# Quantum transport in device (Transiesta code)

I want to understand how the current (or electron) is normally transported in a device like a graphene, under the Transiesta code. It is based on which principle: a gate is applied, or there is a difference in potential between two electrodes is applied?

• +1 but your second question has been commented out since it's different. You can ask that separately. Also why did you write "current vs voltage" in your title, when the quesion asks how the current (not voltage!) is transported in graphene? Commented Sep 15, 2022 at 12:53
• In order to have a current-voltage or transport, you don't need a gate. With the gate you have more control on the mains channel carrier concentration.
– Camps
Commented Sep 16, 2022 at 18:38
• Thank you ^^ 'Camps' Commented Oct 13, 2022 at 9:20

I guess this is more of a physics question than it is specific to TranSIESTA.

I will provide an answer that can help you get a general picture but it is by no means a formal explanation of how transport takes place. For that I suggest you to find resources that explain it in depth (I suggest some at the end).

In transport devices you have a situation like the following:

where the voltage window is the voltage you "apply" between the two electrodes. From this you can already see that there are some electrons in the left electrode that are at a very high energy and would like to go to the empty states of the right electrode at lower energies. However, to go there, they need some electronic state in between that they can use to move from left to right. Certainly, in this specific situation, there are no states in the device at the energies where the electrons would like to move. So basically there is no transport of electrons.

The bottomline is that in order for transport to take place, there must be some states in the device that are inside the voltage window. So, as long as you can put some states in that window you will have transport. The way you get states in the window is specific to your own system and device, but there are two general, easy to understand, approaches:

1. Increasing the voltage. This makes the voltage window larger and therefore you are likely to include more states. This strategy is pictured below:

2. Applying a gate. A gate makes electrons in the device feel "more comfortable" (lower energy) or "less comfortable" (higher energy). Therefore it induces a shift in all the energies of the device's states. In this way you might make some of the states enter the voltage window. This is pictured below:

As I said, you should check resources to understand the issue more in depth. This whole playlist on nanoelectronics I would say is a good start: https://www.youtube.com/watch?v=p5nsWUKiG9k&list=PLtkeUZItwHK6lvGu8kFKBdhz3XaIZQDFj

• Thank you very much , So Increasing the voltage lead to including more states and also can increase the gap Transmission T(E) Commented Oct 15, 2022 at 16:46