【Member Papers】New Breakthroughs by Domestic Joint Research Team have Promote the Development of Gallium Oxide in the Field of RF Devices

      Gallium Oxide is an excellent representative of ultra-wide band gap semiconductor materials. Because its bandgap width and breakdown field strength are much higher than GaN, it can not only work at higher field strength and higher working voltage, greatly improve the output power density, but also realize applications in extreme environments such as high temperature and strong irradiation.

      Johnson’s figure of merit (JFOM) is related to the product of the material's breakdown field strength and the maximum carrier speed, and comprehensively considers the frequency and power characteristics of RF devices. It is the most important indicator to measure the comprehensive performance of RF devices. The JFOM of Gallium Oxide is about 2.6 times that of GaN, indicating that Gallium Oxide also has considerable application prospects in the field of RF devices.

      The United States Air Force Research Laboratory, University at Buffalo and other foreign institutions have successively carried out research on Gallium Oxide RF devices. At present, Gallium Oxide RF devices are faced with many problems. Due to the restriction of short channel effect, mobility, thermal conductivity and other factors, the performance indicators of Gallium Oxide RF devices such as frequency, current density and power density are still at a low level. In particular, the thermal conductivity of Gallium Oxide is extremely low, only 10W/m⋅K, which is 2% of SiC, resulting in extremely poor heat dissipation of Gallium Oxide RF devices. The development of Gallium Oxide RF devices is greatly hampered by the serious self-heating effect, which makes it difficult to improve the power and frequency of devices.

      To address these challenges, a joint research team composed of Nanjing University, Nanjing Institute of Electronic Devices, Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, Suzhou Institute of Nanotechnology and Nanobionics, Chinese Academy of Sciences, and Hangzhou Institute of Xidian University proposed an architecture design based on the heterogeneous integration of high thermal conductivity Silicon Carbide substrate and Gallium Oxide RF devices. Wafer level heterointegration of a 2-inch high-quality SiC substrate with a 50nm ultra-thin Gallium Oxide film was achieved using universal ion knife stripping and transfer technology (FIG. 1a), combined with low-energy ion implantation channel technology (IEEE Electron Device Lett. 44, 1060, 2023). For the first time in the world, it realizes the Silicon Carbide based Gallium Oxide heterogeneous integrated radio frequency device, and the frequency performance of the device reaches the highest value reported publicly at present. Related achievements are titled "Heterointegrated Ga₂O₃-on-SiC RF MOSFETs with f_T/f_max of 47/51 GHz by Ion-cutting Process". Is published online October 24, 2023 in IEEE Electron Devices Letters.

(Screenshot of paper)

DC characteristics of devices

      The Gallium Oxide heterogeneous integrated RF device with gate length of 0.1μm has a current density of up to 661 mA/mm and on-resistance as low as 24Ω·mm. The device turns off well at -8V gate voltage without significant short channel effect, as shown in Figure 1b. At a leakage voltage of 15V, the threshold voltage is -7.5V and the transconductance is 57mS/mm. At -12V offset gate voltage, the off-state current is lower than 1nA/mm, and the breakdown voltage reaches 90V, indicating that the device does not have the leakage barrier decline effect even at high field.

      Comparative study shows that the current density of Gallium Oxide homogeneous substrate devices with the same structure is only 235mA/mm, which shows obvious self-heating effect. This indicates that the heterogeneous integration technology can effectively improve the heat dissipation capacity of the device. Combined with C-V and I-V curves, the electron surface concentration, mobility and sheet resistance of the channel under different gate pressure are extracted. The results show that by making full use of the advantages of low activation temperature of Gallium Oxide silicon-ion and small thermal diffusion length, ultra-thin conductive channels similar to 2DEG with high electron concentration can be achieved through low-energy silicon-ion implantation combined with rapid annealing technology. The channel electrons are mainly distributed within 10nm from the surface of Gallium Oxide, which can effectively solve the short channel effect of RF devices (FIG. 1c).

      When the gate voltage is 8V, the electron surface density is as high as 1.83×10¹³cm^-2, the mobility is 67cm²/V·s, and the channel sheet resistance is as low as 5.07kΩ/sq. When the gate voltage is reduced to -1V, the electron surface density decreases to 1.1×10¹²cm^-2, and the corresponding mobility increases to 118cm²/V·s (Figure 1d). Compared with Gallium Oxide homogeneous substrate devices, the channel mobility is significantly improved due to the effective inhibition of self-heating effect, which is also an important reason for the increase of the current density of Gallium Oxide heterogeneous RF devices.

FIG. 1. Schematic diagram of cross-sectional structure of SIC-based Ga₂O₃ heterogeneous integrated RF device (a); (b) DC output characteristics; (c) C-V curve and extracted electron concentration distribution; (d) electron surface concentration, mobility and sheet resistance of the channel at different gate voltage.

Frequency characteristics and power output performance

      The cutoff frequency f_T and the maximum oscillation frequency f_max of the device with gate length of 0.1μm reach 47GHz and 51GHz respectively, which is the highest level publicly reported for Gallium Oxide RF devices at present, as shown in Figure 2a. Figure 2b and 2c compare cutoff frequency f_T and maximum oscillation frequency f_max of a Gallium Oxide RF device with different gate lengths. When a conventional bulk doped channel is used, the cutoff frequency f_T of the device no longer increases with the decrease of the gate length when the gate length is reduced to less than 0.2μm, indicating that the device has an obvious short channel effect. After using low-energy ion implantation channel and heterointegration technology, the f_T·L_G of the 0.5μm gate length device is 5.45GHz·μm, and the corresponding carrier saturation speed is as high as 3.42×10⁶ cm/s, which is comparable to the RF device with AlGaO/Ga₂O₃ modulated doping structure. The results show that a carrier transport channel with similar performance to 2DEG can be obtained by using low-energy ion implantation channel technology.

      The power output characteristics of a heterogeneous Gallium Oxide integrated RF device with a gate length of 0.1µm at 2GHz were tested by a load traction system. Operating in continuous wave (CW) mode, the device has a saturated output power density of 296mW/mm, a power gain of 11dB, and a maximum power added efficiency (PAE) of 25.7%. With the increase of input power, the power gain remained at a high level, and there was no significant decrease before the output power was saturated.

      FIG. 2d compares the output power density of publicly reported Gallium Oxide RF devices at different frequencies. The output power density is inversely proportional to the operating frequency, and the output power density under continuous wave is generally lower than that under pulse mode. At present, the reported maximum output power of CW at 2GHz is 213mW/mm, and its drain operating voltage is 20V. Thanks to higher saturation current density and better heat dissipation capability, the Gallium Oxide heterogeneous integrated RF device developed in this work can achieve a CW output power density of 296mW/mm at 15V operating voltage.