MoS2 transmission and the gate voltage

[NH]

 I was trying to investigate the effect of the gate voltage on the transmission of monolayer MoS2 FETs. I supposed that the transmission spectrum shifts by changing the gate voltage, but ATK results showed no shifts. Are there any mistakes in my calculation?
 I have attached the python file.

[Anders Blom]

You should add a second gate below the structure which is kept at zero voltage.

[Arya]

Sir,

Tutorials on graphene FETs do not add any gate below with 0V. Why do we require extra gate for MOS2?

[Anders Blom]

Well, a gate is one thing, it's another story if you want to simulate the more macroscopic effect of an electric field applied across the structure like in http://quantumwise.com/publications/tutorials/mini-tutorials/209.

[Arya]

Thanks Anders,

Here is another question. I have attached three images of LDDOS for a GNR FET:

lddos_nogate_nocharge.png: Is for a GNR-device without any gate or bias. (This curve seems good)
lddos_pin.png: Is for a p-i-n structure with GNR no bias (charges are added to both electrodes) (We can see the slope in LDDOS as expected)
lddos_with_gate_volatge: Is the same structure with gate at -1V without any charges on electrode

Now in the third figure we see the conduction band moving up due to gate voltage. Why is the valence band not effected? Shouldn't it move up as well?

I in fact tried to have second gate with 0V but things didn't change.

Thanks in advance
-Arya

[Anders Blom]

There is no immediate reason to expect that the band gap itself is not modified by the gate. The gate will create a net charge in the center of the system which modifies the electronic structure - note that the effect of the gate is selfconsistently taken into account in the calculation, it's not just an external potential that is added afterwards.

[Arya]

Thanks Anders! That does makes sense