Characterization of nonlinear substrate resistance in short-channel MOSFETs

The substrate current conduction in the short-channel MOSFET is described as a virtual JFET coupled with the intrinsic MOSFET. A method to extract the I-V characteristics of the substrate-JFET from I-V measurements on the MOSFET, and pertinent results for a few varied MOSFET structures are presented. The JFET model and characteristics appear valid for the structures investigated, and reveal the fact that modulation by the depletion regions of the source and drain junctions is a major factor in the nonlinearity of the substrate resistance of actual MOSFETs. The nonlinearity of the substrate resistance needs to be considered for an accurate description of the drain-voltage induced limitation of a standard MOSFET. Also, the pinch-off of the substrate-JFET offers a physical explanation to the "early" turn on, a type of drain-voltage limitation present in the short-channel MOSFET when the depletion regions of the source and drain merge in the depth of the channel.