Electrical Characteristics of the 0.1 μm Gate Length Pseudomorphic High-Electron-Mobility Transistors with Low-Dielectric-Constant Benzo-Cyclo-Butene Passivations

We examine the effects of thin-film passivation on the DC, RF and noise characteristics of 0.1-μm-gate-length pseudomorphic high-electron-mobility transistors (PHEMTs) when either conventional Si₃N₄ or low-dielectric-constant benzo-cyelo-butene (BCB) is used for the passivation layer. A significant shift in pinch-off voltage of ∼-0.4 V is observed from the PHEMTs after the Si₃N₄ passivation, whereas no such variation in DC parameters is observed in the case of BCB passivation. Noise figures of the devices are very stable after the BCB passivation and are only ∼60% of those in the case of Si₃N₄ passivation when measured at 55-62 GHz. Other RF parameters are not significantly affected by the passivation materials at a frequency of 50 GHz. We propose that the degradation in DC and noise characteristics can be due to the high density of surface states in the Si₃N₄ passivation layers; on the other hand, excellent stability in electrical characteristics in the BCB passivations can attribute to the minimized surface states in the passivation layer.