I am not sure what your question is. Basically you have spontaneous polarization, and piezoelectric effects from the wurtzite nature of Gallium Nitride. Depending on the orientation of the growth dipoles can be formed in either direction. When you form the heterojunction in addition to the band offsets you can generate sheet charge from the strain. After that it is electrostatics and the surface chemistry. Depending on the surface passivation you can have charge on the surface if there are bonds there to accommodate it. I think there are some detailed papers by the Denbaars or Misra groups at University of Santa Barbara that have nice detailed explanations.