How to distinguish device configuration from pure crystals

[jerry]

Dear,
    I have a problem in distinguishing device configurations from pure crystals (periodic strcures), because both of these two structures are periodic. How should i depict the differences between them? I mean how to use academic words to depict the difference between them. Waiting for your reply!
Thanks a lot!

[Nordland]

There is several ways to do it. A device is only periodic in two direction while a crystal is periodic in three directions.
Another way is: A device consist of two semi-infinite crystals and their interface

[jerry]

Thank you very much!
Then how to depict the differences of calculation methods between devices and crystals?
If the scattering region of a device is a semiconductor, then there will be a energy gap in the transmission spectra of this device. Can this energy gap equals the band gap of the semiconductor? or can they represent each other? Do they have a linear relation?

[Nordland]

If there were nothing called surface effect or interface effects and semi-infinite crystal always shared the same properties as the infinite crystal, then almost yes.

But if the semi-conductor is wide enough, then it can function as a barrier for the electrons transport, and therefore there could be a gap in transmission spectrum corresponding to the band gap of the crystal, but if this picture is true (which it very rarely are), there is really no reason for doing a NEGF calculation of the device.