[DFTB-Plus-User] DFTB+NEGF with a gate contact

Gabriele Penazzi gabriele.penazzi at bccms.uni-bremen.de
Thu Mar 14 12:07:32 CET 2013

On 03/13/2013 03:25 PM, Yuranan Hanlumyuang wrote:
> Hi,
>    I have some basic questions about NEGF calculations with three
> contacts, using the Poisson box
> (1) Why is in the code, the bias direction is set to the x-direction.
> In the manual, it's suggested that
> the transport direction is the z-direction, while the gate is y-dir.  
> However, in the  source code  poisson.F90
> where the gate  direction is  automatically set to y-dir, and the bias
> is set to x-dir.

Hi Yuranan,

the gate should be placed accordingly to the transport direction you
choose (there's a contdir variable taking it into account in the
source). Maybe you refer to the scheme in the in-source documentation,
but I think that's just an example. It should work anyway, but if you
spot a bug send a sample input file in list.

> (2) What are the criteria for the gate distance (i.e. Rmin_Gate)?
> Sometimes, I got a warning message
> that "WARNING: Gate Distance too large!". I have looked around the
> source code, but still couldn't figured
> out which section  decides the value of this parameter.

Rmin_Gate is the radius of the gate, if you choosed a cylindrical one,
therefore it's an input parameter.
Probably  your poisson box is not big enough, and your gate is lying
out. Try to increase the size of the poisson box around your device.

> (3) I've missed the the discussion on the bugs of DFTB+NEGF version
> r4198, and how they
> are fixed in version r4305. Is version r4198 containing bugs on the
> three-contacts sections of the code?
> If you could repost the old threads, I would be really appreciated. I
> am using version r4198...

For three contacts we mean, in that context, three reservoir with open
boundary conditions. In your case the gate is actually imposing a
dirichlet boundary condition on the poisson box, i.e. is mimicking a
contact and your system is still, from the Green's function point of
view, a two contact systems. Therefore that problem shouldn't affect
your calculation (as far as you don't spot some other problem).


> Yuranan Hanlumyuang
> University of Houston
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Dr. Gabriele Penazzi
BCCMS - University of Bremen


phone: +49 (0) 421 218 62337
fax: +49 (0) 421 218 62770

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