  Using cyclotron resonance, we measure the effective mass, $m$*, of electrons
in AlGaN/GaN heterostructures with densities, $n_{2D}\sim
1-6\times10^{12}$cm$^{-2}$. From our extensive data, we extrapolate a band edge
mass of $(0.208\pm0.002) m_e$. By comparing our $m$* data with the results of a
multi-band \textbf{k.p} calculation we infer that the effect of remote bands is
essential in explaining the observed conduction band non-parabolicity (NP). Our
calculation of polaron mass corrections -- including finite width and screening
- suggests those to be negligible. It implies that the behavior of $m*(n_{2D})$
can be understood solely in terms of NP. Finally, using our NP and polaron
corrections, we are able to reduce the large scatter in the published band edge
mass values.
