Photocurrent Measurements of AlGaN/GaN Hemt Under X-Ray Irradiation

Abstract

The photocurrent of AlGaN/GaN HEMT under x-ray exposure was measured. A sharp response in the photocurrent was observed with the exposure and the current showed linear relationship with the x-ray tube current. An increase in the photocurrent was measured, especially for high intensity exposure, and a tail in the current was observed after shutting the exposure. The transient response in the photocurrent may have relationship with the deep defects in the epitaxial wafer. Introduction Gallium nitride (GaN) HEMTs are widely used in the devices for power and RF applications. Due to the epitaxial growth of the layers on foreign substrates, lots of defects are incorporated in the epitaxial wafer. In this study, we measured the photocurrent of the AlGaN/GaN HEMT under x-ray exposure, and its transient response was observed. Experiments AlGaN/GaN HEMT on a Si substrate was fabricated by Ohmic contact to the 2DEG with top electrode (Ti/Al/Ni/Au) with thermal treatment[1], followed by mesa isolation by dry etching. Finally, backside contact was formed by Al evaporation. The area of the exposure was 5×5 cm2. A bias voltage of 40 V was applied to the substrate, where the dark current was as low as 0.5 nA. The x-ray source was WKα and tube current was changed from 10 to 150 mA. Results and discussions Fig.1 shows the photocurrent response to the x-ray exposure. A sharp response of the photocurrent was obtained when the x-ray is exposed to the devices. A linear relationship with the photocurrent and the tube current was obtained, as shown in fig. 2. However, a gradual increase in the photocurrent was observed, especially at high tube current. Moreover, a tail photocurrent was also observed after shutting the x-ray. The magnitude of the tail current was high with large tube current. The gradual increase and the tail in the photocurrent might be caused by the trapping of photogenerated carriers.[2] Conclusion A gradual increase in the photocurrent was observed under high intensity x-ray exposure to the AlGaN/GaN HEMT. Also, a tail current was measured after shutting the exposure. These photocurrent transient responses might have some relationship with the defects in the epitaxial wafer.