The design, fabrication, and characterization of AlGaN/GaN polarization-doped field-effect transistors (PolFETs) with graded heterostructure are presented in this paper. The 3-D profiles of carriers are obtained across the graded AlGaN/GaN heterostructure grown on a sapphire substrate, which brings about some novel features. The dc transfer and frequency characteristics exhibit bias-insensitive throughout the low-and high-voltage operating regions, demonstrating the potential for high linearity applications. In addition, dynamic I-V measurement was carried out to analyze the trapping behaviors. Negligible current collapses were observed in the unpassivated PolFETs with graded heterostructure, which can be explained in the aspect of unique energy band profiles of AlGaN/GaN graded heterostructures.

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A new method to characterize the strain status in the electrical degradation of AlGaN/GaN heterojunction field-effect transistors (HFETs) using micro-Raman spectroscopy at a wavelength of 532 nm was proposed. This method was applied to the devices stressed under different dc biased configurations to find the direct evidence of strain relaxation due to inverse piezoelectric effect (IPE). It was observed that the strain relaxation in AlGaN barrier layers became more severe with the increment of the electric field applied externally, which would lead to the rise of the gate leakage current. The deterioration of the gate leakage current shows a time-dependent feature.

GaN planar Schottky barrier diode (SBD) with an n − /n + structure was grown and fabricated on sapphire substrate. An n + GaN epitaxial layer with doping concentration of 8 × 10 18 cm −3 was employed to reduce the parasitic resistance. An air-bridge structure and 50 μm substrate thinning-down technique were adopted in order to reduce the parasitic capacitance. A record cutoff frequency (f c) of 902 GHz was achieved for GaN planar SBD with 2 μm anode diameter.