Analysis of Operational Characteristics of AlGaN/GaN MIS-HEMT with Different Slant-Recessed-Gate Structures
Authors: Chen Hsin Chu , Liu An-Chen , Lai Yung-Yu , Kuo Hao-Chung 
Published: February 2024

Purpose of Work
Recessed-gate metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) have emerged as a promising technology for normally-off operation because of their ability to withstand high gate voltage sweeps while maintaining low levels of leakage current [1, 2]. Despite their excellent performance, the etch to leave nanometer-thin layers of AlGaN has proven to be a significant challenge due to difficulties in precisely controlling the etching process. Achieving a positive threshold voltage (Vth) while maintaining the high electron mobility characteristic of the two-dimensional electron gas (2DEG) requires atom-level etch control. Additionally, conventional dry etching techniques can potentially cause damage, presenting a second challenge [3]. However, atomic layer etching (ALE) offers a promising solution with its potential for low damage, uniformity, and high-resolution depth control [4]. In this work, we not only focus on the ALE process to leave nanometer-thin layers of AlGaN/GaN as recessed-gate structures but also analyze the breakdown voltage characteristics of the slant-recessed gate structure.
Approach
Normally-off AlGaN/GaN MIS-HEMT with a recessed gate structure was fabricated on a 6-inch commercial Si substrate by MOCVD. The process involved mesa isolation by ICP-RIE, followed by the formation of the ohmic contact (Ti/Al/Ni/Au) at 825 °C annealing for source and drain metals, and the AlGaN/GaN recessed gate structure using ALE technology by O2/BCl3 multiple cycles of oxidation and etching [5]. After the recessed-gate process, the native oxide and carbon residue were removed by a wet cleaning process and O2 plasma treatment, and then the atom layer deposition (ALD) of about 12 nm thickness Al2O3 high-dielectric layer was also deposited. The backend process included gate field plate, SiNx passivation, and contact opening. Finally, the Ni/Au metal stack was deposited as pad electrodes, as shown in Fig. 1. The specific characteristics of the fabricated recessed MIS-HEMT were Lg of 3 μm, Lgs of 5 μm, Lgd of 10 μm, and Wg of 100 μm.
Results and Significance
In Figure 2(a) and (b), TEM profiles of the recessed gate structure were presented. Following the ALE process for the recessed gate structure, the remaining thickness of AlGaN was approximately 5.1 nm and 5.6 nm for Slant A and Slant B, respectively. Although the thickness of AlGaN remaining was similar in both cases, the slant angle of the recessed gate structure differed, significantly. Specifically, Slant A had a 65-degree recessed gate structure, while Slant B had a 45-degree recessed gate structure. In order to analyze of operational characteristics of AlGaN/GaN MIS-HEMT with different slant-recessed gate structures (Slant A and Slant B), the three-terminal OFF-state breakdown voltage (BV) was measured, as shown in Fig. 3. Recessed gate MIS-HEMT feature design of Lg/Lgd/Lgd/Wg of 3/5/10/100 μm, and VG of -10 V was used as this BV measured condition. The Slant B structure demonstrated higher breakdown voltage (BV) performance compared to the Slant A structure. Specifically, the Slant B structure exhibited a BV of 470 V at 1 μA/mm and 750 V at 1 mA/mm, while the Slant A structure had a BV of only 70 V at 1 μA/mm and 140 V at 1 mA/mm. The reason for the superior BV performance of the Slant B structure (45-degree recessed gate structure) was attributed to the gradient of the AlGaN profile, which facilitates the thin film deposition process of materials such as Al2O3 and gate metal (Ni/Au). Additionally, the Slant B structure helps to reduce the high electric field effect between the gate and drain, further contributing to its improved BV performance.
[1] Y. S. Lin, Y. W. Lain, and S. S. H. Hsu, "AlGaN/GaN HEMTs With Low Leakage Current and High On/Off Current Ratio," IEEE Electron Device Letters, vol. 31, no. 2, pp. 102-104, 2010.
[2] Z. Tang et al., "600-V Normally Off SiNx/AlGaN/GaN MIS-HEMT With Large Gate Swing and Low Current Collapse," IEEE Electron Device Letters, vol. 34, no. 11, pp. 1373-1375, 2013.
[3] T. H. Hung, P. S. Park, S. Krishnamoorthy, D. N. Nath, and S. Rajan, "Interface Charge Engineering for Enhancement-Mode GaN MISHEMTs," IEEE Electron Device Letters, vol. 35, no. 3, pp. 312-314, 2014.
[4] T. Y. Yang et al., "A Normally-Off GaN MIS-HEMT Fabricated Using Atomic Layer Etching to Improve Device Performance Uniformity for High Power Applications," IEEE Electron Device Letters, vol. 43, no. 10, pp. 1629-1632, 2022.
[5] I.-H. Hwang, H.-Y. Cha, and K.-S. Seo, "Low-Damage and Self-Limiting (Al)GaN Etching Process through Atomic Layer Etching Using O2 and BCl3 Plasma," Coatings, vol. 11, no. 3, p. 268, 2021.

Figure 1