Comparison of Id/Vds using EB and NEB models

mos2ex05.in : Comparison of Id/Vds using EB and NEB models

Requires: SSuprem 4/S-Pisces/Giga
Minimum Versions: Athena 5.22.1.R, Atlas 5.22.1.R

This example demonstrates fabrication and Id/Vds analysis of a short channel structure with Energy Balance and Non-isothermal Energy Balance Models. The example shows:

  • Creation of a short channel MOSFET in Athena
  • Regridding in DEVEDIT
  • Isothermal Id/Vds analysis in Atlas
  • Non-isothermal Id/Vds analysis in Atlas

Deep submicron devices should be simulated using the Energy Balance Model due to velocity overshoot and nonlocal impact ionization effects, which could substantially influence device characteristics. For high current levels (high gate voltages) the thermal self-heating effects can also play an important role by decreasing mobility and the impact ionization rate. This example demonstrates a comparison of Id/Vds curves obtained between the Energy Balance and Nonisothermal Energy Balance Models.

The structure is formed using a standard LDD NMOS process and regridding is performed in DEVEDIT to refine the mesh in the areas where high electric fields will occur in Atlas. This procedure is described under the Substrate and Gate current example in the MOS example section.

There are two Atlas runs in this example file. The first Atlas run uses Energy Balance Model, and the second run uses both the energy balance and heat flow equations.

In both runs the material statement is used to assign an energy relaxation time for electrons. The models statement is used to select a set of physical models for this simulation. In this case, these models are CONSRH and AUGER recombination, the CVT mobility model, Band Gap Narrowing, two carriers model (carriers=2), and the energy balance equation for electrons only (hcte.el). The impact statement is used to assign the energy relaxation length for the Selberherr model. The contact statement is used to assign the work function on the polysilicon gate.

The gate voltage is ramped to 3V. At this stage, a combined algorithm is used by specifying method gummel newton . This means that if convergence is not reached in decoupled (or gummel) mode, the simulator will automatically switch to coupled (or newton) mode . The the drain voltage is then ramped. The combined algorithm is typically used for low and moderate drain biases, and newton alone for high drain biases.

The drain voltage is ramped to show the Id/Vds curves obtained up to 8V. This demonstrates the convergence improvements possible by using a fully coupled solution for energy balance simulation. A decoupled solution generally would not converge at higher drain biases.

In the second Atlas run, the same set of models is used, except that the solution of the lattice energy balance equation is activated using models lat.temp .

As with all lattice heat flow simulation, thermal boundary conditions must be defined. Thermal boundary conditions are defined in the thermcontact statement. A value of the thermal conductance is specified at the thermal contact located along the substrate, whilst thermal isolation conditions are assumed on the all other surfaces.

The final Id/Vds curves can be compared and overlaid using TonyPlot.

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