We propose two kinds of stacked materials that effectively yield low drain off-state leakage current, high off-state breakdown voltage and low current collapse simultaneously in AlGaN/GaN HEMT. The two kinds of stacked materials, which is NiOX/SiNX and Al2O3/SiNX respectively, act as gate dielectric layer and passivation layer for the devices. The NiOX and Al2O3 are prepared through depositing Ni and Al thin film by electron beam evaporation and then annealing in O2 ambient by rapid thermal processing. The SiNX is deposited by plasma-enhanced chemical vapor deposition. Compared to the single SiNX layer, the NiOX/SiNX stack layer and Al2O3/ SiNX stack layer can distinctly reduce the drain off-state leakage current of devices. Meanwhile, high off-state breakdown voltage is also achieved. The off-state breakdown voltage in the devices with NiOX/SiNX stack layer and Al2O3/SiNX stack layer are 52% and 36% higher than that of the devices with single SiNX layer respectively. Especially, the current collapse under 200 V stress bias is reduced by 90% and 58% in the devices with NiOX/SiNX stack layer and Al2O3/SiNX stack layer compare to the devices with single SiNX layer respectively.

We proposed a preparation method of a TiN capping layer compatible with the ohmic contacts process, and demonstrated the Au-free ohmic contacts of an undoped AlGaN/GaN high-electron-mobility transistor (HEMT) with a Ti/Al/Ni/TiN metal structure. TiN was prepared through depositing Ti thin film by a sputtering system and then annealing in N₂ ambient by the rapid thermal annealing process. The thickness of Ti/Al, annealing temperature, and annealing time were investigated systematically. Using the Ti/Al/Ni/TiN structure, a low contact resistance (3.47 x 10⁻⁵,,Ω cm², 1.1 Ω · mm) was obtained when annealed at 900 °C for 30 s in N₂ ambient, which was comparable with conventional Au-based ohmic contacts (3.12 x 10⁻⁵ Ω · cm², 1.05 Ω · mm). In addition, the Ti/Al/Ni/TiN ohmic contacts showed smooth surface morphology with a surface roughness of 5.89 nm. AlGaN/GaN HEMT, based on Ti/Al/Ni/TiN Au-free ohmic contacts, was also fabricated and exhibited good dc characteristics. The reported Au-free AlGaN/GaN HEMT fabrication process can be used in standard Si fabs without the risk of contamination.

We propose the Ti/Al/Ni/Ti ohmic contacts to improve the breakdown voltage (V _BD ) of GaN-based high electron mobility transistors (HEMTs). Using the same photolithography process, the first Ti/Al metal stack and the second Ni/Ti metal stack were achieved by electron beam (EB) evaporation and magnetron sputter, respectively. Attributed to the better particle filling on the sidewall by magnetron sputtering, the second Ni/Ti metal stack completely wraps the first Ti/Al metal stack and extends to both sides after a lift-off process. Even after high temperature annealing (HTA), the Ti/Al/Ni/Ti metal stack can keep smooth edge acuity and surface morphology, resulting in a uniform electric-field distribution near the ohmic contacts. Compared with Ti/Al/Ni/Au ohmic contacts, HEMTs with Ti/Al/Ni/Ti ohmic contacts have higher V _BD and can improve the uniformity as well as repeatability of V _BD .

This paper reports a novel method for fabricating Schottky/ohmic drain for GaN high electron mobility transistors (HEMTs). Without secondary photolithography or secondary metal evaporation, the Schottky/ohmic drain can be obtained by adjusting the direction of the electron beam evaporation. Besides, different metal/semiconductor interfaces can be simultaneously formed at this state, which are beneficial to enhance the shielding effect of the rough ohmic-drain metal morphology and modulate the electric field distribution on the drain side. By using Schottky/ohmic drain technology, the average breakdown voltage calculated from 50 devices each is improved from 895 V (ohmic drain device) to 1205 V for L _GD = 18 μm, indicating enhanced off-state reliability characteristics for GaN HEMTs.