(Invited) Extraction of Trap Level in GaN Hemt from Transient Current upon Back Gate Voltage Application

Gallium nitride (GaN) have attracted attention as a high frequency and high voltage power device material because they have wide bandgap and high mobility [1]. However, large number of defects exists in the epitaxial layer. Current collapse originated from the traps at the surface and in the buffer layer is one of the results of the defects [2]. To distinguish the bulk traps from the whole current collapse, back-gate voltage application is commonly used [3]. In this study, we measured the bulk traps by measuring the drain transient current by back gate voltage application. TLM pattern ( L /W=120/100 mm) was fabricated on an epitaxial wafer; GaN(2nm)/AlGaN(26nm)/ GaN(150/350nm)/C-GaN/buffer/Si sub. Fig. 1 shows the I d - V B characteristic of the TLM, where a threshold voltage ( V th ) of -300/-450 V can be extracted. A large clock-wise hysteresis suggests large amount of electron trapping in the epitaxial layer. Fig. 2(a) shows the voltage application sequence for measuring the transient I d 2DEG current. A negatively high V B below the V th was applied to clear all the traps initially exists in the epitaxial layers. Then, a step voltage was applied to a certain voltage, and the transient I d was monitored until the value saturates. Fig. 2(b) shows typical transient I d on V B . To analyze the time constant (t) of the transient, the differential of the I d on logarithm time was used, as the time at the peak signal corresponds to the time constant. Fig. 3(a) shows analysis result, and the peak of this graph correspond to t. On the other hand, the amplitude is related to trap density. Fig. 3(b) shows the Alenius plot of the extracted t, and activation energy from 0.2 to 0.3 eV was detected. In conclusion, we have measured the transient I d of GaN-HEMT device with a step voltage application to the substrate back gate. From analysis, we extracted defect levels with activation energy of 0.2 to 0.3 eV. The method is useful for characterizing the trap density in the epitaxial layers. [1] T. Mizutani, et al. , IEEE Trans Electron Devices, 50 , 10, 2015 (2003). [2] G. Meneghesso, et al. , IEEE Trans Electron Devices , 53 , 12, 2932 (2006). [3] X. Chen, et al. , Appl. Phys. Lett., 117 , 26, 263501 (2020). Figure 1