The diffusion thermopower S d of the two-dimensional electron gas at a GaN / AlGaN heterojunction is studied, in detail, in the low-temperature region T Ͻ 30 K. The important mechanisms of scattering of electrons considered are those by edge dislocations, remote and background impurities, interface charges and roughness, and bulk acoustic phonons via deformation potential and piezoelectric fields. The dominant contribution to S d is found to be from dislocations via the coulomb interaction; the contribution from strain field associated with the dislocations is, comparatively, small. The contribution from acoustic phonons is negligible. The calculations, for pure samples without dislocations in the low-temperature region where acoustic phonon scattering dominates, bring out the characteristics of the Bloch-Grüneisen regime, as in mobility studies.