Gallium nitride (GaN) devices have been successfully commercialized due to their superior performance, especially their high-power transformation efficiency. To further reduce the power consumption of these devices, the optimization for the ohmic contacts is attracting more and more attention. In the light of the mature and powerful machine learning (ML) techniques, this work provides a novel method to evaluate the fabrication processes of the ohmic contacts in AlGaN/GaN heterojunction, n-type, and p-type GaN, by establishing a regression-based model. The proposed model can not only investigate the influence weight of each process but also predict the contact resistance by inputting the desired recipes. A website ( http://ohmic.zeheng.wang/ ) containing the successfully trained model for the readers’ interests is also provided, which, we believe, would benefit the society of the process development and optimization.

A Machine Learning-Assisted Model for GaN Ohmic Contacts Regarding the Fabrication Processes

Insulated-gate GaN-based transistors can fulfill the emerging demands for the future generation of highly efficient electronics for high-frequency, high-power and high-temperature applications. However, in contrast to Si-based devices, the introduction of an insulator on (Al)GaN is complicated by the absence of a high-quality native oxide for GaN. Trap states located at the insulator/(Al)GaN interface and within the dielectric can strongly affect the device performance. In particular, although AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) provide superior properties in terms of gate leakage currents compared to Schottky-gate HEMTs, the presence of an additional dielectric can induce threshold voltage instabilities. Similarly, the presence of trap states can be detrimental for the operational stability and reliability of other architectures of GaN devices employing a dielectric layer, such as hybrid MIS-FETs, trench MIS-FETs and vertical FinFETs. In this regard, the minimization of trap states is of critical importance to the advent of different insulated-gate GaN-based devices. Among the various dielectrics, aluminum oxide (Al2O3) is very attractive as a gate dielectric due to its large bandgap and band offsets to (Al)GaN, relatively high dielectric constant, high breakdown electric field as well as thermal and chemical stability against (Al)GaN. Additionally, although significant amounts of trap states are still present in the bulk Al2O3 and at the Al2O3/(Al)GaN interface, the current technological progress in the atomic layer deposition (ALD) process has already enabled the deposition of promising high-quality, uniform and conformal Al2O3 films to gate structures in GaN transistors. In this context, this paper first reviews the current status of gate dielectric technology using Al2O3 for GaN-based devices, focusing on the recent progress in engineering high-quality ALD-Al2O3/(Al)GaN interfaces and on the performance of Al2O3-gated GaN-based MIS-HEMTs for power switching applications. Afterwards, novel emerging concepts using the Al2O3-based gate dielectric technology are introduced. Finally, the recent status of nitride-based materials emerging as other gate dielectrics is briefly reviewed.

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Status of Aluminum Oxide Gate Dielectric Technology for Insulated-Gate GaN-Based Devices

Widegap-channel Al0.65Ga0.35N/Al0.3Ga0.7N/AlN/SiC metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with ultrasonic spray pyrolysis deposition (USPD) grown Al2O3 gate-oxide demonstrating enhancement-mode (E-mode) operation are investigated for the first time. The E-mode operation was achieved by using fluorine ions (F−) implantation. In comparison, conventional Schottky-gate device (sample A) and MOS-HFET (sample B) showing depletion-mode (D-mode) operation were fabricated on the same epitaxial structure. The device characteristics with respect to different gate-to-drain spacings ( $L_{GD}$ ) of $6~\boldsymbol {\mu }\text{m}$ and 14 $\boldsymbol {\mu }\text{m}$ have also been studied. The present E-mode Al0.65Ga0.35N/Al0.3Ga0.7N/AlN MOS-HFET (sample C) with $L_{GD} =6$ (14) $\boldsymbol {\mu }\text{m}$ has demonstrated improved maximum drain-source current density ( $I_{DS,max}$ ) of 206.3 (163.5) mA/mm at $V_{DS} =20$ V, maximum extrinsic transconductance ( $\text{g}_{m,max}$ ) of 32.9 (22.0) mS/mm, on/off-current ratio ( $I_{on}/I_{off}$ ) of 3.7 ${\times }\,\,10^{9}$ ( $1.8\,\,{\times }\,\,10^{9}$ ), two-terminal off-state gate-drain breakdown voltage ( $BV_{GD}$ ) of −370 (−475) V, and three-terminal on-state drain-source breakdown voltage ( $BV_{DS}$ ) of 330 395 V.

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Enhancement-Mode Characteristics of Al₀.₆₅Ga₀.₃₅N/Al₀.₃Ga₀.₇N/AlN/SiC MOS-HFETs

CMOS-compatible GaN-based high electron mobility transistors with gate-first technology

We studied the mechanism of low-temperature-annealed Ohmic contacts to AlGaN/GaN heterostructures via formation and removal of Ta/Al/Ta Ohmic-metals. Multi-probe Hall device measurements show one order increase in the sheet electron concentration after Ohmic-metal formation compared with that before formation, indicating that high-density doping takes place in the AlGaN/GaN heterostructure under the Ohmic-metal. However, after Ohmic-metal removal, the increased sheet electron concentration returns to the value before formation. Moreover, we formed Ni/Au Schottky contacts on the AlGaN/GaN heterostructures before Ohmic-metal formation and after Ohmic-metal removal, and confirmed that the characteristics are almost the same. These results indicate that donors do not exist after Ohmic-metal removal, suggesting that, although high-density doping takes place, high-density donors are not formed under the Ohmic-metal. The high-density doping without high-density donors could be attributed to polarization doping, playing a significant role in Ohmic contact formation. 

Mechanism of low-temperature-annealed Ohmic contacts to AlGaN/GaN heterostructures: A study via formation and removal of Ta-based Ohmic-metals

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssa.202000585

Surface Preconditioning and Postmetallization Anneal Improving Interface Properties and V th Stability under Positive Gate Bias Stress in AlGaN/GaN MIS-HEMTs

Anthony Calzolaro

Papers

Surface Preconditioning and Postmetallization Anneal Improving Interface Properties and V th Stability under Positive Gate Bias Stress in AlGaN/GaN MIS-HEMTs

Status of Aluminum Oxide Gate Dielectric Technology for Insulated-Gate GaN-Based Devices

Material investigations for improving stability of Au free Ta/Al-based ohmic contacts annealed at low temperature for AlGaN/GaN heterostructures