Nominally undoped GaN/Al x Ga1-x N/GaN high electron mobility transistors were grown by plasma-induced molecular beam epitaxy in order to study the formation and electric transport properties of two-dimensional electron gases. By depositing an AlN nucleation layer on sapphire substrates before the growth of the GaN buffer layer, we were able to change the polarity of the wurtzite films from N- to Ga-face. The change in polarity causes a change in the sign of the spontaneous and piezoelectric polarization directed along the c-axis of the strained AlGaN barrier. The sign and the gradient in polarization at one of the GaN/AlGaN interfaces is mainly responsible for the generation and confinement of the two-dimensional electron gas. Ga- and N-face heterostructures with mobilities up to 1050 and 1200 cm2/Vs, respectively, and sheet carrier concentrations of up to 1.2×1013 cm-2 at room temperature were realized. Transistors processed from heterostructures with both polarities show maximum source-drain currents between 800 and 850 mA/mm and a transconductance of up to 250 mS/mm.

Dimitrov, R., Mitchell, A., Wittmer, L., Ambacher, O., Stutzmann, M., Hilsenbeck, J., & Rieger, W. (1999). Comparison of N-face and Ga-face AlGaN/GaN-Based High Electron Mobility Transistors Grown by Plasma-Induced Molecular Beam Epitaxy. Japanese Journal of Applied Physics, 38(9R), 4962.