Boundary Conditions Set between two gate and one gate FET

[395235863]

i think there are three different two gate FET models in the following 3 pictures, as well as the single gate FET.
i want to know which model is suggested? can you tell me the differences among them in details?
after read http://quantumwise.com/forum/index.php?topic=1597.0
i don't know how to set B direction boundary conditions when i use two gate FET and single gate FET.like 1.png and 4.png
desired for your help
thank you !

[Jess Wellendorff]

- The FET models in 2.png and 3.png are not appropriate for computations. Your gates should not extend all the way to the boundaries along B of the supercell.

- As for 1.png and 4.png, I can not really "suggest" any of them over the other. The choice of model device depends on the physical device you want to simulate.

- Neumann boundary conditions along B should work well for both 1.png and 4.png. You want the electrostatic field on the boundaries above and below the device to be constant (finite or zero) but with zero real-space derivative.

[395235863]

- The FET models in 2.png and 3.png are not appropriate for computations. Your gates should not extend all the way to the boundaries along B of the supercell.

- As for 1.png and 4.png, I can not really "suggest" any of them over the other. The choice of model device depends on the physical device you want to simulate.

- Neumann boundary conditions along B should work well for both 1.png and 4.png. You want the electrostatic field on the boundaries above and below the device to be constant (finite or zero) but with zero real-space derivative.

sorry,i don't what is zero real-space derivative. can you explain it in details?

and for Neumann boundary, how to calculate the electric field strength E in single gate and double gate FET.?

Thank you very much!

[Jess Wellendorff]

Let my first correct my previous answer a bit: You can use metal gates that extend all the way to the supercell boundaries. However, this is not likely to change your results.

Real-space derivative: What I mean is that Neumann boundary conditions impose the condition that the field is constant at the boundary, but imposes no conditions on the strength of the field at the boundary.

Field strength: Do you mean the electric field strength between the gates?

[395235863]

Let my first correct my previous answer a bit: You can use metal gates that extend all the way to the supercell boundaries. However, this is not likely to change your results.

Real-space derivative: What I mean is that Neumann boundary conditions impose the condition that the field is constant at the boundary, but imposes no conditions on the strength of the field at the boundary.

Field strength: Do you mean the electric field strength between the gates?

yes, i want to plot i-E curve  , so i need to calculate the electric field strength in one gate and two gates model.

Thank you!

btw, for example , i calculate a ivcurve using bias=0,0.2,0.4,0.6,0.8 , the log file shows that bias=0.2V calculation didn't converge,but others converged.  is the current value of bias= 0.4,0.6,0.8V real(useful)?

[Umberto Martinez]

btw, for example , i calculate a ivcurve using bias=0,0.2,0.4,0.6,0.8 , the log file shows that bias=0.2V calculation didn't converge,but others converged.  is the current value of bias= 0.4,0.6,0.8V real(useful)?

Probably not. You can check how the energy varies for these calculations (from the log file) to have an idea if you were close enough to the convergence or you ended up in a completely crazy solution.