Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face Al x Ga 1Ϫx N/GaN/Al x Ga 1Ϫx N and N-face GaN/Al x Ga 1Ϫx N/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15ϽxϽ0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6ϫ10 12 to 2ϫ10 13 cm Ϫ2 in the GaN channel with increasing Al-concentration from xϭ0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N-and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by piezoelectric and spontaneous polarization effects.

The small-signal equivalent circuit of AlGaN/GaN HEMT is discussed It is shown that an extremely high gate voltage has to be applied to correctly determine series resistances. Effects appearing at high forward gate voltages are discussed. Good agreement between measured and simulated data has been achieved.

No abstract

No abstract

No abstract