We will present the results of the experimental studies of transport properties of two-dimensional electron gas (2DEG) and three-dimensional electrons (that might be responsible for a parallel conduction) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements , cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies . The room temperature high field measurements allow us to clearly separate contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures.
The Quantum Hall Effect measurements are performed in the magnetic fields up to 30 Tesla and temperatures between 2K-80K. This high magnetic field in combination with very high mobilities  in the samples on the bulk GaN substrates allow us to observe very interesting features related both to cyclotron resonance and spin splitting. The temperature dependence of these splittings determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance results, allows us to determine a complete set of 2DEG transport parameters.
1) Acoustic phonon scattering of two-dimensional electrons in GaN/AlGaN heterostructures, W. Knap et al. Appl. Phys. Lett. 80, 1228 (2002)
2) Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface, W. Knap et al. Appl. Phys. Lett. 70, 2123 (1997)
3) High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates, E. Frayssinet et al. Appl. Phys. Lett. 77, 2551 (2000)