Innovative Solutions to Overcome Challenges in Gallium Oxide Technology

In any electronics application today, higher power density is desired. Switches are an integral part of every power converter, impacting its efficiency directly. RF transistors are the backbone of high-frequency power amplifiers for base stations or radars, impacting not only the sustainability of 5G but also national security and resilience. Although Silicon has been the backbone of electronics, it is amply clear when high temperature, high frequency, and high-power density when required simultaneously for an application, wide bandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) are the forerunners. Already commercialized in some RF and power applications, they have pushed the roadmap further beyond Si, owing to their superior material quality. While SiC and GaN are making impressive headway, even wider bandgap semiconductors are being evaluated in labs to understand their role in electronics and optoelectronics. One of the emerging ultra-wide bandgap (UWBG) materials is Ga₂O₃. Vertical MOSFET was a critical milestone in the gallium oxide (Ga₂O₃) roadmap that was recently achieved [1]. Due to the lack of an effective current blocking layer in Ga₂O3, which is essential for any DMOS-like (double-diffused MOSFET) the achievement of a vertical MOSFET has been difficult. Using two novel findings first, a selective diffusion doping technique utilizing magnesium (Mg) doping spin-on-glass (SOG) as a dopant source to form a current blocking layer (CBL), and second, the first demonstration of a vertical Ga₂O₃ MOSFET with the Mg diffused CBL- Vertical Diffused Barrier Field-Effect-Transistor (VDBFET). At its very first stage of development, the device exhibited an excellent field effect-transistor (FET) behavior with a high on-off ratio of 10⁹ and a decent saturation. If successful, this technology can lead to at least a 3x increase in power density in vertical switches over and above SiC and GaN, and at a lower cost per die.<br/>Besides the lack of p-type Ga₂O₃, thermal management has been a challenge. For achieving high power density at high frequencies, with aggressive device scaling heat generation is unavoidable. However, the room temperature thermal conductivity of β-Ga₂O₃ (anisotropic; 11-27 W/mK) is the lowest among the technologically relevant semiconductor materials [2].<br/>Polycrystalline diamond epitaxial growth on β-Ga₂O₃ for device-level thermal management was performed and reported in our recent study. Our study focused on establishing diamond growth conditions on β-Ga₂O₃accompanying the study of various nucleation strategies [3]. A growth window was identified, yielding uniform-coalesced films while maintaining interface smoothness. In this first demonstration of diamond growth on β- Ga₂O₃, a diamond thermal conductivity of 110 ± 33 W m⁻¹ K⁻¹ and a diamond/β- Ga₂O₃ thermal boundary resistance of 30.2 ± 1.8 m²K G⁻¹ W⁻¹ were measured. The film stress was managed by growth optimization techniques preventing delamination of the diamond film. It marked a significant step toward device-level thermal management of β-Ga₂O₃ electronic devices.<br/>These two recent experimental studies set the stage to systematically study and understand the challenges of Ga₂O₃technology and create a road map beyond GaN<br/><b>References</b><br/>1. K. Zeng, R. Soman, Z. Bian, S. Jeong, and S. Chowdhury, "Vertical Ga2O3 MOSFET With Magnesium Diffused Current Blocking Layer," <i>in IEEE Electron Device Letters</i>, vol. 43, no. 9, pp. 1527-1530, Sept. 2022<br/>2. S. Choi, S. Graham, S. Chowdhury, E. R. Heller, M. J. Tadjer, G. Moreno, and S. Narumanchi, "A perspective on the electro-thermal co-design of ultra-wide bandgap lateral devices", Appl. Phys. Lett. vol. 119, p. 170501, 2021.<br/>3. M. Malakoutian, Y. Song, C. Yuan, C. Ren, J. S. Lundh, R. M. Lavelle, J. E. Brown, D. W. Snyder, S. Graham, S. Choi, and S. Chowdhury, “Polycrystalline diamond growth on β-Ga₂O₃ for thermal management,” <i>Applied Physics Express</i>, vol. 14, no. 5, p. 055502, 2021