Modelling of the dynamic threshold MOSFET

Despite the inappropriateness of the depletion approximation when the source-substrate junction is slightly forward biased, conventional SPICE models are being used to simulate digital integrated circuits implemented with dynamic threshold voltage MOSFETs (DTMOS). Based on PISCES simulations and experimental data, the correct modelling of DTMOS is discussed in this work. It is found that the dependence of the threshold voltage on the source-substrate forward bias can differ importantly from the conventional model at voltages above ∼0.4 V for long channel technologies, but it could agree with conventional models for voltages as high as ∼0.5 V for deep submicron technologies. This is explained in terms of the difference in substrate doping levels between such technologies, which is at least one order of magnitude. Mobility and transverse electric field always improve in the dynamic operation. However, the conventional models used by SPICE to calculate these two parameters, which are based on the depletion approximation, lead to important miscalculations. This is proven using the PISCES numerical solution of the Poisson equation considering the presence of mobile charge in the depletion region. With these results, some guidelines for the correct modelling of this device are generated.