【Member Papers】Heterogeneous integration of ultrawide bandgap semiconductors for radio frequency power devices

日期：2025-11-24阅读：118

Researchers from the Xidian University have published a dissertation titled "Heterogeneous integration of ultrawide bandgap semiconductors for radio frequency power devices" in Science Advances.

Project Suppot

The work in Xidian University was supported by the National Natural Science Foundation of China (NSFC) with the grant nos. 62222407 and 62421005 (to H.Z.).

Background

Radio-frequency (RF) semiconductor devices are widely used in communication, consumer electronics, aerospace, and defense applications, with design goals typically including high cutoff frequency, high maximum oscillation frequency, high output power density, high efficiency, and low noise. Wide bandgap (WBG) and ultrawide bandgap (UWBG) semiconductors, owing to their large bandgap, high critical electric field, and high saturation velocity, are considered key materials for enhancing RF device performance. However, a single UWBG material rarely offers both shallow-level dopants and high thermal conductivity—for example, Ga₂O₃ has low thermal conductivity, whereas AlN and diamond possess high thermal conductivity but lack shallow-level dopants—limiting device output power and reliability.

Heterogeneous integration of different UWBG materials offers a promising solution to this challenge. Existing approaches, including mechanical exfoliation, hydrogen ion cutting, and wafer bonding, enable material combination but face limitations such as restricted flake size, material degradation, and high interfacial thermal resistance. In this work, we develop a novel heterogeneous integration technique that combines mechanical exfoliation, arrayed transfer, and wafer-level direct bonding. This approach ensures good film uniformity and enables large-area arrays, providing a new pathway for the fabrication of high-performance RF devices and integrated circuits.

Abstract

Ultrawide bandgap (UWBG) semiconductors offer high critical electric fields and saturation velocities ideal for radio frequency (rf) devices, but achieving both shallow-level doping and high thermal conductivity (k_T) in a single material remains difficult. We demonstrate a scalable, exfoliation-based layer-transfer process to heterogeneously integrate gallium oxide (Ga₂O₃) thin films with shallow dopants onto high-k_T aluminum nitride (AlN) substrates. This method obviates ion implantation and interfacial dielectric layers used in conventional approaches. A large conduction band offset (3.4 electron volts) at the Ga₂O₃/AlN interface improves electron confinement in the Ga₂O₃ channel. T-gate rf power transistors achieve a maximum oscillation frequency of 90 gigahertz and output power densities of 4.6 watts per millimeter at 2 gigahertz and 4.1 watts per millimeter at 6 gigahertz—among the highest for UWBG devices. A minimal noise figure of 0.48 decibels at 8 gigahertz—among the lowest reported in this frequency range—further highlights the platform’s promise for next-generation rf applications.

Conclusion

In summary, this work presents a Ga₂O₃-on-AlN heterogeneously integrated platform for rf power devices, which can leverage the complementary electrical and thermal properties of multiple UWBG materials. The integration method combines exfoliated transfer and wafer-scale bonding, obviating the ion implantation and interfacial oxide required in many conventional methods, at the same time enabling a uniform geometry of the arrayed thin films with the total size scalable to the wafer level. The Ga₂O₃-on-AlN rf transistor leverages the high potential barrier offered by the AlN/Ga₂O₃ junction to realize tight gate control and gate length scalability in a highly doped Ga₂O₃ channel. In addition, the high E_C of the AlN substrate eliminates the premature substrate breakdown. These synergistic advances in the material, process, and device enable impressive frequency and power performance, including an average electric field over 3 MV/cm, a f_T/f_max of 23.8/90 GHz, a P_out of 4.1 to 4.6 W/mm at 2 to 6 GHz, and a NF_min of 0.48 dB at 8 GHz, with the P_out being the highest in all UWBG rf devices and the noise among the one of the lowest in all representative X-band rf devices. These results suggest great potential of the heterogeneous UWBG platform for high-frequency, high-power, and low-noise rf electronics.

Fig. 1. Heterogeneous integration process and material characterization.(A) Illustration of the Ga₂O₃/AlN heterogeneous integration process. The β-Ga₂O₃ belt or film with uniform geometry is cleaved from the substrate edge and then manually transferred onto the blue tape to form an array. The thickness of β-Ga₂O₃ films on blue tape is reduced by multiple wafer-level bonding process until the thickness reaches around 500 nm. The graphite/Si/β-Ga₂O₃/AlN stack is formed and loaded into the bonding chamber under a 3-hour bonding process to transfer the wafer-size β-Ga₂O₃ film array onto the AlN substrate. (B) Top-view optical microscopic image of part of a single β-Ga₂O₃ film on the AlN substrate. (C) Thickness mapping of 120 β-Ga₂O₃ films in a wafer-size array after being transferred onto the 1-inch AlN substrate. (D) Atomic force microscopy (AFM) image of the surface of β-Ga₂O₃ film transferred on the AlN substrate, revealing an atomic flat surface with an RMS roughness of 0.2 nm. (E) Zoomed-in and HRTEM image at the atomic interface between the β-Ga₂O₃ channel and AlN substrate. (F) XRD scan characteristics of β-Ga₂O₃ on AlN. a.u., arbitrary units. (G) High-resolution rocking curve of transferred and bonded β-Ga₂O₃ with an FWHM of 80 arc sec.

Fig. 2. Electrical and thermal properties of the hetero-UWBG platform and device design.(A) Leakage current characteristics between two mesa structures fabricated on the β-Ga₂O₃ on the AlN sample and the β-Ga₂O₃ on the SiC sample, revealing a 2.5 times higher breakdown voltage in the β-Ga₂O₃ on the AlN sample. (B) Band alignment of SiC, β-Ga₂O₃, and AlN, where a large conduction band discontinuity of 3.4 eV is presented between β-Ga₂O₃ and AlN. (C) Cross-sectional schematic image of β-Ga₂O₃ rf power MOSFETs on the AlN substrate. (D) Top-view SEM and cross-sectional TEM images of a T-gate shaped β-Ga₂O₃ rf power MOSFET with L_G = 150 nm and a gated channel thickness of 30 nm. (E) Extracted thermal resistance (R_T) versus dc power density of identical-geometry Ga₂O₃ MOSFETs fabricated on SiO₂/Si, SiC, and AlN substrates; the same heterogeneous integration process is applied to three substrates. (F) R_T benchmark for this work and other reported Ga₂O₃ transistors on a variety of substrates.

Fig. 3. dc characteristics of the Ga₂O₃-on-AlN MOSFET.(A) Linear-scale I_D-V_DS of a representative Ga₂O₃-on-AlN MOSFET with L_G = 300 nm. (B) Log-scale I_D-V_GS-I_G characteristics at V_DS = 10 V and (C) linear-scale I_D-V_GS-g_m characteristics at V_DS = 1/10/20 V of the same device. (D) Pulsed I_D-V_DS characteristics of the device under three quiescent bias conditions with 5-μs pulse width, 1% duty cycle, and V_GS = 0 V. (E) Three-terminal off-state leakage current and breakdown characteristics of β-Ga₂O₃ MOSFETs fabricated on SiC and AlN substrates. (F) Simulated electric field contour within the Ga₂O₃-on-AlN MOSFET structure at a device breakdown voltage of 216 V, as well as the electric field profiles along the two cutlines in the Ga₂O₃ channel and AlN substrate. Although the breakdown occurs in Ga₂O₃, a high field of 4.1 MV/cm is present in the AlN substrate; if such a substrate field cannot be withstood by the substrate material, premature breakdown will occur in the substrate.

Fig. 4. Small-signal rf characteristics and linearity characteristics of the Ga₂O₃-on-AlN MOSFET.(A) Small-signal gain characteristics of a Ga₂O₃-on-AlN MOSFET with L_G = 300 nm when biased at V_DS = 45 V and at the V_GS corresponding to the peak g_m and (B) extracted f_T and f_max versus V_DS. A f_max = 90 GHz is extracted. (C) Two-tone load-pull P_in sweeps at f = 2 GHz to study the linearity of a device at V_DS = 15 V. A high linearity with OIP3 = 28 dBm is extracted.

Fig. 5. Large-signal rf characteristics of the Ga₂O₃-on-AlN MOSFET.Large-signal performance of a Ga₂O₃-on-AlN MOSFET with L_GS/L_GD/L_G= 400/700/300 nm and W_G = 2 × 30 μm and the (B) extracted P_out and PAE dependence on V_DS. The device is biased at class-AB condition with a frequency of 2 GHz, pulse width of 100 μs, and duty cycle of 10%. A record P_out = 4.6 W/mm at 2 GHz and PAE = 50.5% are achieved. (C) Large-signal class-AB performance of a Ga₂O₃-on-AlN MOSFET with L_GS/L_GD/L_G = 400/700/300 nm and W_G = 2 × 15 μm, with a power sweep at f = 6 GHz and V_DS = 45 V. Inset: Schematic of a Ga₂O₃-on-AlN MOSFET for an amplifier with rf signal amplification (D) P_out and PAE dependence on V_DS from 25 to 45 V at f = 6 GHz. A P_out = 4.1 W/mm at 6 GHz is extracted.

Fig. 6. Benchmark of the rf metrics for the Ga₂O₃-on-AlN MOSFET against the state-of-the-art rf transistors. P_outversus PAE of rf transistors reported in various UWBG material platforms, including diamond, Ga₂O₃-on-Ga₂O₃, Ga₂O₃-on-SiC, Al₇Ga_0.3N, and representative GaN rf transistors, reported at a frequency from 1 to 6 GHz. (B) f_max versus f_T × V_DS of the best data reported in rf transistors based on various oxide materials, including Ga₂O₃, In₂O₃, ITO, IGZO, and IZO, and representative GaN rf transistors. The device with L_G/W_G/L_GD = 0.3/2 × 30/0.7 μm has f_T = 23.3/23.8 GHz at V_DS = 40/45 V and BV = 216 V.

DOI：

doi.org/10.1126/sciadv.adw6167