Compact Modeling of Body Effect for “Extrinsic” MOSFETs

One of the MOSFET compact modeling challenges is a correct account of the finite output resistance in saturation due to different short channel effects. Previously, we proposed a new “improved” smoothing function that ensures a monotonic increase in output resistance from the minimum value at the beginning of the triode regime to the maximum value at saturation. We used this smoothing function for compact modeling of an “ intrinsic” (with neglecting the contacts parasitic resistances) MOSFET. Later, we proposed a linear approximation for the drain current dependence on drain bias in the saturation regime for the “extrinsic” (taking into account contact parasitic resistances) MOSFET without taking into account body effect. This approximation is based on the proposed by us equations for the output resistance of the “extrinsic” MOSFET in the saturation regime (one for Level l and second for BSIM3 /4 models). Later, we generalized this equation on the case with accounting for the body effect, that was considered in the linear approximation. Note that modern transistors with steep retrograde body doping profiles exhibit approximately linear relationship between a threshold voltage and a source-to-body bias. In addition, we have shown how it is possible to convert a transistor with a body terminal to an equivalent transistor without a body terminal. In this paper, we use an “improved” smoothing function for compact modeling of the drain current of an “extrinsic” MOSFET operating in the above threshold regime that accounts for the body effect in the linear approximation. The resulting model yields a monotonic decrease in output conductance, which is important for improving the accuracy of MOSFET compact modeling in CAD software. Furthermore, we analyze in detail the relationship between the equation obtained for the output resistance of the “extrinsic” MOSFET in the saturation regime with the output resistance of a common-source amplifier with source degeneration. Note that in the theory of a common-source amplifier the circuit that consists of an NMOS transistor with a resistor in series with its source terminal is known as a transistor with source degeneration.