| The emergence of III-V semiconductor heterostructures has enabled the study of a broad range of two-dimensional phenomena. These heterostructures are the building blocks of high frequency field-effect transistors (FETs).;GaN-based heterostructures will have great impact in FET applications as they have the highest carrier densities achievable. In this thesis we studied electron transport in AIGaN/GaN samples covering a broad range of carrier density, from 0.5x1012 cm-2 to 1.0x10 13 cm-2. We found at carrier densities below 5.0x10 12 cm-2, electron scattering from threading dislocations were the dominant scattering mechanism. The electron mobilities can be greatly enhanced by reducing the density of threading dislocations based on novel patterned growth techniques. We extended our studies to even higher carrier densities using lattice matched In0.16A10.84N/GaN heterostructures. In particular, we investigated the magnetotransport regime in the presence of two subbands by using gated Hall bar samples.;In addition to scattering processes, we also studied the spin-orbit interaction and phase coherence in GaN 2DEG samples by performing weak antilocalization (WAL) measurements. The spin-splitting energies extracted from WAL were found to be much smaller than the previous reports based on Shubnikov-de Haas measurements. By studying the spin splitting energies as a function of Fermi wave-vector we obtained the linear and cubic spin-orbit parameters for GaN 2DEG system. Furthermore, we used the WAL feature as a thermometer for the electron system, which allowed us to study energy relaxation processes in GaN.;In contrast to GaN heterostructures, we used GaAs/AIGaAs heterostructures to study two-dimensional electron transport at very low carrier densities. We focused on the deep insulation regime of integer quantum Hall liquids (IQHLs), a transport regime not accessible by ordinary transport measurements. We explored the charge dynamics in this regime by using single electron transistors (SET) in both unpatterned and antidot structures. SETs allowed us to monitor slow motion of charges in and out of the IQHLs. We found that, near zero temperature, IQHLs can be viewed as magnetic flux-to-charge transformers and confirmed our ideas by making a SET based magnetometer with an SET placed on an AlGaAs/GaAs hetcrostructure with 25 identical quantum wells. |
