The wireless communication technology has entered the 5G era，while the corresponding system needs to support higher throughput and faster information transmission. Meanwhile，the advantages of 5G technology，such as high peak-to-average power ratio of signal，wide bandwidth，and small size of hardware system，has raised new demands to the radio frequency system. The broadband power amplifier is a critical module in the system，and power added efficiency （PAE）and bandwidth performance have been considered to be key parameters that reveals its efficiency^［1］. Many approaches have been proposed to improve the efficiency in last decade ^{［2］-［6］}. Lu Y. et al. developed achieved AlGaN/GaN HEMT by the Recess-arrayed Ohmic Contact technology ^［7］，with the PAE of 71.6% at 5 GHz，which further attained 85.2% after the third harmonic tuning. Wienecke ^［8］ et al. developed an AlGaN/GaN MIS-HEMT structure using N-Polar GaN material with SiC as substrate，and its PAE attained 54.9%，52.5%，and 26.9% with continuous output power of 8 W/mm at 10 GHz，30 GHz，and 94 GHz，respectively.

For the broadband circuit，when the device works in narrow band，both efficiency and bandwidth can be taken into account；however，when the device works at high frequency and reaches the power index，the low-frequency area is often in a saturated state，where the harmonic component increases，resulting in the increase of gate current，hence the biased gate voltage leading the power decline of the circuit at the low-frequency area. To meet the requirements of broadband work，traditional HEMT devices with Schottky gate metal contacts have been widely used to improve the quality of the gate because the low Schottky forward barrier reduces the reverse leakage of the gate，which is one of the current leakage channels that aggravates the device loss and reduces the effective voltage loaded on the gate. However，when the device works in a saturated state，the gate voltage exceeding the Schottky barrier will lead to a rapid increase in the gate current，and even cause the device to fail.

In this study，a SiN_x gate dielectric was introduced to compose an AlGaN/AlN/GaN MIS-HEMT structure，which is supposed to reduce the gate reverse leakage and improve PAE. Effects of the dielectric layer on swing of the gate forward working voltage，interfacial status，and frequency characteristics of devices were investigated，thus its feasibility of applying in broadband circuits was demonstrated.

The AlGaN/GaN epitaxial material structure used in this study is shown in Fig. 1（a）and described from bottom to top as the following. A SiC substrate with thickness of 280 nm was used for heat dissipation. An18-nm AlN nucleation layer was used to release the lattice stress generated by the lattice and thermal adaptation，and to reduce the background carrier concentration. Then，a high resistance GaN buffer layer with a thickness of 2 µm was deposited to reduce device leakage and avoid efficiency loss caused by device leakage in high fields. Another AlN layer with a thickness of 1 nm was deposited to form a deeper and narrower quantum well，which is beneficial to improve the channel electron density. With the AlN layer，the following undoped AlGaN barrier layer，with its thickness of 14 nm，were prevented from penetration of two-dimensional electron gas，so that the disordered scattering of carriers and the channel electron mobility could be improved ^［9］. A SiN_x cap layer was then deposited to generate compressive stress on the barrier layer and negative charge caused by polarization，strengthening the positive charge on the upper surface of the AlGaN barrier layer，so as to shield the influence of AlGaN surface state on channel electrons，and help to suppress the current collapse effect ^［10］. The square resistance of the final material is 310 Ω/square，and the mobility is more than 2 000 cm²/Vs.

The structure of the device is shown in Fig. 1（a-c）. The source-drain distance （L_SD），gate-drain distance （L_GD），gate length，and gate cap of the MIS-HEMT are 2.4 µm，1.15 µm，0.25 µm，and 0.7 µm，respectively. The recessed-gate width and depth are 0.8 µm and 6 nm，respectively. The three-dimensional shape of the gate adopts a U-shaped gate structure ^［11］，which is helpful to reduce the electric field between the gate and drain and to improve the breakdown voltage，thereby the output power density and power additional efficiency of the device are improved.

The device is fabricated by a 0.25 µm gate length process：started by W metal as the marker，then source and drain recess was etched，Ti/Al/Ni/Au layers as the ohmic contact metal stack were deposited. Finally，the structure was rapid thermal annealed at 800 ℃ for 30 s in N₂ ambient，yielding a contact resistance of 0.3 Ω·mm，and device isolation was formed utilizing multiple-energy nitrogen ion implantation.

In order to improve the gate control ability and reduce the distance from the gate to the channel，a low-damage atomic layer etching method was used to obtain the recessed-gate ^［12］. The 5 nm SiN_X layer was deposited by PEALD method and was annealed at 650 ℃ for 4 minutes in O₂ atmosphere ^［13］. T-shaped gate was subsequently accomplished by electron beam lithography （EBL）of UVIII/Al/PMMA resist stack. Width of the T-gate foot and head are 0.25 μm and 0.7 μm respectively. Ni/Au layers were deposited as gate metal，then the device was passivated with 200 nm SiN_x grown by PECVD method. Finally，the electrodes were electroplated to 3 μm. The finished device is shown in Fig. 1（b），and Fig. 1（c）shows a TEM picture of the gate area.

The capacitance-voltage（C-V）measurement is employed to estimate interface trap density. The frequency dispersion of the second slope in the C-V curves ^［14］ was analyzed with F_M varying from 10 kHz to 500 kHz at R.T.，as shown in Fig. 2. The slight dispersion （∆V =0.24 V）was used to calculate the interface trap density D_it，which falls below 7.1×10¹² cm^-2eV^-1 with E_C decrease from -0.33 eV to – 0.45 eV，as shown in the inserted image；this low D_it indicates high interface quality of the MIS-HEMT.

The DC current-voltage characteristics of the AlGaN/GaN MIS-HEMTs were measured by keithly 4 200，and the output characteristics of the device are shown in Fig. 3（a）. When V_GS = 2 V，the maximum output density was 1.2 A/mm，and the knee voltage （V_KNEE）was 2.9 V，which reveals a good ohmic contact that promise low energy consumption and high device efficiency.

Transfer characteristics were measured at V_DS= -6 V with the V_GS ranging from -6 V to 2 V. A maximum G_M of 435 ms/mm was attained when V_GS = -0.2 V，and threshold voltage （V_TH）was measured to be -1.6 V，while V_GS = 2 V，I_DS = 1.2 A/mm，as shown in Fig. 3（b）.

The validity of the MIS-HEMT is generally reflected by gate performance. As shown in Fig. 3 （c），with V_GS sweeping from -30 V to 6 V，the leakage current merely reached 1×10^-8 A/mm when the reverse V_GS = -30 V，which is 2~3 orders of magnitude lower than that of the normal Schottky gate structure. Meanwhile，when the positive gate voltage V_GS = 6 V，the forward current reached 3.59×10^-6 A/mm，that demonstrated the prior reliability of the improved MIS-HEMT device even under a high V_GS. Meanwhile，the Schottky barrier is maintaining only 1.6 V～1.8 V^［12］. Therefore，the gate voltage of the MIS-HEMT has a wider control range，which ensures its reliability under condition of saturated power serving.

The breakdown voltage was measured with the V_GS fixed at -4，the leak voltage V_DS set from 0 V to 150 V，and the breakdown current limited to 10 mA/mm. When V_DS varied from 0 V to 53 V under the off-state operation，the current was below 10^-4 A/mm. However，the leakage rose sharplywhen V_DS reached 105 V，which was the drain-source breakdown voltage of the device.

The small signal characteristics of the device were measured by Agilent E8363B Vector Network Analyser，with a test range from 100 MHz to 40 GHz and V_GS=-0.5 V. As shown in Fig. 4，when V_DS = 10 V，the current-gain cutoff frequency f_T= 49.5 GHz，the unit-power-gain frequency f_max= 88.1 GHz. When V_DS = 30 V，the f_T= 39.2 GHz，f_max = 100.9 GHz. The increasing initial gain results in an increase in the corresponding MAG. The f_T and f_max under different V_DS，as depicted in the inserted diagram，suggests that as V_DS grows，f_T drops because the increasing V_DS enhances the electric field strength between the gate and drain，hence the deterioration of the electron saturation velocity. The decrease of trans-conductivity reduces f_T，whereas f_max，despite the current decreases，the increase of RF power caused by the higher V_DS increases the gain of the device，and the corresponding f_max，up to power gain equal to one also increases. As the V_DS approaches V_BR/2，the power reaches its saturation point，the f_max also reaches the highest，where V_DS = 30 V and f_max = 102 GHz，as shown in the insert diagram.

The pulse I-V test，with the pulse cycle time，pulse signal width，and the duty cycle set to 10 μs，200 ns，and 2%，respectively，was conducted to estimate current collapse of the device. The static offset point was set to （V_GSQ，V_DSQ）= （0 V，0 V）and the electrical stress for the gate area was set to （V_GSQ，V_DSQ）= （-6 V，0 V），and the results are shown in Fig. 5. The extent of the I_DS deterioration with different electrical stress reveals the effect of the traps related to the gate on the frequency dispersion effect，namely the gate-lag. The gate-lag of this device was 1.83% at the knee point voltage position，and it was almost completely restored when V_DS = 10 V. When the drain voltage stress increased to （V_GSQ，V_DSQ）= （-6 V 10 V）/ （-6 V，15 V）/ （-6 V 20 V），the drain-lag values were 8.46%，12.77%，and 17.93% at the V_KNEE respectively；the results suggest that the current collapse effect of the device is well controlled.

Based on the Load-Pull test system，a 4×50 μm gate-width device was tested by a continuous wave （CW）signal with a frequency of 5 GHz. Load pull at the input and output impedance points was carried out on all devices for optimal efficiency. Device was biased in class AB at I_BIAS=10% I_MAX. The device power curve obtained is shown in Fig. 6（a），when V_GS= -0.5 V，V_DS=10 V，the input impedance point Zsource =43.84 + j95.28，and the output impedance point Zload =147.18 + j76.48，the linear gain of the device was 17.2 dB，the saturated output power was 24.3 dBm （1.4 W/mm），and the PAE reaches 76.1%. A similar pattern was observed when V_DS increased to 30 V，as shown in Fig. 6（b），the linear gain of the device was 20.1 dB，the saturated output power was 30.7 dBm （5.9 W/mm），and the PAE reached 63.2%. Output power and efficiency under wider V_DS range was further evaluated，and a comparison diagram is shown in Fig. 6（c），which suggests that the increasing V_DS enhanced the output power with only a slight decrease in the efficiency. The above results indicate the excellent efficiency performance of the device at 5 GHz.

Additionally，noted from Fig. 6（a）and （b）that the maximum efficiency of the device was obtained at the P_3dB compression point. Since the static voltage of the device is V_GS= -0.5 V and the V_PP is 2 V when P_IN = 10 dBm，the voltage on the gate is higher with the increase of P_IN. The positive wide swing of the MIS-HEMT device ensures the stability of the gate and the device efficiency continues to increase.

As mentioned，when the device is further compressed，the harmonic component increases and the gate current fluctuates. Fig. 7 shows the gate current recorded by power sweeping in the load-pull test. When P_IN = 15 dBm，the power was compressed within 5 dB，and the gate current maintained at 10^-5 A/mm. When the device is further compressed to 6 dB，the gate current rises to 1.7 mA/mm，and the device still worked normally. The excellent gate characteristics of MIS-HEMTs ensure the power flatness working in full frequency band.

The device was also evaluated by a CW large signal with a frequency of 5，10 and 30 GHz. Device was biased in class AB at 0.1 A/mm（～10% I_DS，max），and V_DS was fixed at 30 V. The load reflection coefficient （Γ_load）was tuned considerating both efficiency and output power. The output power of all frequency points exceeded 5 W/mm. As shown in Fig. 8，with the frequency of 5，10，and 30 GHz，the lineal gains are 20.1 dB，16.9 dB and 10.3 dB，respectively，and the efficiency under high output power are 63.2%，59.4% and 50.4%，respectively. The results demonstrate the outstanding performance of the device for broadband application.

In this study，a SiN_x dielectric layer was stacked on the Schottky gate of HEMT to improve the characteristics of the gate project. Performance results in DC，small signal，and large signal tests suggest that the high-quality dielectric layer not only reduced the reverse leakage of the gate，but also increased the positive swing of the gate bias voltage，hence the guarantee for device serving reliably under saturated output power condition. Based on the device design and process conditions in this study，the additional power efficiency reached 75.6% at C-band when low drain bias voltage V_DS=10 V，and 63.2% when V_DS=30 V. Even when the frequency increased to 30 GHz with V_DS kept at 30 V，PAE still maintained at 50.4%. The high efficiency of devices is the prerequisite for the communication system，providing device-level guarantee for stable power system and design of broadband circuit.