【Domestic News】Combination of Gallium Oxide Power Devices and DUV Optoelectronic Devices (2)

04 Experimental investigation on the instability for NiO/β-Ga₂O₃ heterojunction-gate FET under negative bias stress

      Ultra-wide band gap β-Ga₂O₃ has high critical breakdown field strength and Baliga’s figure of merit, which meets the demand of power system for higher power, higher power density and higher conversion efficiency, and has attracted great attention in the field of power electronics. However, the lack of effective P-type doping makes the breakdown voltage and power optimum devices well below the theoretical limit. To overcome this challenge and gain the advantage of PN junction devices, multiple P-type oxides have been investigated and heterointegrated with n-type β-Ga₂O₃, among which, P NiO has been widely studied because of its large band width and controllable doping. However, the mutant NiO /β-Ga₂O₃ heterojunction may produce multiple complex trap effects, and may lead to the degradation of electrical parameters such as threshold voltage (V_TH) and current under negative bias stress (NBS). Currently, studies on the degradation mechanism of NiO /β-Ga₂O₃ heterojunction devices under stress are insufficient, which is unfavorable for its further high-power applications.

      Recently, Professor Luo Xiaorong's research group of UESTC cooperated with the School of Microelectronics of USTC of China studied the instability mechanism of NiO /β-Ga₂O₃ heterogate FET under different gate stress voltages (V_G,s) and stress time (t_s), and proposed two different degradation mechanisms of devices under NBS. The results indicate that stress application causes a slight forward shift of the threshold voltage drift (∆V_TH). After the recovery time (t_r) of 1000s, the V_TH almost returns to the initial value. The observed V_G,s = -10 V than V_G,s = -5 V resulted in greater V_TH. Meanwhile, the stress-induced gate-off current degradation gradually recovers with increasing t_r. It is inferred that at low V_G,s and short t_s, bulk trap capture and release of electrons in NiO lead to reduction and recovery of current, respectively. Moreover, it was found that the degradation mechanism will change at high V_G,s. High V_G,s can cause a near-permanent negative movement of the V_TH. Significant recovery of the electrical properties was not observed,which is also observed at lower V_G,s and long t_s. This is because the heterojunction interface dipoles are almost permanently ionized, and the resulting electron-hole pairs bind to the ionization donor-acceptor of the spatial charge region at the heterojunction, resulting in narrowing of the spatial charge region. This study explains the mechanism of degradation of NiO /β-Ga₂O₃ heterojunction devices under NBS, and provides theoretical guidance for the development of NiO / β-Ga₂O₃ heterojunction devices.

      This paper is published in Journal of Semiconductors , titled “Experimental investigation on the instability for NiO/β-Ga₂O₃ heterojunction-gate FET under negative bias stress”.

Figure 1. As measured at the V_{G, s} = -5 V and-10V, t_s = 10s (a) ∆V_TH、(b) I_{GS, off} degradation rate;(c) ∆V_TH measured at V_G,s  = -15 V and-20 V, t_s = 10s;(d) in the V_G,s = -10 V,curve changes of V_G,s-I_DS when t_s ranges from 0 to 1000s, and recovery at t_r = 2000s.
doi: 10.1088/1674-4926/44/7/072803

05 A large-area multi-finger β-Ga₂O₃ MOSFET and its self-heating effect

      β-Ga₂O₃ is an ultra-wide band gap semiconductor material, which is considered a preferred material for the next generation of high-power applications due to its 4.8 eV band gap width, high critical breakdown field strength of 8 MV/cm, and Baliga’s figure of merit of 3444. However, one of the material drawbacks of β-Ga₂O₃ is its very low thermal conductivity (11-27 W/(mK) at 300 K), which leads the device to serious self-heating effect (SHE) in the working state and affects the reliability and stability of the device. Several solutions have been proposed for the severe self-heating effects of β-Ga₂O₃ field-FET devices, such as ion-cutting techniques, transfer to substrates, and structural design. Novel measurements have also been used to characterize the transient temperature distribution of the β-Ga₂O₃ MOSFETs. However, most reports on β-Ga₂O₃ MOSFETs have focused on pursuing high power factors and exploring novel structures, but practical applications require large area structures to meet circuit application requirements. For large area structures, due to the small surface area-volume, the self-heating effect will be more serious than the small volume devices, which is more worthy of study.

      Recently, the team of Professor Long Shibing from the University of Science and Technology of China reported a large-area multi-finger β-Ga₂O₃ MOSFET with an output current of 0.5 A. The paper studied the DC characteristics and used infrared thermal imaging to study the effect of self-heating on β-Ga₂O₃ MOSFET. By extracting the change of transient temperature distribution (Figure 1) and the heat equilibrium temperature under different leakage pressures (Figure 2), the law of heat production and heat dissipation of β-Ga₂O₃ MOSFET are obtained. The highest temperature in the β-Ga₂O₃ MOSFET channel reached 250℃ at 8 V leakage pressure, indicating a severe self-heating effect of the device. This work is the first study on the self-heating effect of large-area β-Ga₂O₃ field-effect transistors, which can be useful for the implementation of gallium oxide in the future.

      This paper is published in Journal of Semiconductors, titled “A large-area multi-finger β-Ga₂O₃ MOSFET and its self-heating effect”.

Figure 1. The drain bias voltage captured by the infrared thermal imaging camera is set to 2 V, and the bias time is (a) 0 second, (b) 10 seconds, and (c) 20 seconds, and (d) 50s respectively, (e) time-dependent characteristic curve of the highest temperature in the β-Ga₂O₃ channel with different drain voltage.

Figure 2. In the thermal equilibrium state(a) temperature distribution image when the drain voltage is 8 volts; (b) The relationship curve of the highest temperature in the β-Ga₂O₃ channel with the applied drain voltage.
doi: 10.1088/1674-4926/44/7/072804

06 Large-area β-Ga₂O₃ Schottky barrier diode and its application in DC-DC converter

      Power devices and circuits are an important part of the electric energy conversion system, At the same time, power devices and circuits based on ultra-wide band semiconductors have more potential to reduce power loss in the conversion process. β-Ga₂O₃ is considered to have great potential in power electronics applications due to its band gap width of about 4.8 eV, high critical electric field of 8 MV/cm and high Baliga’s figure of merit of 3444. These characteristics make β-Ga₂O₃ power devices promising in high voltage, high power and other fields. In the past decade, β-Ga₂O₃ devices, especially the Schottky barrier diode (SBD), have developed rapidly, and their performance has been significantly improved, close to the performance of SiC and GaN. At present, the work of large area devices mainly focuses on the combination with edge terminals, and there are few studies on baseline devices or called terminal-free SBD for high current applications. Our recent work shows that the performance of small area SBD can be greatly improved by interface engineering, and therefore this is an opportunity for large area devices. The high-performance SBD without terminals may better reflect the application potential of Ga₂O₃ SBD. In conclusion, Ga₂O₃ SBD is more mature in its application, and its application potential also needs to be further demonstrated.

      Recently, the research group of Professor Long Shibing / Associate researcher Xu Guangwei of University of Science and Technology of China realized the high-performance large-area β-Ga₂O₃ SBD without terminal structure through the surface treatment process combined with dry-wet method, and realized the efficient conversion in DC-DC. In this work, they removed the unreliable layer on the surface of the epitaxial layer by dry etching, and then repaired the damage formed by etching by wet corrosion, and then completed the growth of ohm electrode and Schottky electrode. The contact area of prepared β-Ga₂O₃ SBD Schottky is 11 mm², with good forward characteristics of 8 A@5 V, low on-resistance of 0.46 Ω and high breakdown voltage of 612 V, and the device performance is at the advanced level. The β-Ga₂O₃ SBD was separately packaged by TO-220 and used to build DC-DC converter. At the input voltage of 200 V, the conversion efficiency of the circuit reached 95.81%, and good circuit performance was achieved.

      The considerable performance of β-Ga₂O₃ SBD devices and their circuits without terminal structure shows that it has great application potential in the field of power electronics, and has important reference value for the development of β-Ga₂O₃ SBD devices and applications.

      This paper is published in Journal of Semiconductors, titled “Large-area β-Ga₂O₃ Schottky barrier diode and its application in DC-DC converter”.

Figure 1. (a) cross-section diagram of β-Ga₂O₃ SBD, (b) positive guide feature and (c) reverse breakdown feature; (d) Performance comparison of the prepared β-Ga₂O₃ SBD device and the reported high-current Schottky diode.

Figure 2. β-Ga₂O₃ SBD-based (a) schematic diagram of the DC-DC converter, (b) test bench;  The β -Ga₂O₃ SBD-based DC-DC converter （c）V_Gs、V_out、I_D and（d）the waveform curve of V_Gs、V_D、I_D

doi: 10.1088/1674-4926/44/7/072805