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We review our state-of-the-art GaN-based device technologies for power switching at low frequencies and for future millimeter-wave communication systems. These two applications are emerging in addition to the power amplifiers at microwave frequencies which have been already commercialized for cellular base stations. Technical issues of the power switching GaN device include lowering the fabrication cost, normally-off operation and further increase of the breakdown voltages extracting full potential of GaN-based materials. We establish flat and crack-free epitaxial growth of GaN on Si which can reduce the chip cost. Our novel device structure called Gate Injection Transistor (GIT) achieves normally-off operation with high enough drain current utilizing conductivity modulation. Here we also present the world highest breakdown voltage of 10400V in AlGaN/GaN HFETs. In this paper, we also present high frequency GaN-based devices for millimeter-wave applications. Short-gate MIS-HFETs using in-situ SiN as gate insulators achieve high f_max up to 203GHz. Successful integration of low-loss microstrip lines with via-holes onto sapphire enables compact 3-stage K-band amplifier MMIC of which the small-signal gain is as high as 22dB at 26GHz. The presented devices are promising for the two future emerging applications demonstrating high enough potential of GaN-based transistors.

This paper presents technology computer-aided design (TCAD) modeling of an enhancement-mode aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistor (HEMT) with extensive π-gate optimization for high-power and radio frequency (RF) applications. Effects of the gate voltages on threshold (V_th), transconductance (g_m), breakdown voltage (V_BR), cutoff frequency (f_T), maximum frequency of oscillation (f_max) and minimum noise figure (NF_min) are systematically investigated with different gate structures (π–Shaped p-GaN MISHEMT, π–Shaped p-GaN HEMT, π–Gate HEMT). A comparative study demonstrates that π–Gate with additional p-GaN and insulating layer makes the device effectively operate in the enhancement mode having a threshold voltage (V_th) = 1.72 V with a breakdown voltage (V_BR) = 341 V, exhibiting better gate control with maximum transconductance (g_m,max) of 0.321 S/mm. In addition, the proposed device architecture with an optimized gate structure maintains a balance between a positive device threshold and a high breakdown voltage and achieves a better noise immunity with the minimum noise figure of 0.64 dB while operating at 10 GHz with a cutoff frequency (f_T) of 33.4 GHz, and a maximum stable operating frequency (f_max) of 82.3 GHz. Moreover, the device achieved an outstanding V_th, g_m,max, V_BR, f_T, f_max and NF_min making it suitable for high-power, high-speed electronics, and low-noise amplifiers.

In the present study, AlGaN/GaN high-electron-mobility transistors (HEMTs) were fabricated through metal–organic chemical vapor deposition. Gate recess etching, combined with inductively coupled plasma reactive ion etching, was adopted, and etching time was controlled to manipulate the threshold voltage $(V_{\mathrm{\text{th}}})$. The DC characteristics of devices etched for 0–25 s were investigated. $V_{\mathrm{\text{th}}}$ exhibited a 1.9-V positive shift in the device with the AlGaN layer etched for 25 s. The effect of an AlN buffer layer on the $V_{\mathrm{\text{th}}}$ shift was also investigated. The $V_{\mathrm{\text{th}}}$ of the HEMT etched for 25 s and without an AlN buffer layer exhibited a positive shift of 3.1 V.