A method of spot welding a sheet overlapped to a tube with a servomotor-driven electrode applying a clamping force to the sheet. The upper side and lower side idlers bias upper side and lower side gate voltages by generating upper side and lower side bias currents so as to turn on the upper side and the lower side transistors at the same time in the crossover region between an input voltage and a reference voltage respectively. The first and second sourcedrain regions extend at least to a depth d1, wherein the depth d1 is measured from the top surface of the substrate.