We report on AlInGaN/GaN heterostructure field effect transistors (HFETs) and the effect of different barrier material compositions. The analytical model for the interface charge in quaternary nitride heterostructures is described in detail and is applied in the calculation of the expected sheet carrier density. Experimental results from different lattice-matched AlInGaN/GaN heterostructures are presented and compared with the analytical predictions. Three heterostructures with AlInGaN barriers grown on sapphire substrates were processed and have been investigated. Each barrier layer was lattice-matched to GaN and the gallium content was 0.1, 0.15 and 0.2 at a barrier thickness of 13.5, 12.8 and 11.3 nm, respectively. Additionally, from these experiments, the basic trends for quaternary nitride Schottky barrier contacts are discussed. Finally, comprehensive dc characterizations have been performed. All devices had a gate length of 1 μm and exhibited a good transconductance of around 260 mS mm −1 at nearly the same current density level. An increase in threshold voltage as well as a decrease in gate leakage current for increasing GaN content has been observed. The nearly constant electron mobility in the range of 1700 cm 2 V −1 s −1 at room temperature is within the best reported so far for HFETs with InN-containing barriers.

Back-gated measurements on conductive silicon substrates have been performed to investigate the effect of stress voltage on the dynamic behaviour of GaN-on-silicon (GaN-on-Si) transistors. Two comparable samples were studied with the only difference being the vertical dislocation density. Results show a clear correlation between dislocation density and the ability of the GaN buffer to dynamically discharge under high stress conditions.

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AlGaN/GaN/Si metal-insulator-semiconductor heterostructure field-effect transistors (MISHFETs) with SiN and Al 2 O 3 gate insulators are characterized by static and high-frequency measurements, and their performance is compared with nonpassivated and SiN-passivated heterostructure field-effect transistors (HFETs). The saturation drain current increased from ∼500 mA mm −1 for the HFETs to ∼770 mA mm −1 for the MISHFETs. The peak extrinsic transconductance of the MISHFETs (147 mS mm −1 for 8 nm SiN and 220 mS mm −1 for 4 nm Al 2 O 3 ) is higher than expected due to the increased gate-to-channel separation. Similarly, small signal microwave characterization yielded an increase in the current gain cut-off frequency (from 3.2 to 7.2 GHz) and the maximum frequency of oscillation (from 12.3 to 20.4 GHz) for the MISHFETs with 2 μm gate length compared to the HFET counterparts. Finally, the density of trap states, evaluated from the frequency-dependent conductance measurements, was ∼ =3 × 10 12 cm −2 eV −1 for the HFETs but only ∼ =1.8 × 10 12 cm −2 eV −1 for the MISHFETs. All of these demonstrate the capability of AlGaN/GaN MISHFETs of preparing high-performance and cost-effective devices for high-power microwave applications on a Si substrate.when compared with sapphire. Unfortunately, high-resistive SiC substrates are a costlier alternative to AlGaN/GaN devices. Devices on Si 8 , on the other hand, are the best solution considering their low cost, high crystalline perfection, good thermal conductivity and large-area availability of Si substrates.Recently, an excellent millimetre-wave performance (e.g. maximum frequency of oscillation of 110 GHz and noise figure of 1.3 dB at 20 GHz) has been reported on AlGaN/GaN-on-Si HFETs [1].