Impact of Parasitic Gate Capacitance on RF Performance in GaN-Based HEMTs for X-Band Applications
Authors: Chang Sung-Jae , Jeong Hyeon-Seok , Jung Hyun-Wook , Choi Su-Min , Choi Il-Gyu , Noh Youn-Sub , Kim Seong-Il , Lee Sang-Heung , Ahn Ho-Kyun , Kang Dong-Min , Kim Dae-Hyun , Lim Jong-Won 
Published: February 2024

GaN-based high-electron-mobility transistors (HEMTs) are intensively researched for high-power radio frequency (RF), low noise, and aerospace applications, thanks to their high breakdown electric field, high carrier mobility and density at the hetero-interface, and wide bandgap [1-2]. For the improvement of RF performance, various approaches have been reported, including T-gate structure [3-4], surface passivation [5], n+-regrown source and drain contact [6], thin barrier structure [7], and graded-channel [8]. Among them, the T-gate structure has been extensively employed for the reduction of the gate resistance (Rg
) and thereby improvement of the RF performance. However, the usage of the wide T-gate structure enlarges parasitic gate capacitance components, which deteriorates the RF performance. As a result, one should pay careful attention to the dimension of the wide T-gate structure to reduce Rg as much as possible, not to enlarge the parasitic gate capacitance components. This motivates us to explore the optimum dimension of the wide T-gate structure in GaN-based HEMTs for X-band RF applications.

To investigate the detailed impact of the gate resistance and parasitic gate capacitance components, we fabricated LG
= 0.15 μm GaN-based HEMTs with various dimensions of the T-gate head from 0.5 to 0.8 μm during an e-beam lithography process step. The unit gate width (WG
) and number of gate finger (NF
) were 100 μm and 2, respectively.

For comparison of the device characteristics with respect to the dimension of the gate head, DC and RF measurements were conducted for the fabricated GaN-based HEMTs with various dimensions of T-gate head. The DC characteristics in terms of the threshold voltage (VTH
), the maximum transconductance (gm,max
), and the saturation drain current (Id,sat
) were almost identical, regardless of the dimension of the T-gate head. On the contrary, the current-gain cut-off frequency (fT
) and maximum oscillation frequency (fMAX
) were improved by 26.5% and 21.0%, respectively, when the T-gate head size was reduced from 0.8 to 0.5 μm.

In an effort of explore the origin of the dependence of the T-gate head on the RF performance, we have carried out small-signal modeling to extract the parasitic gate capacitance and gate resistance components from the measured RF data. As the T-gate head size was reduced from 0.8 to 0.5 μm, the gate-source parasitic capacitance component (Cgs_par
) was reduced by 19%, while the gate resistance component was increased by 32%. In our device design and fabrication, it turned out that the reduction of Cgs_par
was more beneficial than the increase of Rg
, leading to the improved RF performance with the gate head size decrease.

In this work, our systematic research revealed that, unlike earlier reports [3-4], the reduction of the parasitic gate capacitance was essential for the improvement of the RF performance as opposed to the reduction of the gate resistance in LG
= 0.15 μm GaN-based HEMTs for X-band RF applications.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (No. NRF-2021M3C1C3097672).

References

[1] T. Palacios, A. Chakraborty, S. Rajan, et al., “High-power AlGaN/GaN HEMTs for Ka-band applications,” IEEE Electron Device Lett., 2005, 26, 781-783.

[2] K. Shinohara, D. C. Regan, Y. Tang, et al., “Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Applications,” IEEE Trans. Electron Devices, 2013, 60, 2982-2996.

[3] M. S. Cho, J. H. Seo, S. H. Lee, et al., “Fabrication of AlGaN/GaN Fin-Type HEMT Using a Novel T-Gate Process for Improved Radio-Frequency Performance,” IEEE Access, 2020, 6, 139156-139160.

[4] X. W. Zhang, K. J. Jia, Y. G. Wang, et al., “AlNGaN HEMT T-Gate Optimal Design,” Applied Mechanics and Materials, 2013, 347, 1790-1792.

[5] S.-J. Chang, H. W. Jung, J. W. Do, et al., “Enhanced Carrier Transport Properties in GaN-based Metal-Insulator-Semiconductor High Electron Mobility Transistor with SiN/Al2O3 bi-layer Passivation,” ECS J. Solid State Sci. Technol., 2018, 7, N89-N90.

[6] T. Tang, K. Shinohara, D. Regan, et al., “Ultrahigh-speed GaN high-electron mobility transistors with fT/fmax of 454/444 GHz,” IEEE Electron Device Lett., 2015, 36, 549-551.

[7] Y. Yue, Z. Hu, J. Guo, et al., “InAlN/AlN/GaN HEMTs with Regrown Ohmic Contacts and fT of 370 GHz,” IEEE Electron Device Lett., 2012, 33, 988-990.

[8] J.-S. Moon, J. Wong, B. Grabar, et al., “360 GHz fMAX Graded-Channel AlGaN/GaN HEMTs for mmW Low-Noise Applications,” IEEE Electron Device Lett., 41, 1173-1176.

Figure 1