Investigation of Multi-Mesa-Channel-Structured AlGaN/GaN MOSHEMTs with SiO 2 Gate Oxide Layer

: In this study, an electron-beam lithography system was employed to pattern 80-nm-wide and 980-nm-spaced multi-mesa-channel for fabricating AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs). Since the structure of multi-mesa-channel could enhance gate control capabilities and reduce the self-heating effect in the channel, the performance of the MOSHEMTs could be obviously improved. The direct current performance metrics of the multi-mesa-channel-structured MOSHEMTs, such as a saturation drain-source current of 929 mA/mm, maximum extrinsic transconductance of 223 mS/mm, and on-resistance of 2.1 Ω -mm, were much better than those of the planar-structured MOSHEMTs. Moreover, the threshold voltage of the multi-mesa-channel-structured MOSHEMTs shifted toward positive voltage from − 2.6 to − 0.6 V, which was attributed to the better gate control capability. Moreover, the multi-mesa-channel-structured MOSHEMTs also had superior high-frequency and low-frequency noise performance. A low Hooge’s coefﬁcient of 1.17 × 10 − 6 was obtained. that could be by using the


Introduction
Recently, due to the rapid requirement of emerging applications such as electric vehicles, fifth generation (5G) wireless systems, and renewable energy systems, high-power and high-frequency devices have attracted significant attention. Although both gallium nitride (GaN)-based and silicon carbide (SiC)-based materials are used in high-power and high-frequency amplifiers [1][2][3][4], SiC is much more expensive than GaN [5]. Moreover, due to the polarization effect in the AlGaN/GaN heterostructure, a two-dimensional electron gas (2-DEG) with high sheet electron density and high electron mobility can be obtained. To improve the power-handling capabilities and enhance operation voltage by reducing gate leakage current, AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) have been used to replace metal-semiconductor HEMTs and have become mainstream high-frequency devices and high-power devices used in various systems [6]. To fabricate AlGaN/GaN MOSHEMTs, several dielectric materials, such as SiO 2 [7][8][9][10], Si 3 N 4 [11,12], Al 2 O 3 [13,14], Ga 2 O 3 [6,15], ZnO [16], and LiNbO 3 [17], have been inserted between gate metal and GaN-based semiconductors as the gate oxide layer. Of the dielectric materials, SiO 2 film is a promising gate oxide material due to its stable properties, leakage current suppression, and commonly easy deposition. By using an SiO 2 gate oxide layer, AlGaN/GaN planar-structured MOSHEMTs have been previously reported [7][8][9][10]. Despite the success of planar-structured devices,

Devices Fabricated Procedure and Methods
In this study, epitaxial wafers grown on sapphire substrates by a metalorganic chemical vapor deposition (MOCVD) system were obtained from Nippon Telegraph and Telephone Advanced Technology Co. (NTT-AT), Japan. The epitaxial layers included a 20-nmthick AlN nucleation layer, a 4-µm-thick carbon-doped GaN buffer layer, a 300-nm-thick undoped GaN (i-GaN) layer, and a 35-nm-thick AlGaN barrier layer. A 2-DEG channel with a sheet electron density of 1.1 × 10 13 cm −2 and an electron mobility of 1700 cm 2 /V-s was formed at the polarized AlGaN/GaN hetero-structured interface. Figure 1a depicts the three-dimensional schematic configuration of the AlGaN/GaN MMC-MOSHEMTs with 30nm-thick SiO 2 gate oxide layers. Figure 1b depicts a high-resolution transmittance electron microscope (HR-TEM) image of the multi-mesa-channel. To fabricate the AlGaN/GaN MOSHEMTs, after a positive photoresist (GL-2000) was firstly spun on the samples with a spin coater, parallel nanostrip patterns with a width of 80 nm and a spacing of 980 nm were created using an electron-beam lithography system (ELS-7500). The photoelectrochemical (PEC) etching method was utilized to etch the 80-nm-wide nanostrip-patterned samples to isolate the 2-DEG channel and form a multi-mesa-channel. The PEC etching method and processes were previously discussed [29,30]. According to the HR-TEM image shown in Figure 1b, under a 50-µm-wide gate region, the total channel width of the multi-mesa-channel was approximately 3.76 µm. After an isolation mesa region (area = 310 µm × 320 µm) was patterned by a 500-nm-thick Ni metal mask, a reactive-ion etching system with a BCl 3 etchant was utilized to etch the region without Ni metal mask protection down to a depth of about 600 nm in the carbon-doped GaN buffer layer. Subsequently, an ammonium sulfide ((NH 4 ) 2 S x ) chemical solution was used to treat the sample's surface at 60 • C for 30 min, which was able to completely remove the undesired native oxide residing on the surface of the AlGaN barrier layer [31].
After the surface treatment, an electron-beam evaporator was employed to sequentially deposit Ti/Al/Pt/Au (25/100/50/400 nm, respectively) laminated metals on the patterned samples as the source and drain electrodes of the MOSHEMTs. To obtain better ohmic contact performance, the samples were annealed in a nitrogen atmosphere at 850°C for 1 min using a rapid-thermal annealing system. The separation between the source electrode and the drain electrode was 10 µm. Prior to depositing the 30-nm-thick SiO 2 gate oxide layer, a patterned gate region with a width of 50 µm and a length of 1 µm was etched about 10 nm away by PEC etching method to form the gate-recessed structure. After the surface treatment of (NH 4 ) 2 S x chemical solution again, the 30-nm-thick SiO 2 gate oxide layer was deposited on the gate-recessed samples with a radio frequency (RF) magnetron sputtering system using an Si target (99.999%) purchased from Admat Inc., Japan. The RF power, chamber pressure, and argon/oxygen gas flow were 125 W, 10 mtorr, Coatings 2021, 11, 1494 3 of 8 and 30/20 sccm, respectively. Subsequently, the gate electrode of Ni/Au (20/300 nm) metals was deposited on the SiO 2 gate oxide layer using an electron-beam evaporator. Finally, after using the lift-off technique to remove the redundant SiO 2 layer and Ni/Au metals, AlGaN/GaN MMC-MOSHEMTs with SiO 2 gate oxide layers were fabricated. The AlGaN/GaN planar-structured MOSHEMTs with a channel length of 10 µm, channel width of 50 µm, and gate length of 1 µm were also fabricated as comparison devices. After the surface treatment, an electron-beam evaporator was employed to sequentially deposit Ti/Al/Pt/Au (25/100/50/400 nm, respectively) laminated metals on the patterned samples as the source and drain electrodes of the MOSHEMTs. To obtain better ohmic contact performance, the samples were annealed in a nitrogen atmosphere at 850 ℃ for 1 min using a rapid-thermal annealing system. The separation between the source electrode and the drain electrode was 10 μm. Prior to depositing the 30-nm-thick SiO2 gate oxide layer, a patterned gate region with a width of 50 μm and a length of 1 μm was etched about 10 nm away by PEC etching method to form the gate-recessed structure. After the surface treatment of (NH4)2Sx chemical solution again, the 30-nm-thick SiO2 gate oxide layer was deposited on the gate-recessed samples with a radio frequency (RF) magnetron sputtering system using an Si target (99.999%) purchased from Admat Inc., Japan. The RF power, chamber pressure, and argon/oxygen gas flow were 125 W, 10 mtorr, and 30/20 sccm, respectively. Subsequently, the gate electrode of Ni/Au (20/300 nm) metals was deposited on the SiO2 gate oxide layer using an electron-beam evaporator. Finally, after using the lift-off technique to remove the redundant SiO2 layer and Ni/Au metals, Al-GaN/GaN MMC-MOSHEMTs with SiO2 gate oxide layers were fabricated. The Al-GaN/GaN planar-structured MOSHEMTs with a channel length of 10 μm, channel width of 50 μm, and gate length of 1 μm were also fabricated as comparison devices.

Experimental Results and Discussions
Following the measurements of an Agilent 4156C semiconductor parameter analyzer, Figure 2a,b depicts drain-source current (IDS)drain-source voltage (VDS) characteristics of the AlGaN/GaN planar-structured and MMC-MOSHEMTs, respectively, operating at various gate-source voltages (VGS). At the operating voltages of VDS = 10 V and VGS = 5 V, the saturation drain-source current (IDSS) values of the planar-structured and MMC-MOSHEMTs were 475 and 929 mA/mm, respectively. Since the total real channel widths were 50 μm and 3.76 μm for the planar-structured and MMC-MOSHEMTs, respectively, the associated total real drain-source current values were 23.75 mA and 3.49 mA, respectively. After calculating the on-resistance (Ron) from dV dI ⁄ | , , the Ron values of the planar-structured and MMC-MOSHEMTs were found to be 13.9 and 2.1 Ω-mm, respectively.

Experimental Results and Discussion
Following the measurements of an Agilent 4156C semiconductor parameter analyzer, Figure 2a,b depicts drain-source current (I DS )-drain-source voltage (V DS ) characteristics of the AlGaN/GaN planar-structured and MMC-MOSHEMTs, respectively, operating at various gate-source voltages (V GS ). At the operating voltages of V DS = 10 V and V GS = 5 V, the saturation drain-source current (I DSS ) values of the planar-structured and MMC-MOSHEMTs were 475 and 929 mA/mm, respectively. Since the total real channel widths were 50 µm and 3.76 µm for the planar-structured and MMC-MOSHEMTs, respectively, the associated total real drain-source current values were 23.75 mA and 3.49 mA, respectively. After calculating the on-resistance (R on ) from dV DS /dI DS | V DS =0 V, V GS =5 V , the R on values of the planar-structured and MMC-MOSHEMTs were found to be 13.9 and 2.1 Ω-mm, respectively.    obtained for the planar-structured and MMC-MOSHEMTs, respectively. Moreover, the corresponding threshold voltages (V TH ) were −2.6 and −0.6 V, respectively. The value of V TH was defined as the V GS corresponding to I DS = 1 µA/mm. According to the experimental results, the direct current performance, g mmax , and R on of the MMC-MOSHEMTs were better than those of the planar-structured MOSHEMTs. In addition to the smaller heat-release power in the multi-mesa-channel-structured devices due to the smaller total real drain-source current, the improvement mechanisms were attributed to the fact that the lateral heat flow within the space between the multi-mesa-channel could be easily driven to achieve better heat dissipation [22,32]. Consequently, the self-heating effect could be reduced in the multi-mesa-channel structure. Moreover, compared to the planar-structured MOSHEMTs, the V TH of the MMC-MOSHEMTs moved toward the positive voltage direction. This phenomenon was attributed to the better gate control capability enabled by modulation of the two additional lateral electric fields from the wall of the multi-mesachannel, in addition to the vertical electric field modulation from the top surface [24,33]  . Drain-source current-drain-source voltage characteristics of (a) planar-structured MOSHEMTs and (b) multi-mesa-channel-structured MOSHEMTs operating at various gate-source voltages. Figure 3a,b depicts the dependence of drain-source current and the extrinsic transconductance (gm) on the gate-source voltage of both the MOSHEMTs operating at VDS = 10 V. The maximum extrinsic transconductance (gmmax) values of 79 and 223 mS/mm were obtained for the planar-structured and MMC-MOSHEMTs, respectively. Moreover, the corresponding threshold voltages (VTH) were −2.6 and −0.6 V, respectively. The value of VTH was defined as the VGS corresponding to IDS = 1 μA/mm. According to the experimental results, the direct current performance, gmmax, and Ron of the MMC-MOSHEMTs were better than those of the planar-structured MOSHEMTs. In addition to the smaller heat-release power in the multi-mesa-channel-structured devices due to the smaller total real drainsource current, the improvement mechanisms were attributed to the fact that the lateral heat flow within the space between the multi-mesa-channel could be easily driven to achieve better heat dissipation [22,32]. Consequently, the self-heating effect could be reduced in the multi-mesa-channel structure. Moreover, compared to the planar-structured MOSHEMTs, the VTH of the MMC-MOSHEMTs moved toward the positive voltage direction. This phenomenon was attributed to the better gate control capability enabled by modulation of the two additional lateral electric fields from the wall of the multi-mesachannel, in addition to the vertical electric field modulation from the top surface [24,33]   To assess high-frequency performance, small-signal scattering parameters were evaluated with an Agilent 8510C network analyzer. Figure 5 presents the high-frequency performance as a function of frequency for both the MOSHEMTs operating at VDS = 10 V. The  To assess high-frequency performance, small-signal scattering parameters were evaluated with an Agilent 8510C network analyzer. Figure 5 presents the high-frequency performance as a function of frequency for both the MOSHEMTs operating at V DS = 10 V. The unit gain cutoff frequency (f T ) and maximum frequency of oscillation (f max ) were, respectively, obtained from the short-circuit current gain of 0 dB and the maximum available power gain of 0 dB. For the planar-structured MOSHEMTs, the f T and f max were 4.9 and 9.4 GHz, respectively. For the MMC-MOSHEMTs, the f T and f max were 6.9 and 13.7 GHz, respectively. Compared to the high-frequency performance of the AlGaN/GaN planar-structured MOSHEMTs, the high-frequency performance was improved by using the multi-mesa-channel structure.  To assess high-frequency performance, small-signal scattering parameters were evaluated with an Agilent 8510C network analyzer. Figure 5 presents the high-frequency performance as a function of frequency for both the MOSHEMTs operating at VDS = 10 V. The unit gain cutoff frequency (fT) and maximum frequency of oscillation (fmax) were, respectively, obtained from the short-circuit current gain of 0 dB and the maximum available power gain of 0 dB. For the planar-structured MOSHEMTs, the fT and fmax were 4.9 and 9.4 GHz, respectively. For the MMC-MOSHEMTs, the fT and fmax were 6.9 and 13.7 GHz, respectively. Compared to the high-frequency performance of the AlGaN/GaN planarstructured MOSHEMTs, the high-frequency performance was improved by using the multi-mesa-channel structure. In general, the low-frequency noise performance of MOSHEMTs was significantly affected by the quality of gate oxide layer and the interface quality between gate oxide layer and semiconductor [34]. The normalized low-frequency noise power (SIDS(f)/IDS 2 ) spectra of both the MOSHEMTs biased at VDS = 1 V and at various VGS voltages were measured and are shown in Figure 6. The curves of SIDS(f)/IDS 2 for both the MOSHEMTs were well-fitted by the 1/f fitting line at the frequency range from 1 Hz to 1 kHz. Therefore, it was deduced that the dominant noise was flicker noise, which implied that low surface In general, the low-frequency noise performance of MOSHEMTs was significantly affected by the quality of gate oxide layer and the interface quality between gate oxide layer and semiconductor [34]. The normalized low-frequency noise power (S IDS (f)/I DS 2 ) spectra of both the MOSHEMTs biased at V DS = 1 V and at various V GS voltages were measured and are shown in Figure 6. The curves of S IDS (f)/I DS 2 for both the MOSHEMTs were wellfitted by the 1/f fitting line at the frequency range from 1 Hz to 1 kHz. Therefore, it was deduced that the dominant noise was flicker noise, which implied that low surface damage occurred on the etched gate-recessed AlGaN surface using the PEC etching method and there was good interface between the SiO 2 gate oxide layer and the gate-recessed AlGaN surface when using the (NH 4 ) 2 S x surface treatment. The S IDS (f)/I DS 2 values of the planarstructured and MMC-MOSHEMTs biased at V GS = 5 V and f = 10 Hz were 4.97 × 10 −13 and 3.02 × 10 −14 Hz −1 , respectively. Moreover, we used the mobility fluctuation model to calculate the Hooge's coefficient α, which is another important factor for appraising the low-frequency noise performance of MOSHEMTs. In general, the α value was found to be proportional to the S IDS (f)/I DS 2 and the gate width. The α values were 7.99 × 10 −5 and 1.17 × 10 −6 for the planar-structured and MMC-MOSHEMTs operating at V GS = 5 V and f = 10 Hz, respectively. Compared to the planar structure, the MMC-MOSHEMTs had lower normalized low-frequency noise power and Hooge's coefficient values. It was indicated that better low-frequency noise performance could be achieved by using the multi-mesa-channel structure.
praising the low-frequency noise performance of MOSHEMTs. In general, the α value was found to be proportional to the SIDS(f)/IDS 2 and the gate width. The α values were 7.99 × 10 −5 and 1.17 × 10 −6 for the planar-structured and MMC-MOSHEMTs operating at VGS= 5 V and f = 10 Hz, respectively. Compared to the planar structure, the MMC-MOSHEMTs had lower normalized low-frequency noise power and Hooge's coefficient values. It was indicated that better low-frequency noise performance could be achieved by using the multi-mesa-channel structure.

Conclusions
In this work, using an electron-beam lithography system and the PEC etching method, AlGaN/GaN MMC-MOSHEMTs with a gate-recessed structure and an SiO2 gate oxide layer were manufactured. According to the improved transconductance, subthreshold swing, threshold shift, and high-frequency performance, it was deduced that the MMC-MOSHEMTs had better gate control capability and better self-heating dissipation in comparison to those of the planar-structured MOSHEMTs. Moreover, the lower normalized low-frequency noise power and Hooge's coefficient values of the MMC-MOSHEMTs confirmed that the PEC etching method and (NH4)2Sx surface treatment could form low etching damages and defects existed on the gate-recessed AlGaN surface. Therefore, it is expected that the PEC etching method is a more suitable method and better technique for etching an AlGaN semiconductor than the reactive-ion etching system. Furthermore, according to the superior low-frequency noise performance of the planar-structured and MMC-MOSHEMTs, the SiO2 material is a suitable gate oxide layer for fabricating AlGaN/GaN MOSHEMTs. Compared to the performance of AlGaN/GaN planarstructured and MMC-MOSHEMTs, the multi-mesa-channel is a more promising structure in various-materials-based MOS devices. However, under the same channel width, the total real drain-source current of the multi-mesa-channel-structured devices was found to be smaller than that of the planar-structured devices. Although the total real drain-source current could be enhanced by using a wider channel width, the power performance of the multi-mesa-channel-structured devices still needs work.

Conclusions
In this work, using an electron-beam lithography system and the PEC etching method, AlGaN/GaN MMC-MOSHEMTs with a gate-recessed structure and an SiO 2 gate oxide layer were manufactured. According to the improved transconductance, subthreshold swing, threshold shift, and high-frequency performance, it was deduced that the MMC-MOSHEMTs had better gate control capability and better self-heating dissipation in comparison to those of the planar-structured MOSHEMTs. Moreover, the lower normalized low-frequency noise power and Hooge's coefficient values of the MMC-MOSHEMTs confirmed that the PEC etching method and (NH 4 ) 2 S x surface treatment could form low etching damages and defects existed on the gate-recessed AlGaN surface. Therefore, it is expected that the PEC etching method is a more suitable method and better technique for etching an AlGaN semiconductor than the reactive-ion etching system. Furthermore, according to the superior low-frequency noise performance of the planar-structured and MMC-MOSHEMTs, the SiO 2 material is a suitable gate oxide layer for fabricating AlGaN/GaN MOSHEMTs. Compared to the performance of AlGaN/GaN planar-structured and MMC-MOSHEMTs, the multi-mesa-channel is a more promising structure in various-materials-based MOS devices. However, under the same channel width, the total real drain-source current of the multi-mesa-channel-structured devices was found to be smaller than that of the planarstructured devices. Although the total real drain-source current could be enhanced by using a wider channel width, the power performance of the multi-mesa-channel-structured devices still needs work.