The threshold voltage of "normally off" Si-MESFET's for simple dc-coupled circuits is defined and computed with a two-dimensional device model, taking into account carrier velocity saturation and diffusion. The influence of the main parameters, gate length, conducting-layer thickness, and doping is investigated outside the range of Shockley's equations. A threshold voltage of a MESFET with a 1-µm gate lengthU_{T} = 0.2V±10 percent requires a conducting-layer thickness toleranced =0.15µm±3 percent.