error in I-V for different gate voltages

[ramkrishna]

Dear Sir,
        I have calculated Id-Vd characteristics of graphene junction device (http://www.quantumwise.com/documents/tutorials/latest/GrapheneDevice/index.html/) for two different gate voltage( using Vg= -1V and Vg= 1V) but I have got same current for these cases. Please see the files in the attachments. In general, it should give different values. Please help me.

Regards
Ramkrishna

[Anders Blom]

In order to get correct results you should run the calculation self-consistently, your current script does it non-self-consistently (default for Huckel).

This is not entirely obvious, we're looking at ways it can be clearer which semi-empirical models are self-consistent and which are not.

In case you are relying on the Graphene Device tutorial, please note that it has been updated recently, to cover this point (see here).

[ramkrishna]

Dear Sir,
       Thank you for your kind reply. I have calculated again by considering the same approach (taking all the parameters same as it is given) which is given in the updated tutorial, but I have faced the same problem. To calculate the self consistent I-V characteristics, I have used "ivscan-6-6.py" and "ivscan_current.py" scripts which are given in the tutorial. In these script, I have changed only the temperature range and bias voltage range, but have got same current for Vg = -1V and Vg = 1V. I can't understand where is the problem exactly. Please find the attachments. Please help me.

Thanking you,
Ramkrishna   

[Anders Blom]

As far as I can see, the calculations are still non-self-consistent. You must go back into the Scripter and UNMARK "No SCF iteration". Or else, you can in your script add this manually, i.e.

Code: python

left_electrode_iteration_control_parameters = IterationControlParameters()        # <-------- add this
right_electrode_iteration_control_parameters = IterationControlParameters()        # <-------- add this
device_iteration_control_parameters = IterationControlParameters()        # <-------- add this

# go down some lines

left_electrode_calculator = HuckelCalculator(
    basis_set = basis_set,
    numerical_accuracy_parameters=left_electrode_numerical_accuracy_parameters,
    iteration_control_parameters=left_electrode_iteration_control_parameters,        # <-------- add this
    poisson_solver=left_electrode_poisson_solver,
    )

right_electrode_calculator = HuckelCalculator(
    basis_set = basis_set,
    numerical_accuracy_parameters=right_electrode_numerical_accuracy_parameters,
    iteration_control_parameters=right_electrode_iteration_control_parameters,        # <-------- add this
    poisson_solver=right_electrode_poisson_solver,
    )

#----------------------------------------
# Device Calculator
#----------------------------------------
calculator = DeviceHuckelCalculator(
    basis_set = basis_set,
    iteration_control_parameters=device_iteration_control_parameters,        # <-------- add this
    poisson_solver=device_poisson_solver,
    electrode_calculators=
        [left_electrode_calculator, right_electrode_calculator],
    )

[ramkrishna]

Dear Sir,
        I am waiting for your kind reply. Please help me.

Regards
Ramkrishna

[Anders Blom]

Why do you think the current should remain constant after a certain bias? NDR is a common effect in nanoscale devices. Your transmission spectrum depends on the bias and the plateau you have around the Fermi level clearly changes width with bias, so this would explain the observed I-V behavior which thus is correct (provided all parameters are converged in k-points etc, of course).

[ramkrishna]

But with the increase of gate voltage , the maximum current should increase (which I have seen in Graphene also). It is not coming although in my simulation, the run converges well.