Comparison of N-face and Ga-face AlGaN/GaN-based high electron mobility transistors grown by plasma-induced molecular beam epitaxy

Nominally undoped GaN/AlxGa1-xN/GaN high electron mobility transistors were grown by plasma-induced molecular beam epitaxy in order to study the forma tion and electric transport properties of two-dimensional electron gasps. B y depositing an AlN nucleation layer on sapphire substrates before the grow th of the GaN buffer layer, we were able to change the polarity of the wurt zite films from N- to Ga-face. The change in polarity causes a change in th e sign of the spontaneous and piezoelectric polarization directed along the c-axis of the strained AlGaN barrier. The sign and the gradient in polariz ation at one of the GaN/AlGaN interfaces is mainly responsible for the gene ration and confinement of the two-dimensional electron gas. Ga- and N-face heterostructures with mobilities up to 1050 and 1200 cm(2)/Vs, respectively , and sheet carrier concentrations of up to 1.2 x 10(13) cm(-2) at room tem perature were realized. Transistors processed from heterostructures with bo th polarities show maximum source-drain currents between 800 and 850 mA/mm and a transconductance of up to 250 mS/mm.