question on the gate voltage

[rose]

hi,
I just reading the tutorial

https://docs.quantumwise.com/tutorials/graphene_nanoribbon_device/graphene_nanoribbon_device.html#graphene-nanoribbon-device

on the gate of graphene nanoribbons and I do not understand when I apply -1V as the gate voltage, what is the zero reference that the value of the gate is -1V compared with it? Is it the applied zero voltage reference that considered for both electrodes(source and drain in the case of FET)? 
thanks
Rose

[Petr Khomyakov]

Please take a look at the following tutorial, https://docs.quantumwise.com/tutorials/inas_p-i-n_junction/inas_p-i-n_junction.html.