Error with multigrid residual

[Hassan]

I'm running a simulation of a transistor device and i get the following error:

The computed multigrid residual is greater than the required accuracy.       #
#                                                                              #
# Computed residual :         inf                                            #
# Required accuracy :   1.00000e-12


The simulator also throws an exception:
* Back Engine Exception : Singular value decomposition failed
 33 ** Location of Exception : mathutils.cpp:1604
 34


I'm using  Atomistix ToolKit 2014.2 [Build 811a2e4]. This error is really unpredictable. I use very similar geometry on other devices and i dont get the same error. I am using Neumann Boundary conditions on the A/B axes and Dirichlet on the C ends of the device.

        left_electrode_poisson_solver = MultigridSolver(
            boundary_conditions=[[NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [PeriodicBoundaryCondition,PeriodicBoundaryCondition]]
            )

        right_electrode_poisson_solver = MultigridSolver(
            boundary_conditions=[[NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [PeriodicBoundaryCondition,PeriodicBoundaryCondition]]
            )

        device_poisson_solver = MultigridSolver(
            boundary_conditions=[[NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [NeumannBoundaryCondition,NeumannBoundaryCondition],
                                 [DirichletBoundaryCondition,DirichletBoundaryCondition]]
            )

I'm attaching my device geometry. Note that the distance between the dielectric and any atom is exactly 1A as suggested by QW experts here.


Any help would be nice.

Thanks

[Umberto Martinez]

Decrease the width (along C direction) of your metallic regions or increase the central region size by maintaining the gate size.
Gates in your structure have the same width as the scattering region of your device. This will not provide enough freedom for the potential to converge properly.
Indeed, with Neumann conditions you assure that the gradient of the potential is zero at the boundary while it also has to coincide with the metallic gate potential.

Given you set up, you can also use a cylindrical region around your nanotube.
Please, refer to the Silicon nanowire field effect transistor tutorial.

[sahar]

Hi
i have this error.the size of gate is same size of canal and but this error exist yet.
Too i don't know exactly "how many  distance between the graphene and dielectric region".
PLZ help me.
thank u.

[Jess Wellendorff]

We will probably need to see the script you are running before we can give you advice on this.