5.1 Device Modeling

Circuits operating at less than 1 V will probably have to operate in the weak and moderate inversion. Accurate MOSFET models in this regime will be indispensable for reliable circuit simulation. Many specialized models (e.g. BSIM [44], EKV [45] and PCIM [46]) have been developed for keeping up with these new demands, but the solutions led to complex models which require complex and time-consuming parameter extractions [47]. In fact, many parameters in BSIM and EKV models are fit parameters, and the models become ``unphysical''. Another aspect to have in consideration, is the performance of models in terms of computation efficiency. Very accurate models leading to long circuit simulation times are of little interest. This is aspecially true when performing transient analyses where simulation times can be very long.

A good compromise between accuracy and computational efficiency is given by table-based device models. The best results are obtained with physically motivated interpolation, such as piecewise polynomial and/or exponential splines, of the terminal currents and charges [48]. An advantage of this approach is that it allows a simple interface with experimental data or simulation results. These simulations can be performed accurately by a device simulator (available in TCAD frameworks).

A method using simple transient simulations with the VISTA's MINIMOS [49] device simulator suitable for the creation of the model data table is presented in [50]. Basically, it consists of the creation of a table with terminal currents and charges for a large set of biasing conditions (the bulk, gate and drain voltages - $v_b$, $v_g$ and $v_d$). This data is then supplied to MINISIM [51], a circuit simulator which uses a charge conservative model and piecewise polynomial and/or exponential splines interpolation according to the axis: Piecewise polynomial in the $v_b$ and $v_d$, or exponential in the $v_g$ direction.