Generation-recombination noise in gallium nitride-based quantum well structures

Electronic noise has been investigated in AlxGa1-xN/GaN modulation-doped field-effect transistors of submicron dimensions, grown by molecular beam epitaxy techniques. Some 20 devices were grown on a sapphire substrate. Conduction takes place in the quasi-two-dimensional (2D) layer of the junction (xy plane) which is perpendicular to the triangular quantum well (z direction). A nondoped intrinsic buffer layer separates the Si-doped donors in the AlxGa1-xN layer from the 2D transistor plane. Since all contacts must reach through the AlxGa1-xN layer to connect internally to the 2D plane, parallel conduction through this layer is a feature of all modulation-doped devices. The excess noise has been analyzed as a sum of Lorentzian spectra and 1/f(alpha) noise. The Lorentzian noise is ascribed to trapping of the carriers in the AlxGa1-xN layer. The trap depths have been obtained from Arrhenius plots of log(tauT (2)) versus 1000/T. Comparison with previous noise results for GaAs devices shows that: (a) many more trapping levels are present in these nitride-based devices and (b) the traps are deeper (farther below the conduction band) than for GaAs, as expected for higher band-gap materials. Furthermore, the magnitude of the noise is strongly dependent on the level of depletion of the AlxGa1-xN donor layer. We also note that the trap-measured energies are in good agreement with the energies obtained by deep level transient spectroscopy. (C) 2003 American Institute of Physics.