Threshold and punchthrough behavior of laterally nonuniformally doped short-channel MOSFET´s

Predictions of gate threshold voltage and punchthrough voltage have been made for short-channel VDMOS and UMOS field-effect transistors using exact, two-dimensional numerical analysis. In these devices the doping concentration varies laterally from source to drain. The threshold voltage is found to be related to the maximum value of channel doping. This correspondence becomes poorer as the channel length is diminished since punchthrough current begins to influence the threshold voltage for short-channel devices. Surface punch-through is predicted for the VDMOSFET whereas bulk punchthrough is found in the UMOS device. A correspondence between the results of two-dimensional computer simulation of punchthrough and the estimations of one-dimensional simplified theory is found.