Effect of Gamma Irradiation on DC Performance of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors
AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). 1-µm gates were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing nitrogen ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to 60Co γ-rays with different doses of 50, 300, 450, or 700 Gy. Irradiations were performed at temperatures <50°C. During device irradiation, the samples were held in nitrogen ambient. The device DC performance was characterized using HP 4156 parameter analyzer.
As shown in Figure 1, IDSS after irradiation was increased from 293mA/mm to 339mA/mm after irradiation. The increase of IDSSincreases with dose before 450 Gy and saturates after that. From the transmission line method (TLM), there is no significant change in sheet resistance after irradiation. However, the device total resistance, the reciprocal of the current current voltage (I-V) curve slope in the lower drain bias voltage, decreased significantly, as illustrated in Figure 1. The total resistance consisted of the channel resistance and the source resistance as well as the drain resistance. Both The source resistance and the drain resistance determined from the transmission line method change less
than 2%. While the channel resistance decreased around 5% as shown in Figure 2. The reduction of the channel resistance was mainly due to the increase of mobility and carrier concentration, which were extracted from the drain IV curve in the low field region and TLM data. Kurakin et. al1 found a similar effect that mobility increases after irradiation due to the relaxation of strain. Berthet et. al2 attributed the drain current increases after irradiation to the introduction of donor type defects through the implantation. From gate lag measurement, the dispersion of the drain current measured at 100kHz increased as a function of the gamma irradiation dose indicating additional traps generated between gate and drain electrode and enhanced the current collapse. There was no obvious current dispersion during the drain pulse measurement showing the defects created by the gamma irradiation were less than the original defects through the material growth in the buffer layer.
In conclusion, low dose gamma irradiation improved the AlGaN/GaN HEMT dc performance. The effect may be due to the strain relaxation effect and due to the donor type defects generated during the gamma irradiation.