Nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structures showed an isolation blocking voltage of 900 V with a leakage current at 1 μA/mm across an implanted isolation-gap of 10 μm between two Ohmic pads. The effect of implanted gap distance (1.7, 5, or 10 μm) between two Ohmic contact pads was evaluated. The isolation current density was determined to be solely dependent on the applied field between the contact pads. A model using a combination of resistive current and Poole–Frenkel current is consistent with the experimental data. The resistance of the isolation implantation region significantly decreased after the sample was annealed at temperatures above 600 °C.

The effects of source field plates on AlGaN/GaN high electron mobility transistor reliability under off-state stress conditions were investigated using step-stress cycling. The source field plate enhanced the drain breakdown voltage from 55 to 155 V and the critical voltage for off-state gate stress from 40 to 65 V, relative to devices without the field plate. Transmission electron microscopy was used to examine the degradation of the gate contacts. The presence of pits that appeared on both source and drain sides of the gate edges was attributed to the inverse piezoelectric effect. In addition, a thin oxide layer was observed between the Ni gate contact and the AlGaN layer, and both Ni and oxygen had diffused into the AlGaN layer. After step-stress cycling, additional threading dislocations were observed.

By replacing the commonly used Ni/Au gate metallization with Pt/Ti/Au, the critical voltage for degradation of AlGaN/GaN High Electron Mobility Transistors (HEMTs) during off-state biasing stress was significantly increased. The typical critical voltage for HEMTs with Ni/Au gate metallization was around À55 V. By sharp contrast, no critical voltage was observed for the HEMTs with Pt/Ti/Au gate metallization, even up À100 V, which was the instrumental limitation in this experiment. Both Schottky forward and reverse gate characteristics of the Ni/Au degraded once the gate voltage passed the critical voltage of À55 V. There was no degradation exhibited for the HEMTs with Pt-gated HEMTs.AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for high power and high frequency operation 1,2 and for applications such as high frequency wireless base stations and broad-band links, commercial and military radar and satellite communications. [3][4][5] In recent reports, hot electron induced degradation has been observed in undoped AlGaN/GaN HEMTs on sapphire, SiC and Si substrate during DC and RF stresses. 6-8 Several degradation mechanisms that suppress device performance and reliability have been reported, ranging from hot-electron-induced trap generation to field-driven mechanisms. 9-14 Ni/Au metallization has been widely employed as the gate contact for AlGaN/GaN HEMT and consequently it has been used in most HEMT reliability studies to date.Pt-based metal schemes have been used as p-Ohmic contacts on p-type GaAs to improve the reliability of AlGaAs/GaAs heterojuction bipolar transistors. 15,16 By alloying at low temperature, Pt diffused into GaAs and reacted with GaAs to form a thin PtAs 2 layer to achieve low contact resistance and excellent device stability. Pt-based metallization in the form of Pt/Ti/Pt/Au was also employed as the gate contacts for InGaP/GaAs, InAlAs/InGaAs, 17,18 and AlGaAs/InGaAs HEMT to improve the device reliability, and an intentional annealing step at 250-350 C was used to sink the Pt into the gate contact semiconductor layer to adjust the threshold voltage of the HEMTs. 19 This allowed both depletion and enhancementmode mode HEMTs to be fabricated on the same wafer. Pt/Ti/Au metallizaton has also been used as the gate contacts on GaN and AlGaN. The thermal stability of Pt/Ti/Au was much higher as compared to Ni/Au gate contacts. 20 The thickness of the Ti layer in the Pt/Ti/Au played an important role in the resulting gate leakage current upon thermal stress. 21 In this work, we compared the critical voltages for off-state stress, Schottky gate forward and reverse current characteristics, as well as drain I-V characteristics for HEMTs fabricated with the Ni/ Au and Pt/Ti/Au gate metallization. X-ray photoelectron Spectroscopy (XPS) was used to examine the thermal stability of the Ni and Pt metal contacts deposited on GaN.AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by metal-organic chemical vapor deposition (MOCVD). The epi-layers consisted of a 1 m thick car...

A subnanometer thick interfacial oxide layer present between the Ni/Au gate metal stack and semiconducting epilayers of an AlGaN/GaN high electron mobility transistor was characterized using high-angle annular dark-field scanning transmission electron microscopy and laser-assisted atom probe tomography. It was revealed that the oxide is composed of distinct Ni-oxide-rich and Al-oxide-rich layers with no Ga-oxide detected. The results provide information that is of potential importance in determining failure mechanisms and improving reliability of AlGaN/GaN high electron mobility transistors.

Sub-bandgap optical pumping with wavelengths of 671, 532, or 447 nm was employed to study traps in AlGaN/GaN high electron mobility transistors. The trap energies were determined from the Arrhenius plots of transient drain current at different temperatures. Prominent states were located around 0.7 eV below the conduction band, and these are commonly reported to be nonradiative traps due to defects trapped on dislocations or possibly Ga interstitials. In addition, traps located at 1.9 and 2.35 eV below the conduction band were found, which have been reported as NGa antisite and VGa–ON complexes, respectively. The postillumination drain current decays were analyzed with a persistent photoconductivity method, and time constants were extracted and associated with the recapture process in the AlGaN barrier and GaN channel layers.