Energy Balance and Drift Diffusion Comparison

mesfetex03.in : Energy Balance and Drift Diffusion Comparison

Requires: Blaze
Minimum Versions: Atlas 5.22.1.R

This example demonstrates electrical analysis of a 0.2um MESFET structure with Energy Balance (EB) and Drift-Diffusion (DD) Models. It shows:

  • Structure definition using Atlas syntax
  • Specification of energy balance models
  • Simulation of Id/Vds with Vgs=0.0V
  • Repeat simulation using drift-diffusion models to compare results.

The example file consists of two Atlas runs. Both use the same device structure defined in a similar manner to the previous example. The first uses energy balance models to simulate Id/Vds and the second does the same electrical simulation using the classical drift-diffusion models. The aim of this example is to compare the effects of the different models.

Energy balance models provide a more accurate description of physical device effects, in particular the effect of velocity overshoot and non-local impact ionization. These are not handled by the classical drift-diffusion model. These two effects can be observed in sub-micron MESFET simulation. This example concentrates on the velocity overshoot effect in GaAs. Non-local impact ionization effects can be seen in breakdown simulations. Since velocity overshoot is not accounted for in drift-diffusion the results from simulations with this model will underestimate the current severely. This discrepancy gets worse as the channel length decreases.

The sequence of simulation syntax is similar to the previous example. The structure differs from the previous only in the addition of heavily doped source/drain regions.

The energy balance models are defined using model hcte.el . Since this is a unipolar device, only electron energy balance is selected. A key parameter in the energy balance model is the relaxation time. Typical GaAs values are set on the material statement.

A choice of numerical methods exists for energy balance simulation. Here, full coupling of the 4 equations to be solved is selected by method newton. The block method may also be added if convergence problems are seen at the initial stage of the simulation.

The second run repeats the first but without 'models hcte.el'. Results from the two runs can be compared by overlaying the two log files in TonyPlot. Two solution files are also saved and these can be compared to see the difference in simulated electron concentration between the two models. The statement output e.velocity was used in both runs to save the electron velocity information to the solution files. Selecting to plot this for the energy balance solution file will show the overshoot effect.

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.