Effect of Proton Irradiation on DC Performance and Reliability of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Circular devices, with smaller junction areas and higher effective gate width/gate length, are better suited for high power, high frequency application. Since the circular devices have been shown to improve the device performance, the effect of radiation on their reliability is an important topic. There are many studies characterizing the performance under proton, neutron, and electron irradiations for rectangular AlGaN/GaN HEMTs. However, the studies for circular-shaped devices are limited. In this work, we report the effect of proton irradiation on circular-based AlGaN/GaN HEMT. DC performance, breakdown voltage, and gate lag measurement were done to characterize the devices.

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). Gates with the dimension of 1.5 µm × 314 µm 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 N₂ 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 protons with 5 MeV and different doses of 2×10¹⁴ proton/cm².  The device DC performance was characterized using an HP 4156 parameter analyzer. The breakdown voltage was conducted with a Tektronix 370A curve tracer. FLOODS TCAD finite-element solver was used to simulate the electrical field around the gate edge to confirm the experimental result.

As shown in Figure 1, drain current IVs, I_DSS, decreased from 454 to 398 mA/mm after irradiation with a proton dose of 2×10¹⁴ cm^-2. Moreover, resistance between source and drain increased and extrinsic transconductance decreased after irradiation. However, the drain breakdown voltage increased from 150 to 230 V. The improvement of the breakdown voltage was due to the reduction of electric field at the edge of the gate, caused by the defects generated by proton irradiation at AlGaN/GaN interface, which was confirmed with simulation, as shown in Figure 2.

Both gate and drain pulse measurements were also performed to verify the introductions of implanted defects into the HMET structure, which will be discussed in the talk.