of a circuit is provided. The circuit is configured to be operated under a power supply. The semiconductor device of the circuit includes a first transistor and a second transistor. The first transistor includes a first source region in a first bulk region; a first drain region defined by a well and a doped region, wherein the first source region and the doped region are separate by a distance, which is a factor which determines a breakdown voltage of the first transistor, the breakdown voltage being associated with the power supply; and a first gate. The second transistor includes a second source region in a second bulk region, the second source region electrically connected with the first source region and the first gate.