Binary AlN/GaN high electron mobility transistors (HEMTs) were grown by plasma-assisted molecular beam epitaxy (PA-MBE) as well as metal-organic chemical vapor deposition (MOCVD) and compared with regard to their structural and electrical properties. The investigated structures differ in Al distribution and composition of the AlN barrier due to characteristic differences of the two growth methods such as growth temperature and interface sharpness. While we observe a nearly pure AlN layer and an abrupt interface for MBE growth, a graded "AlN" barrier with a significant amount of Ga is found for the MOCVD grown structures which is reflected by the electrical properties of the HEMT structures. Si-implanted ohmic contacts were formed on MBE as well as MOCVD grown structures. The activation anneal step subsequent to implantation at temperatures ~1100°C changes the observed Al profiles of MBE structures and damages the active region, whereas MOCVD samples react insensitively and thus were able to be further processed. A maximum drain current of ~1.46 mm(-1) at a gate source voltage of +3 V is observed for the processed devices. A current-gain cut-off frequency of 89 GHz and maximum oscillation frequency of 208 GHz were measured, which demonstrate an excellent small-signal performance of AlN/GaN devices with 100nm gate length
Through implementation of the 3-D tri-gate topology, GaN-based high-electron mobility transistors (HEMTs) have been fabricated and high-frequency performances as well as the short-channel effects are investigated. The designed tri-gate transistors are highly-scaled having 100 nm of gate length, which introduces the condition of a short channel. It is demonstrated that higher sub-threshold slopes, reduced drain-induced barrier lowering and better overall off-state performances have been achieved by the nano-channel tri-gate HEMTs with an AlGaN barrier. A lattice-matched InAlGaN barrier with the help of the fin-shaped nano-channels provide improved gate control, increasing current densities, and transconductance g m . In a direct comparison, very high drain current densities (∼3.8 A/mm) and g m (∼550 mS/mm) have further been obtained by employing a pure AlN barrier.INDEX TERMS High-electron mobility transistor (HEMT), fin-shaped field-effect transistor (FinFET), Gallium nitride, short channel.
In this paper, we demonstrate the fabrication of current aperture vertical electron transistors (CAVET) realized with two different epitaxial growth methods. Templates with a p-GaN current blocking layer (CBL) were deposited by metal organic chemical vapor deposition (MOCVD). Channel and barrier layers were then regrown by either molecular beam epitaxy (MBE) or MOCVD. Scanning electron microscope (SEM) images and atomic force microscope (AFM) height profiles are used to identify the different regrowth mechanisms. We show that an AlN interlayer below the channel layer was able to reduce Mg diffusion during the high temperature MOCVD regrowth process. For the low-temperature MBE regrowth, Mg diffusion was successfully suppressed. CAVET were realized on the various samples. The devices suffer from high leakage currents, thus further regrowth optimization is needed.
Heterostructures with lattice matched Al(Ga)InN barriers have been widely investigated as alternative to standard AlGaN/GaN based high electron mobility transistor structures for high power applications. Mostly these heterostructures comprise a thin AlN based spacer between GaN channel and lattice matched barrier. One key issue for high quality plasma-assisted molecular beam epitaxy (PA-MBE) of these structures is the control of the AlN-Al(Ga)InN interface since optimal growth conditions for high quality AlN differ significantly from those for growth of indium containing material. In this paper, a detailed analysis and a deduced model of the interface growth is presented. The Al/N ratio during AlN spacer growth is likely to influence the subsequent growth of quaternary Al(Ga)InN. Ideal Al/N ratio leads to high performance heterostructures, while slightly Al-rich conditions lead to the formation of Al residues on the substrate surface, which hinder subsequent epitaxial growth. Al/N ratios below unity lead to the deposition of ternary AlGaN instead of binary AlN spacers and to increased alloy scattering. An insertion of a thin GaN layer between spacer and barrier can hinder the formation of Al residues and leads to improved wafer homogeneity
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