Threshold voltage shift induced by intrinsic stress in gate metal of AlGaN/GaN HFET

Abstract

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.