A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time-resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.
Electrical properties of Mg-doped p-type GaN grown by metalorganic chemical vapor deposition have been investigated by Hall effect and conductivity measurements. Metastability and persistent photoconductivity effects have been observed in GaN. It was found that at low temperatures, it takes several hours for the free hole concentration to reach its equilibrium value in the dark as well as in the photoexcited state, implying a bistable nature of impurities in p-type GaN. Temperature dependence of these behaviors have been studied, from which the energy barrier for free hole capture by ionized impurities as well as between the metastable and the stable states of neutral impurities have been obtained.
We report on the fabrication and characterization of Al0.1Ga0.9N/GaN heterojunction field effect transistors, both an enhancement mode and a depletion mode with a low pinchoff voltage, suitable for digital integrated circuit applications. For an enhancement mode device with a 1 μm gate length and 5 μm drain-to-source separation, the dc transconductance is around 23 mS/mm. Connecting the enhancement mode device as a switching transistor and a depletion mode device as a load, we demonstrate an AlGaN/GaN inverter.
We report on the fabrication and characterization of Al0.15Ga0.85N/GaN heterostructure field-effect transistors (HFETs) with transconductance as high as 120 mS/mm and saturated current density of 0.35 A/mm for a device with a gate length and width of 1 and 100 μm. This represents one of the best results for such device. A comparison of the maximum transconductance of devices on wafers with different channel conductance is presented to analyze the factors limiting the performance. Our data indicates the series resistance between the source and drain to be the limiting factor for the maximum dc transconductance.
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