Mechanical stress/strain is altering the charging properties of piezoelectric materials. For AlGaN and GaN heterostructures, this phenomenon has been described theoretically (Ambacher et al 2000 J. Appl. Phys. 87 334–44). Recently it was shown that a mechanically stressed SiN x passivation layer may produce local strain in the AlGaN/GaN layers underneath the gate at certain conditions. This results in a threshold voltage (V th) shift (Osipov et al 2018 IEEE Trans. Electron Devices 65 3176–84). It is therefore straightforward to also investigate the influence of an intrinsically stressed gate metal on the device characteristics. In this study, AlGaN/GaN HFETs were fabricated on nominally identical wafers with variations of intrinsic stress in the gate metallization and in the first passivation. It is verified that the built-in mechanical stress of the gate metal can shift the threshold voltage. In detail we varied the intrinsic stress of both the first passivation layer (SiN x ) and the gate metallization (Ir) by about 1 GPa. These variations result respectively in 0.65 and 0.20 V threshold voltage shift. We have additionally analyzed the thermal stability of SiN x and iridium films with respect to their mechanical properties and the resulting threshold voltage shift.
Vertical silicon carbide transistors and lateral gallium nitride (GaN) transistors for power-electronic applications currently target applications with different voltage and power ratings. Meanwhile, research and development activities continue on vertical GaN transistors and new gallium oxide (Ga 2 O 3) transistors. What are their perspectives in the application and how do they compete against each other and against established transistor technologies? This study discusses the specific characteristics of lateral and vertical GaN and Ga 2 O 3 transistors to assess their strengths and weaknesses.
Herein, the influence of mechanical strain induced by passivation layers on the electrical performance of AlGaN/GaN heterostructure field‐effect transistor is investigated. We studied the physical mechanism of a threshold voltage (Vth) shift for the monolithically fabricated on/off devices reported earlier by our group. For that, theoretical calculations, simulation‐based analysis, and nano‐beam electron diffraction (NBED) measurements based on STEM are used. Strain distribution in the gate vicinity of transistors is compared for a SiNx passivation layer with intrinsic stress from ≈0.5 to −1 GPa for normally on and normally off devices, respectively. The strain in epitaxial layers transferred by intrinsic stress of SiNx is quantitatively evaluated using NEBD method. Strain dissimilarity Δε = 0.23% is detected between normally on and normally off devices. Using this method, quantitative correlation between 1.13 V of Vth shift and microscopic strain difference in the epitaxial layers caused by 1.5 GPa intrinsic stress variation in passivation layer is provided. It is showed in this correlation that about half of the reported threshold voltage shift is induced by strain, i.e., by the piezoelectric effect. The rest of Vth shift is caused by the fabrication process. Therefore, various components/mechanisms contributing to the measured Vth shift are distinguished.
Herein, the fabrication of Au‐free ohmic contacts for mm‐wave GaN heterojunction field‐effect transistors is investigated. To find an optimum metal stack and annealing recipe, different metallization and rapid thermal annealing temperature/duration combinations are tested. They are compared with the well‐known Ti/Al/Ni/Au ohmic contact scheme optimized for AlGaN/GaN epitaxial structures. Herein, a Ta/Al/Ta metal stack is initially fabricated and analyzed. Subsequently, further developments for improving the contact resistance and surface roughness of Ta‐based ohmic contacts are carried out. The best achieved Au‐free contact resistance is ≈0.28 ± 0.18 Ω mm for Ta/Al/W metallization after annealing at 600 °C. The root mean square (RMS) surface roughness and edge definition in this contact are improved significantly to 7.5 nm RMS and about 30 nm edge accuracy compared with 16 nm RMS and 160 nm in Au‐based contacts.
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