||Taek Lim, Patrick Waltereit, Rolf Aidam, Rüdiger Quay, Lutz Kirste, Peter Brückner, Rudolf Kiefer, Oliver Ambacher
High-electron-mobility-transistors based on GaN can provide the highest power density among all contemporary semiconductor power-amplifying devices for microwave frequencies. Up to now, such devices have mostly been manufactured using AlGaN/GaN heterostructures which suffer from the inherent tensile stress in the AlGaN barrier and corresponding relaxation effects for high Al-content required for high-frequency operation. As an alternative material, we present quaternary AlGaInN at compositions nearly lattice-matched to GaN but with high spontaneous polarization difference with respect to GaN giving high sheet electron densities around 1.9E13cm-2. Compared to the often used AlInN system, these materials have a better miscibility and thus offer larger growth windows.
For epitaxial growth, molecular-beam-epitaxy in a Veeco GEN20A system with a nitrogen plasma source has been performed. Using 4H-SiC substrates, an AlN nucleation layer, a thick GaN buffer including the channel, an AlN/GaN/AlN spacer, and a thin (around 5nm) Al0.40Ga0.53In0.07N-barrier have been grown. The quaternary alloy composition has been provided by X-ray diffraction analyses on an AlGaInN layer and an auxiliary AlGaN layer with the same Al/Ga-ratio,. The surface of the samples is smooth (roughness around 0.5nm) with atomic steps clearly visible. As an integral part of the transistor structure, the AlN/GaN/AlN spacers have a total thickness of 2.5-3nm and serve to ensure an electronic separation between channel and barrier to improve electron mobility.
Heterostructures and devices using the triple-layer structure achieve an electron mobility up to 1590cm2/Vs, a maximum current density up to 2.3A/mm and peak transconductance up to 675mS/mm clearly outperforming similar structures with an AlN single interlayer. We do not observe a significant current collapse under pulsed conditions indicating a low trap density. Leakage currents and breakdown behavior are addressed as one of the main challenges with the AlGaInN-barrier devices. First, an increase in the growth temperature of the AlGaInN barrier from 580°C to 600°C shows very effective in reducing the leakage currents. Second, the addition of a 2nm GaN cap layer on top of the barrier also results in significantly better breakdown properties. By combining both an increased barrier growth temperature and an additional GaN cap, a three-terminal breakdown voltage of more than 50V for devices with 4750nm source-drain spacing is achieved. Power measurements (10GHz,30V) on devices (Lg=250nm) processed with our standard gate technology yield a power-added-efficiency beyond 40%, an output-power-density close to 6W/mm and a linear-gain around 15dB. For these devices we arrive at transit frequencies ft around 35GHz. Very recently, we have also processed wafers with a 100nm gate length technology with reduced parasitic capacitances, thus enabling to explore the potential of thin barriers and high transconductance for high frequency operation. Devices processed using this technology show an ft around 110GHz and a maximum frequency of oscillation fmax around 130GHz.