The traditional G-polar consists of AlGaN on top and GaN channel layer at the bottom.

However, for the N-polar GaN Hemt, all the discussion or research papers found online show the structure of GaN(channel)/AlGaN(back barrier)/GaN (buffer )instead of just GaN(channel)/AlGaN(back barrier), which is so confusing to me.

Can anyone here explains to me why?

Greatly appreciated!