Comparison of CVT, SHIRAHATA and WATT Mobility Models

mos2ex14.in : Comparison of CVT, SHIRAHATA and WATT Mobility Models

Requires: S-Pisces
Minimum Versions: Atlas 5.22.1.R

THe example demonstrates a comparison between three different surface mobility models used in MOSFETs. THe example consists of four Atlas runs that:

  • Define optimum MOS mesh using a variable for surface mesh spacing
  • Select in turn one of three MOS Mobility Models
  • Run a Id/Vgs simulation at low Vds
  • Overlay of Id/Vgs curves and mobility in Tonyplot

The mesh generation and structure deinfition part of this input file follows the standard syntax outlined in the Getting Started Chapter of the Atlas manual. A variable surf_a is used to set the mesh spacing at the silicon surface. This mesh spacing is important in resolving the high electric field and carrier concentration gradients at the surface. The sensitivity of the mobility models to mesh variations can also be easily examined by re-running this example with different settings of surf_a.

The three models compared in this example are CVT, Shirahata and Modified Watt . The CVT model is selectred simply with the syntax models cvt since it contains doping and parallel field dependence. The Shirahata model contains only a perpendicular field dependence. It must be combined with Klaassen's model and the standard field dependent mobility model using models kla shi fldmob .

The modified Watt model also needs to be combined with other models. The syntax is:
models conmob fldmob
mobility watt.n mod.watt.n

Extra parameters are also required to determine the range of the modified Watt model. These parameters are ymaxn.watt that specifies the depth of the Watt model and min.surf that specifies the lateral extent by restricting the model to minority regions. In this case this means the channel and not the LDD or source and drain regions.

The Id/Vgs simulation is similar to that in the previous section. For the initial drain biasing, the syntax method gummel newton is used. This provides better convergence at low biases. The simulation reverts to method newton once the drain is biased. Note that in this case only electrons are needed. The syntax method carriers=1 electrons is used to specify that only electron continuity and poisson equations are to be solved.

The probe statement is used to save the surface mobility and perpendicular field at the center of the channel. Values for both these parameters are saved to the log file. TonyPlot can be used to display graphs of mobility vs. field. Note that the probe statement reports the exact mobility used in Atlas calculations. The value is calculated at the centre of the side of each triangle. The nearest such point to the location of the probe is used. Values of mobility or field from the probe might differ from values saved in the solution file . The solution file saves the average value of such quantities at a node point. With coarse or bad meshes, this may result in inaccurate plots.

To load and run this example, select the Load example button in DeckBuild. This will copy the input file and any support files to your current working directory. Select the run button to execute the example.