In recent years，high electron mobility transistors（HEMTs）based on GaN have attracted more attention，due to their high thermal conductivity，high breakdown voltage，and high-power density for millimeter-wave（mm-wave）power amplifiers. In an AlGaN/GaN HEMTs structure，the working voltage may reach 28 V or even higher^{［1］［2］}，such high voltage will enhance the longitudinal electric field to increase the gate leakage^［3］_. Additionally，the internal electric field intensity will reach 10⁶~10⁷ V/cm when the 20~30 V is applied to drain bias，leading to current collapse，reduction of breakdown voltage，and increase in leakage^［4］. In order to achieve high-performance GaN HEMT at low operating voltage，the energy-band theory is used to design new epitaxial structures to increase the electron gas density meanwhile preventing the gate from losing its control ability for the short T-gate. Therefore，the ultra-thin barrier layer technology has shown great advantages in ultra-high frequency and high power^{［5］［6］}.

In millimeter-wave applications，the gate length is shrunk to deep-submicron size，and the transverse dimension of the device needs to be scaled down at the same proportion. To avoid the short channel effect，the material structure with an ultra-thin barrier layer is used to solve the aspect ratio of the gate. The issue primarily results from the much stronger spontaneous and piezoelectric polarization of AlN/GaN compared to AlGaN/GaN，leading to a much higher drain current in the HEMT channel，also allowing the use of a much thinner barrier layer. While along with the shrink of vertical device dimensions，increased gate leakage necessitates the use of a gate insulator^［7-10］.

AlN barrier has been shown highly sensitive to the air and vapor for oxidation，consequently，surface treatment and passivation techniques play a significant role in the surface state. To achieve a low gate leakage current，materials with a wide bandgap are necessary，such as SiO₂ and Al₂O₃^{［7］［9］}. However，it is inevitable that these materials are deposited on the AlN surface when it is exposed to air，becoming contamination at the interfaces. On the other hand，in-situ deposition of SiN_x is a promising way to realize proper interfaces，which guarantees the insensitivity of AlN surfaces to temperature change.

In this work，we demonstrated the AlN/ GaN MIS-HEMTs. By using in-situ SiN_x insulator，a maximum drain current I_D，max of 2.2 A/mm was obtained at V_GS=2 V，it doubled I_D，max of the AlGaN/GaN HEMTs under the same condition. Transconductance G_m，ext of 509 mS/mm are also achieved. Moreover，the OFF-state drain leakage，as well as gate leakage current in the HEMTs，was reduced by the low interface state between AlN barrier and insulator，contributing to a low Schottky gate leakage of 4.7×10^-6 A/mm at V_GS = -30 V and a low OFF-state drain leakage of 8.2×10^-5 A/mm. Owing to the suppressed current collapse，when V_DS = 8 V，a high output power density of 2.3 W/mm with peak power-added-efficiency（PAE）of 45.2%，and a power gain of 10.2 dB are achieved at 40 GHz in the continuous-wave（CW）mode.

The schematic cross section of MIS-HEMTs is shown in Fig. 1（a）. The AlN/GaN heterostructures in this study were grown on semi-insulating SiC substrates by metal-organic chemical vapor deposition（MOCVD），consisting of a Fe-doped GaN buffer layer，an unintentionally doped GaN channel layer，1 nm AlN spacer layer，a 5 nm AlN barrier layer，and 5 nm SiN_x insulator layer. Device fabrication was started with source/drain ohmic contact formation by Ti/Al/Ni/Au stack，and subsequent rapid thermal annealed at 800 ℃ for 30 s in N₂ atmosphere，to yield a contact resistance of 0.3 Ω·mm. Device isolation was then formed utilizing multiple-energy nitrogen ion implantation. A T-shaped gate was subsequently accomplished by electron beam lithography（EBL；model manufacturer）of UVIII/Al/PMMA resist stack. The width of the T-gate foot and head are 0.15 and 0.6 μm，respectively^［2］. A Ni/Au metal layer was generated by e-beam evaporation（EVA450）on SiN_x’s surface for the gate contact. Finally，the AlN/GaN HEMT devices were passivated with 60 nm stress-free SiN_x grown by plasma-enhanced chemical vapor deposition（PECVD）. The fabricated MIS-HEMTs have a source-drain distance（L_SD）of 2.4 μm and a gate-drain distance（L_GD）of 1.15 μm. An SEM picture of the T-gate is shown in Fig. 1（b）.

As a comparison，AlGaN/GaN HEMT devices are also developed，with the barrier and cap layers replaced with a 21-nm Al_0.25Ga_0.75N and a 3-nm GaN layers，respectively，as Ref.［16］. The gate recessed process，which differs from the AlN/GaN device’s，uses inductively coupled plasma（ICP）dry etching with chlorine-based plasmas of BCl₃ and Cl₂ to fabricate recessed-gate with a width of 0.8 μm and depth of 6 nm. Then the same T-shaped gates were fabricated on it. The remaining process steps are the same as for AlN/GaN devices.

The fabricated devices yielded in this study exhibit a typical static characterization，as shown in Fig. 2（a）. Due to the much stronger spontaneous and piezoelectric polarization of AlN/GaN，a maximum drain current of 2.2 A/mm at V_GS=2 V was observed. The thickness of the in-situ SiN_x cap layer is critical for highly scaled GaN devices to avoid gate leakage current contributing to a reverse density of 4.7×10^-6 A/mm at V_GS = -30 V，as shown in Fig. 2（b）. The short-channel effect was effectively suppressed by the thin barrier，as shown by the transfer curves in Fig. 2（c），and the OFF-state drain leakage is merely 1.0×10^-6 A/mm. Meanwhile the corresponding G_m，ext at V_DS = 6 V is 509 mS/mm（Fig. 2（c））. Based on AlGaN barrier device，G_m，ext is 294 mS/mm and the OFF-state drain leakage is 8.2×10^-5 A/mm under the same test condition（Fig. 2（d））.

The small-signal RF characteristics of the fabricated MIS-HEMTs were measured using a network analyzer in a frequency range from 100 MHz to 40 GHz. Values of current-gain cutoff frequency f_T and unit-power-gain frequency f_MAX，as shown in Fig. 4，were determined by 20 dB/dec line extrapolated from the small-signal current gain |h21|and maximum stable gain（MSG）. At V_DS=10 V，f_T and f_MAX are 98 GHz and 165 GHz，respectively（Fig. 3）. It implies that in-situ SiN_x technology effectively suppresses the RF-G_m collapse in mm-wave AlN/GaN HEMTs.

To determine the quality of in-situ SiN_x，the capacitance-voltage（C-V）measurement was employed to realize interface trap density. The frequency/temperature dispersions of the second slope in C-V curve were analyzed^［11-13］，and the results are shown in Fig. 4. With f_m varying from 1 KHz to 1 MHz（Fig. 4（a）），and T increasing from 25 ℃ to 150 ℃（Fig. 4（b）），the C-V characteristics of AlN/GaN MIS-HEMT exhibits a slight（∆V less than 0.05 V）dispersions in multi-f/T ac-CV characteristics，indicating low Dit and high interface quality in MIS-HEMT. Accordingly，D_it at the in-situ SiN_x/AlN interface was mapped against E_T^［14-15］. From E_C-0.58 eV to E_C -0.29 eV，D_it falls between 3.4×10¹¹ and 1.1×10¹² cm^-2eV^-1（Fig. 4（c））.

The low interface state density ensures the low dc-RF dispersion，the pulse I-V characteristic of the devices is shown in Fig. 5（a）. The pulse period and width were set to 10 μs and 200 ns，respectively. The gate-lag effect under a quiescent bias of（V_GSQ，V_DSQ）=（-6 V，0 V）barely changes in the MIS-HEMTs. The drain-lag ratio under a quiescent bias of（V_GSQ，V_DSQ）=（-6 V，15 V）is pretty weak in the saturation region（collapse ratio：1.5%，Fig. 5（a））. It is probably due to the N in the SiN_x rather than the AlN barrier that leads N vacancies creating a conducting channel through the AlN barrier，hence low annealing temperature and time. The in-situ SiN_x impeded the formation of nitrogen deficiency and oxidation of bare AlN surface when conventional process of ohmic annealing at above 800 °C，and suppressed damage to the AlN barrier during the process of extra SiN_X ex-situ passivation. The ultralow dispersion implies that in-situ SiN_X effectively obstructed the bombardment of ion when the plasm was generated. As shown the pulsed transfer characteristics curves in Fig. 5（b），hysteresis is less than 100 mV after sweeping from -8 V to 0 V，indicating significant suppression of deep interface traps with in-situ insulator.

Figure. 6 depicts the large-signal power performance of the mm-wave AlN/GaN MIS-HEMTs，evaluated at 40 GHz in CW mode，in comparison with AlGaN/GaN HEMTs. The devices were biased at Class-AB condition with low operation voltage，V_DS = 8 V，V_DS = 10 V，and V_DS = 15 V，respectively. Load and source impedance were optimized for the best PAE before the evaluation.

Owing to the enlarged current density and minimized forward gate leakage current of AlN/GaN MIS-HEMTs，a record high PAE of 45.2% is achieved at V_DS = 8 V，and the corresponding output power density and associated gain are 2.3 W/mm and 10.8 dB gain. By contrast，the PAE，output power density，and gain of AlGaN/GaN HEMTs are merely 42.6%，1.2 W/mm，and 9.1 dB respectively. when V_DS = 10 V，P_out of AlN/GaN MIS-HEMTs reached 3.3 W/mm while that of AlGaN/GaN HEMTs is 1.5 W/mm；when V_DS = 15 V，P_out of AlN/GaN MIS-HEMTs increased to 5.2 W/mm while that of AlGaN/GaN HEMTs is 2.8 W/mm. In previous research using the AlGaN HEMTs structure，P_out of 5.1 W/mm can be only obtained under V_DS over 25 V^［16］. The high performance of AlN/GaN HEMTs is believed to attribute to the wide conduction band between AlN and GaN，as well as the high-quality SiN_x/AlN interface.

At low voltage，the power density of AlN / GaN thin barrier MIS-HEMTs based on in-situ SiN growth is nearly double that of AlGaN barrier devices，making them promising for low voltage applications.

With in-situ SiN_x technique on AlN/GaN epi-structure and T-gate process，high-performance MIS-HEMTs have been fabricated for low V_DS applications at Ka-band. A high-quality SiN_x/AlN interface has been obtained，which was verified by analyzing the frequency and temperature-dependent of the second slope in the C-V characteristics. Using 0.15 μm Γ-shaped gate technology，the developed MIS-HEMTs show a maximum drain current of 2.2 A/mm at V_GS=2 V，an extrinsic peak G_m，ext of 509 mS/mm，extra-low dc-RF dispersion. The drain-lag ratio of 1.5% under a quiescent bias of（V_GSQ，V_DSQ）=（-6 V，15 V）collapse-ratio in the saturation region. the MIS-HEMTs can yield an output power density of 2.3 W/mm associated with power-added efficiency（PAE）of 45.2% at 40 GHz under the drain voltage V_DS=8 V in continuous-wave mode. Furthermore，when V_DS=10 V，the power density was 3.3 W/mm，and PAE maintain 43.8%；when V_DS= 15 V，power density increased to 5.2 W/mm with PAE decreasing to 42.2%. The results suggest that the in-situ AlN/GaN MIS-HEMTs are promising for low bias voltage applications requiring high-efficiency and high-power density at Millimeter Waves.