Properties of N-polar AlGaN/GaN heterostructures and field effect transistors grown by metalorganic chemical vapor deposition.

Smooth N-polar GaN/Al_xGa_1-xN/GaN heterostructures with a different Al mole fraction were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates with a misorientation angle of 4° toward the a-sapphire plane. The sheet electron density of the two-dimensional electron gas (2DEG), which formed at the upper GaN/Al_xGa_1-xN interface increased with an increasing Al-mole fraction in the Al_xGa_1-xN layer and increasing silicon modulation doping, similar to the observations for Ga-polar heterostructures. The transport properties of the 2DEG, however, were anisotropic. The growth on vicinal substrates led to the formation of well ordered multiatomic steps during Al_xGa_1-xN growth and the sheet resistance of the 2DEG parallel to the steps was about 25% lower than the resistance measured in the perpendicular direction. The fabricated devices exhibited a drain-source current, I_DS, of 0.9 A/mm at a gate-source voltage +1 V. At a drain-source voltage of 10 V and I_DS=300 mA/mm, current-gain and maximum oscillation frequencies of 15 and 38 GHz, respectively, were measured. [ABSTRACT FROM AUTHOR]