Novel T-Shaped Gate With Air Gap for AlGaN/GaN HEMTs on Silicon With High Johnson’s Figures of Merit
Xiaoyi Liu, Jingxiong Chen, Yuanxi Jiang, Kairan Bian, Hong Wang
Abstract
We present high-performance AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on a silicon substrate featuring a novel T-shaped gate. The incorporation of the air gap between the gate and the low- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> passivation layer effectively reduces the gate capacitance and thus improves the RF characteristics of the device. The prepared device with a gate length of 180 nm has a unit current gain cutoff frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{T}}}$</tex-math> </inline-formula> ) of 55.2 GHz, a maximum oscillation frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{f}_{{\text{MAX}}}$</tex-math> </inline-formula> ) of 73.7 GHz, and a three-terminal OFF-state breakdown voltage (B <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{gd}}}$</tex-math> </inline-formula> ) of 143 V at the gate–drain distance of 2.2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m. The estimated Johnson’s figure of merit (J-FOM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$=$</tex-math> </inline-formula> B <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{gd}}}\ttimes\textit{f}_{{\text{T}}}$</tex-math> </inline-formula> ) is 7.9 THz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot$</tex-math> </inline-formula> V, representing a significant 30% improvement in comparison to the device with only stacked passivation layers. Besides, the J-FOM surpasses that of the single HfO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{2}}}$</tex-math> </inline-formula> passivation and single SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{2}}}$</tex-math> </inline-formula> passivation devices by 64.6% and 58%, respectively. These results clearly indicate that the AlGaN/GaN HEMTs with novel T-shaped gate design have great potential for radio frequency (RF) power device applications.