Novel T-Shaped Gate Structure of AlGaN/GaN HEMTs on Si for RF Application
Authors: Jung Hyun-Wook , Choi Il-Gyu , Chang Sung-Jae , Jeong Hyeon-Seok , Choi Su-Min , Ahn Ho-Kyun , Kang Dong-Min , Kim Dae-Hyun , Lim Jong-Won 
Published: February 2024

Due to the high electron saturation velocity and breakdown electric field, GaN-based high electron mobility transistors (HEMTs) have been widely used in high frequency and power application, such as telecommunication, satellite communication, and radar system [1]. Since first demonstration of AlGaN/GaN HEMTs in 1993 [2], the RF performance of these device has continuously improved by the progress on epitaxy and device scaling technology [3].
The current gain cut-off frequency (fT) and the maximum oscillation frequency (fmax) from the small-signal equivalent circuit of the HEMT are derived by the following equations [4].
From these equations, it can be concluded that in order to achieve high frequency operation, HEMTs should be designed with a T-shaped gate structure that simultaneously achieves low gate resistance (Rg) and low parasitic capacitance (Cgs and Cgd). However, there are complex fabrication processes to form the T-shaped gate. Typically, the T-shaped gate structure is implemented through electron beam lithography process using a tri-layer resist such as PMMA/PMGI/PMMA, which requires multiple exposures or developments. Multiple exposures can cause overlay errors that can significantly affect the T-shaped gate with smaller gate lengths [5]. Also, it is difficult to obtain an undercut structure that enables a good lift-off and guarantees a high yield due to the need for precise dose control. In addition, since the T-shaped gate has an unstable structure in which a narrow foot supports the weight of a wide head, the gate may collapse during a subsequent process.
In this work, we fabricated a novel T-shaped gate using the negative e-beam resist HSQ, which offers a simple T-shaped gate fabrication process. The HSQ that was exposed to e-beam is located underneath the gate head and on both sides of the gate foot in our novel T-shaped gate design, providing a robust structure. The HSQ-assisted gate HEMTs exhibit comparable DC/RF performance compared to conventional T-shaped gate HEMTs.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (No. NRF-2021M3C13097672).

References

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[4] D. H. Kim, B. Brar, J. A. del Alamo, “fT = 688 GHz and fmax = 800 GHz in Lg = 40 nm In0.7 Ga0.3As MHEMTs with gm_max > 2.7 mS/μm,” in Proc. IEEE IEDM, 319, (2011).
[5] B. Sun, P. Zhang, T. Zhang, S. Shangguan, S. Wu, and X. Ma, “Single step electron-beam lithography archiving lift-oﬀ for T-gate in high electron mobility transistor fabrication”, Microelectron. Eng., 229(111337), (2020).

Figure 1