Polysilicon versus metallic floating gate

eprmex05.in : Polysilicon versus metallic floating gate

Requires: S-Pisces
Minimum Versions: Atlas 5.28.1.R

This example creates a simple EEPROM cell with a polysilicon floating gate. It then highlights the difference between treating this as a floating contact (perfect conductor) or a floating electrode (semiconductor). It demonstrates

  • Single gate EEPROM structure formation in Atlas.
  • Mesh refinement in Atlas.
  • Threshold voltage simulation before charging of polysilicon.
  • Assignment of a charge to perfectly conducting contact.
  • Threshold voltage simulation after charging.
  • Assignment of a charge to a semiconductor contact.
  • Threshold voltage simulation after charging.

The FLOATING parameter on the CONTACT statement causes the specified electrode to be treated as a perfect conductor, and therefore as an equipotential. Any stored charge is assumed to be evenly distributed and coupled to the device potential by using Gauss's flux theorem. For a polysilicon electrode this can be an imperfect approximation, because in the isolated semiconductor the potential and charge distributions will be position dependent in order to maintain the condition of zero steady state current flow. The FLOATING parameter on the ELECTRODE statement allows to to model the floating gate as a semiconductor.
This example has four sections, each staring with
go atlas
The first creates the EEPROM cell using Atlas syntax, giving the polysilicon floating gate a donor density of 1e18 /cc. A simple mesh refinement is then carried out to give better resolution near the p-n junctions, and the structure is saved
In the next section the structure is loaded and the device with zero floating gate charge is simulated to give the unprogrammed Id-Vg curves.

The next section loads the structure and assigns the floating gate to be a perfect conductor using the FLOATING parameter on the CONTACT statement, as well setting the workfunction to be that for n-doped polysilicon by using the N.POLY flag. The floating gate is given a negative charge of -1e-15 C/cm using the Q<elec> syntax of the SOLVE statement. This is an alternative to charging the floating gate during a transient solve. The Id-Vg curve of this charged device is then obtained, and the structure file with a control gate bias of 3 V is saved.
The final section loads in the structure and uses the syntax
ELECTRODE MODIFY FLOATING
to specify that the named electrode should be treated as an isolated semiconductor. It is still necessary to specify the workfunction of the contact using the N.POLY parameter on the CONTACT statement, but the FLOATING parameter should not be specified on the CONTACT statement in this case. The floating gate is given a negative charge of -1e-15 C/cm using the Q<elec> syntax of the SOLVE statement, as previously, and the Id-Vg curve is obtained. The PROBE statement saves the integrated floating gate charge to the logfile. The structure file of the device with a control gate bias of 3V is saved.
The visualisation of the structure files shows a cutline through the gate stack, with the floating electrode case having a position dependent electron concentration and the floating contact case showing no concentration. The potentials in the floating gate can also be compared to see an equipotential in the floating contact case and potential changes in the floating electrode case. The Id-Vg curves are slightly different between the two cases, due to the different charge distributions. The integrated total charge of the floating electrode, as obtained from the PROBE statement, can be seen to be bias independent. The threshold voltages for the three curves can be obtained from the results.final file.

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