Breakdown Voltage Extraction

mos1ex07.in : Breakdown Voltage Extraction

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

This is an Athena/DevEdit/Atlas interface example simulating the breakdown voltage of an NMOS transistor. This example demonstrates:

  • Process simulation of a MOS transistor in Athena
  • Process parameter extraction (eg. oxide thicknesses)
  • Autointerface between Athena and DevEdit
  • Remeshing using DevEdit
  • Autointerface between DevEdit and Atlas
  • Solution for a Vds ramp with Vgs=0.0V to get breakdown

The process simulation, process parameter extraction and electrode definition for this example are exactly as described in the first example in this section.

The grid requirements for Atlas simulation of impact ionization effects are more stringent than for the low electric field cases described earlier. DevEdit is used to remesh the Athena structure before proceeding to Atlas. The remeshing commands are described in the previous example in this section.

The Atlas simulation contains similar syntax to the simpler examples described earlier in this section. The models, contact and interface parameters are the same, except that the concentration dependent SRH model is used. This provides a more accurate simulation of the pre-breakdown leakage current. The Selberherr impact ionization model is also selected.

On the method statement, two parameters are chosen to restrict the use of the current convergence criteria used in Atlas. Since the pre-breakdown leakage current is very low it is necessary to tighten the tolerances on current convergence. The parameter setting method climit=1e-4 is also recommended in cases where the mesh is not as tight as the one used here.

The sequence of SOLVE statements shows a ramp in drain voltage. Small steps are taken at first, but the main simulation is done in 0.5V steps. A compliance limit of 5.0e-8A/um is set on the drain. Compliance limits are useful in breakdown simulations to stop the simulation once the breakdown point is reached.

The value used here might seems rather low compared to typical values used in measurements. This is simply an issue of CPU time. Running the simulation up into microamp or milliamp ranges is possible, however the extra information gained is usually not worth the CPU time spent. No solutions will be possible once the voltage exceeds the breakdown voltage. Atlas will cut the voltage step and try again. It does this four times resulting in a minimum step of (0.5)^4 or 0.0625V. This is sufficient to resolve most breakdown voltages. It is possible for users to ramp using smaller voltage steps, to use current boundary conditions or curve tracing to further trace the IV curve to higher values of current. However, this is usually not needed. Much extra CPU time might be required and yet the value of breakdown voltage remains the same to within the accuracy expected by simulation. Examples demonstrating snapback and curve tracing are included in other sections.

The extract syntax used to measure the breakdown voltage is of the current search type. This is preferred over the simple max(v."drain") syntax that could be used as it gives more consistent results.

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.