【Member Papers】University of Electronic Science and Technology of China Professor Luo Xiaorong's team β-Ga₂O₃ Latest Progress

      Recently, the joint research team of Professor Luo Xiaorong from UESTC and Special Researcher Xu Guangwei from USTC has made the latest progress in the degradation model and reliability reinforcement technology of NiO/β-Ga₂O₃ heterojunction transistors. The study was titled as “Experimental investigation on the instability for NiO/β-Ga₂O₃ heterojunction-gate FETs under negative bias stress", was published in the Journal of Semiconductors. Dr. Zhuolin Jiang, UESTC, is the first author, Professor Xiaorong Luo of UESTC and Researcher Guangwei Xu of UESTC are co-corresponding authors.

Research Content and Graphics

      NiO proved to be very suitable for β-Ga₂O₃ power devices, but the mutated NiO/β-Ga₂O₃ heterojunction may produce serious trap effects, including degradation of threshold voltage (V_TH) and current under negative bias stress, which seriously affect the operation and use of the device. The instability mechanism of NiO/β-Ga₂O₃ heterojunction gate field-effect transistor (HJ-FET) under different gate stresses and stress times has been investigated experimentally by UESTC and USTC, which provides important theoretical guidance for studying the reliability of NiO/β-Ga₂O₃ heterojunction devices in power electronics applications.

FIG. 1 (a) Schematic diagram of NiO/β-Ga₂O₃ HJ-FET cross-section and its manufacturing process; (b) monopulse; (c) delayed multi-pulse experimental process

      In this paper, NiO/β-Ga₂O₃ heterojunction gate FET is manufactured according to the experimental steps shown in FIG. 1(a), and then the pre and post stress test shown in FIG. 1(b) and the delayed multi-pulse stress test shown in FIG. 1(c) are carried out respectively.

      As shown in Figure 2(a), when V_G,Str=-5V, the V_GS-I_DS curve hardly changes in the voltage region from -7.5V to -5V, while in the region from -10V to -7.5V, the I_DS current is reduced by the impulse, but the V_GS-I_DS curve gradually returns to the initial value when t_r=1000s. A similar phenomenon can be observed in the V_GS-I_GS curve, as shown in FIG. 2(b). The ΔV_TH extracted from FIG. 2(c) shows an increase after NBS and a gradual decrease with the increase of t_r. After stress withdrawal t_r=1000s, V_TH almost returns to the initial value. Figure 2(d) shows that I_GS,off decreases by 84% at V_G,Str=-5V and then gradually returns to its initial value. In addition, V_G,Str=-10V causes I_GS,off to drop by about 87%. It can be seen from Figure 3(a) that the degradation mechanism of the device changes under high V_G,Str stress. When V_G,Str=-20V, the subthreshold and linear region currents of the transfer characteristics increase after NBS, while the current in the off state region (-10V<V_GS<-9V) hardly changes. A high V_G,Str stress results in a decrease in ∆V_TH and a near-permanent negative degradation, compared to an increase in ∆V_G,Str caused by NBS, while a higher V_G,Str results in a greater permanent negative drift, as shown in Figure 3(b).

FIG. 2 Monopulse experiment. (a) V_GS-I_DS; (b) V_GS-I_GS changes; (c) ΔV_TH; (d) I_GS,off degradation

FIG. 3 Monopulse experiment. (a) The change of V_GS-I_DS at V_G,Str=-20V; And (b) ΔV_TH extracted under V_G,Str=-15V/-20V stress

      In this study, NBS experiments for a long time_s were also performed on another NiO/β-Ga₂O₃ HJ-FET with the same structure, which was not treated with any electrical stress. The curves at t_r=2000s almost coincided with those immediately after completing NBS t_s=1000s, indicating that no recovery process had occurred, as shown in Figure 4(a). The extracted ∆V_TH indicates that when t_s<20s, the device characteristics are relatively stable and do not start to drift. However, when t_s≥20s, V_TH starts to move negatively and finally shifts negatively by about 2V, while no recovery sign appears when t_r=2000s, as shown in FIG. 4(b).

FIG. 4 NBS experiment with ts increase. (a) V_GS-I_DS change; And (b) extracted ΔV_TH

      In this paper, an interfacial dipole ionization model is proposed to explain the mechanism of HJ-FET's almost permanent decline under NBS conditions. As shown in Figure 5(a), the presence of interfacial dipoles cancels out the internal electric field generated by ionized impurities in the space charge region (SCR) due to the discontinuity of the interfacial energy band of the NiO/β-Ga₂O₃ heterojunction. The larger V_G,s creates an extremely large electric field at the interface, causing some of the interfacial dipoles to ionize into electrons and holes, as shown in Figure 5(b). Under the stress electric field, the electrons and holes move in the direction of β-Ga₂O₃ and NiO, respectively, and compound with the ionizing charge in the SCR, causing the SCR to thin, the on-resistance to decrease, and the V_TH to shift negatively and not recover, as shown in Figure 5(c).

FIG. 5 Schematic diagram of degradation mechanism of NiO/β-Ga₂O₃HJ-FET. (a) Initial state; (b) at NBS; And (c) after NBS

      The effect of stress on the electrical characteristics of NiO/β-Ga₂O₃ HJ-FET under different stress voltages and stress times is studied. At lower V_G,s and t_s values, the NiO trap traps electrons, which reduces gate injection of electrons, resulting in lower leakage current. Fortunately, it fully recovers after a longer t_r. As the stress voltage or stress time increases, ionization of the interfacial dipole widens the current conduction path, resulting in an increase in current and a negative V_TH  shift. This is a near permanent degradation for the NiO/β-Ga₂O₃ heterojunction and should be taken into account in engineering.