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27 October 2023

Research Progress in Breakdown Enhancement for GaN-Based High-Electron-Mobility Transistors

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China Astronautics Standards Institute, Beijing 100071, China
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Electronics2023, 12(21), 4435;https://doi.org/10.3390/electronics12214435
This article belongs to the Special Issue New Semiconductor Materials, Devices, Power Applications, and Radiation Effects
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Abstract

The breakdown characteristics are very important for GaN high-electron-mobility transistors (HEMTs), which affect the application voltage, power density, efficiency, etc. In order to further enhance the breakdown voltage of the device, it is necessary to carry out research on the breakdown mechanisms of the device. This article summarizes several breakdown mechanisms of GaN devices, including electric field concentration, buffer leakage current, gate leakage current, and vertical breakdown. In order to suppress the breakdown mechanisms, techniques such as the use of a field plate, reduced surface field (RESURF), back barrier, gate dielectric, substrate removal, and addition of AlGaN channels can be developed. With the continuous development of various technologies, the breakdown characteristics of GaN devices can be fully explored, laying the foundation for improving the performance of power electronic systems.

1. Introduction

Power electronic systems are of crucial importance for generating and converting energy in a sustainable way. As one of the key components for efficient and energy-saving power electronic systems, power semiconductors must be updated with high-voltage devices featuring outstanding performance in static and dynamic energy loss, current density, operating voltage and temperature, etc. [1,2,3,4]. Although silicon (Si) power devices have been widely used in diverse fields and industries, their application prospects are hindered by the limited bandgap and lower critical breakdown electric field.
As the most representative material of the wide bandgap semiconductors, gallium nitride exhibits overwhelming superiorities in terms of band gap, critical breakdown electric field, electron saturation speed, etc., attracting more attention for various applications. Emerging as the most attractive key device in high-power and high-efficiency power conversion systems, the existing fabricated GaN power transistors are characterized by the advantages of high breakdown voltage, high switching speed, low conduction resistance, and high conversion efficiency, and have become an ideal option for future power switch applications.
With respect to power devices, high breakdown voltage properties can improve their potential for different applications. Gallium nitride high-electron-mobility transistors (HEMTs) have prospered ever since M. Asif Khan et al. demonstrated the first AlGaN/GaN HEMT on a sapphire substrate in 1993 [5]. Later, GaN chips gradually became commercialized in 2010, after which, the research interest in GaN HEMTs primarily focused on the devices’ breakdown voltage, normally off characteristics, and reliability. However, the maximum operating voltage is only 650 V for pGaN HEMTs, and 900 V for cascode structures [6,7,8].
This study mainly summarizes the research progress in high-breakdown-voltage nitride HEMTs in recent years. This paper is organized as follows. Section 2 introduces the factors restricting the breakdown performance of GaN HEMT devices, which involve electric field concentration, buffer leakage current, gate leakage current, and vertical breakdown. Section 3 outlines the related approaches and research progress for solving breakdown problems. Section 4 summarizes the conclusions of this paper, presenting an outlook for the future research and applications of gallium nitride power electronics.

2. Breakdown Mechanisms

2.1. Electric Field Concentration

When a conventional gallium-nitride-based HEMT device is biased in off state (VGS < Vth), the electric field line existing in the depletion region directly underneath the gate is perpendicular to the gate electrode. Meanwhile, the shape of the depletion region at the gate edge will bend, causing more electric field lines at the gate edge, where the electric field value is greater than the electric field directly underneath the gate. In real situations, the electric field distributes unevenly in the channel, and the electric field on the drain side of the gate is more concentrated. For this reason, the peak value of the electric field is far higher than that of the access region, as in the peak electric field ① shown in Figure 1.
Figure 1. Possible breakdown mechanisms in GaN-based HEMTs.

2.2. Buffer Leakage Current

Under normal circumstances, gallium-nitride-based materials are epitaxially grown on silicon, sapphire, silicon carbide, or GaN substrates. Due to the lattice mismatch of heterogeneous substrates, defects and impurities can be produced, and the dislocation density of GaN materials is approximately 108 cm−2 [9]. In unintentionally doped GaN films, C, Si, and O are the most common impurities. The concentration of unintentionally introduced carbon is usually in the range of 1015–1018 cm−3, depending on the growth conditions [10,11]. Furthermore, owing to the non-negligible concentration of background electron carriers and those defects and impurities, a potential conductive path may be created, thereby inducing leakage current. When a high voltage is applied to the drain of the device, an excessive leakage current in the buffer layer may result in the pre-breakdown of the device, as the shown path ② in Figure 1. In recent years, researchers have made a lot of optimizations to the buffer layer, such as using a back barrier, AlGaN channel, superlattice, and so on. Other researchers have found that Si doping promotes a reduction in dislocation density [12].

2.3. Gate Leakage Current

Gallium-nitride-based HEMT devices primarily adopt Schottky contacts to form gate electrodes. Due to the limited breakdown capability of Schottky gates and the drastic increase in the leakage current with the increased operating temperature, an excessive gate leakage current from the drain to the gate may result in the pre-breakdown of GaN-based HEMT devices, as shown by path ③ in Figure 1.

2.4. Vertical Breakdown

The quality of substrates and epitaxial materials prevent the vertical breakdown performance of gallium-nitride-based HEMT devices from being improved. Due to the critical breakdown electric field of silicon being 0.3 MV/cm, adopting silicon substrates for fabricating high-breakdown-voltage (>1.5 kV) devices still remains almost unfeasible [4,13,14]. In addition, a large number of dislocations exist in the epitaxial buffer layer, causing a conductive path (④ in Figure 1). Under this circumstance, the leakage current during substrate grounding must not be neglected. Amongst the diverse approaches for improving the breakdown voltage capability of the devices, the most efficient and effective ones are to opt for appropriate substrates and to improve the quality of epitaxial layers.

4. Conclusions

This paper summarizes the research progress of breakdown enhancement for GaN-based HEMTs. The future of high-voltage GaN devices is mainly focused on electric field modulation, reducing leakage current, improving vertical breakdown characteristics, etc.
It is still necessary to continue to study the growth technology of high-quality epitaxial materials, new device structures, new terminal technologies, and improved packaging technologies. The field plate structure can induce more electric field peaks, thereby alleviating the peak electric field at the gate edge and improving breakdown. The RESURF structure can make the electric field more uniform through the pn junction. The back barrier structure or doping process can reduce the leakage of the buffer layer. Adding a gate dielectric can reduce gate leakage. The use of SiC or sapphire substrate can avoid the vertical breakdown of Si substrates. The AlGaN channel adopts materials with a higher critical breakdown electric field. By using these methods, GaN devices can achieve better breakdown characteristics. With the development of technology, the breakdown characteristics of GaN devices will be fully explored, resulting in the higher performance of related power supplies.
Table 1 summarizes the typical voltage enhancement methods for the GaN HEMT devices mentioned in this paper and compares their breakdown voltage, average electric field, and figure of merit. In general, GaN-based HEMTs adopt composite field plates, high-quality passivation, substrate selection, and other technologies to modulate the electric field distribution and reduce leakage current. The highest value of the breakdown voltage of the device has already reached 10 kV, and the FOM has reached 2 GW/cm2, but the average electric field is mostly at the level of 1–2 MV/cm.
Table 1. High-voltage technology summary.
GaN-based HEMTs have attracted much attention in power supply, electric vehicle, data center, aerospace, and other fields due to their large bandgap, high critical breakdown field strength, high mobility of 2DEG, and high saturation speed. GaN-based power electronic devices have broken through the theoretical limit of Si materials and gradually approached the theoretical limit of SiC materials, but are far from reaching the theoretical limit of GaN materials. However, due to the limitations of the breakdown voltage and on-state resistance of the device, GaN products are currently concentrated below 650 V. It is still necessary to continue to study the growth technology of high-quality epitaxial materials, novel device structures, device characteristics, and reliability. High-voltage GaN HEMT devices are likely to be realized on sapphire substrates using modulated electric fields and high-quality passivation.

Author Contributions

Conceptualization, P.L.; methodology, X.J.; formal analysis, D.Y.; investigation, X.J. and D.Y.; data curation, Z.T.; writing—original draft preparation, Z.T.; writing—review and editing, Z.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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