This page updated 30 November 2008

MOSCALC Extension Module: MOSFET Simulation

1 General Description

The purpose of the MOSCALC extension module is to provide prototype MOSFET simulation compatible with the fast execution time and ease of use of BIPOLE3. It should be noted that Bipole3 was developed for BJT and HBT simulation and has been extensively tested and calibrated in industry and universities on these bipolar semiconductor devices. This has not yet been done with MOSCALC for MOSFET structures. The simulation is for a self aligned gate MOSFET structure and is based on vertical numerical integration of Poisson's equation including free carrier charge and threshold adjustment implants coupled to horizontal numerical integration of the majority carrier drift equation in the conducting channel. Poisson's equation in polar coordinate solution is used for the drain-substrate space charge region. Mobility dependence on both vertical and horizontal electric fields in included.

The output consists of tables for a given Vds including Ids, gm, source-drain transit time tds and figure of merit Cox/gm.

BIPOLE3 graphs are provided for the following quantities:


2 Outline of numerical method used in Bipole3 MOSFETsimulation

Poisson's equation is solved numerically in the plane and sidewall regions for the diffused source and drain junctions neglecting free carriers in order to determine the zero bias junction capacitances.

For a specified source-drain current IDS, the equation for horizontal (y direction) charge-current potential is solved numerically for Qe(y):

IDS = µe(Ex,Ey) Z Ey(y) Qe(y)

The mobility is a function of doping level, horizontal and vertical electric fields. The limit condition on Qe(y) as the drain space charge region is approached is given by the saturated velocity condition. Beyond this point Poisson's equation is solved numerically using the drain doping profile.

The surface charge is related to surface potential by solving Poisson's equation numerically at each value of horizontal distance x' integrating from the surface to the substrate:

dEx/dx = [(qN(x)/e ][1 + exp([V - 2f F]/Vt)]

where:

Qs = - eoxEox

Qs = -(Qi + QB)

and where QB is obtained from vertical numerical integration.

The above equations are solved numerically for a given IDS and VGS, with VDS being the resultant potential.

3. Physical models used in MOSFET simulation

The physical models for bulk properties are those used for devices simulated for BJT structures and all the model parameter values are user accessible. In addition, physical models for inversion layer mobility due to surface scattering, and field dependent mobility in the inversion layer are special to MOSCALC.

4. MOSCALC output examples

The Bipole3 Tutorial Guide available from this web site contain many examples of MOSFET simulation results. These include graphs for both internal results (electric field and velocity, carrier concentration, band diagrams, inversion layer charge versus bias) and terminal characteristics (Ids versus Vgs, Ids versus Vds for various Vgs, hf and lf capacitance versus bias).