Research On Annealing And Photoresponse Of AlGaN/GaN HEMT With Transparent Gate

AlGaN/GaN high electron mobility transistor(HEMT) has been widely used in the fields of high temperature, high frequency and high power devices and sensors, as well as high performance detectors. Because of the sapphire substrate and AlGaN/GaN heterojunction are all transparent materials, AlGaN/GaN HEMT has certain potential in the field of transparent devices. In order to obtain transparent devices with excellent performance, the AlGaN/GaNHEMT devices were fabricated by using transparent AZO material as gate electrode. The basic characteristics, interface characteristics and optical response of these devices were studied.The basic operating principles of AlGaN/GaN HEMT and the basic theories of Schottky contact are firstly introduced. The preparation technology of the AlGaN/GaN HEMT and the preparation method of the transparent gate electrode are introduced. Finally, an excellent transparencyis obtained by the AZO-gate electrode.Transparent devices with AZO-gate also showed great grid control ability. But the oxygen donors and traps between thegate electrode and the Al Ga N layerintroduced by oxygenatmosphere during the fabrication of AZO electroderesult inlarger ideality factor, andthe higher Schottky barrier brings about the threshold voltage drifting forward,compared with traditional N i / Au-gate HEMTs.Nearly one order of magnitude has been decreased in the saturation current of AZO-gate devices and the frequency characteristics are decreased. However, the reverse leakage current is lower, and a high breakdown voltage(~100V) is obtainedby AZO-gate devices with a relative of N i/Au-gate devices(~50V). In addition, the characteristics of the Ni/AZO-gate devices are similar to that of the Ni/Au device.In this paper, the annealing characteristics of devices with different gate materials were studied. After annealing, the ideality factor of AZOgate is reduced from 2.60 to 1.63, and the Schottky barrier height is increased by 0.1e V, and the forward current is increa sed by two orders of magnitude. Although the Schottky characteristicsare obviously improved, the characteristics of device are degraded.Because of the increase ofthe trap levels and trap density after annealing, particularlythe increaseof thetrap levels during 0.28~0.30 e V, leads to the increase of leakage current in devices, and the significantdecrease of saturation current of the devices.Finally, the effect of the optical response and the effect ofillumination on short-term electric stress recovery are studied. The results show that: the effects of blue light and green light on the transparent gate electrodesare relatively large, which make the trap density of deep levelsabove0.35 e V decrease. However the shallow level trapsarealmost not affected. Blue and green light can make the effects of short-term stress on the device characteristicsrecover in 100 s.Sincepenetration of illumination throughthe edgegap of gate electrode, the N i/Au-gate devices also showed similar optical response results with that of AZO-gate. Althoughthe blue response of the N i/Au device is slightly obvious, the overall optical response is relatively weak.