A model for hydrogen-induced piezoelectric effect in InP HEMTs and GaAs PHEMTs

Abstract

This thesis would not have been possible without the help of many people. First, I would like to thank Prof. Jesus del Alamo for imparting me with an ethically sound approach to research. Throughout the past six years, he provided great answers and also posed the right questions. I would also like to thank my readers, Prof. Clifton Fonstad and Prof. Rajeev Ram, for their comments and suggestions to improve this thesis. Roxann Blanchard established a solid foundation upon which I based my research. I would like to thank her for her efforts and for her invaluable help during the past six years. I learned a lot about InP HEMTs through my discussions with Tetsuya Suemitsu and I would like to thank him and Takatomo Enoki for providing the advanced InP HEMTs which added an important experimental component to this research. This thesis would not have been possible without the tremendous support of the MTL and its expert staff and amazing students. The radius-of-curvature and Auger Electron Spectroscopy measurements were very successful thanks to the expert help of Tim

Hydrogen Sensitivity of InP HEMTs With WSiN-Based Gate Stack

“…The active device structure consists, from top to bottom, of a 5-nm InP etch stop, a 10-nm InAlAs insulator with Si planar doping, a 15-nm InGaAs channel, and a 200-nm InAlAs buffer layer [11]. The layers in the heterostructure are significantly thinner than previously studied devices that did not feature the InP etch stop [6]. In contrast with conventional InP HEMTs, these devices feature a WSiN-Ti-Pt-Au gate stack and an InP gate recess etch-stop layer in the intrinsic heterostructure [1].…”

“…In state-of-the-art InP HEMTs with Ti-based gate stack, H-induced threshold voltage shifts can be as large as 150 mV depending on the gate length and the exposure conditions [6]- [10]. Since the Ti-layer is believed to be the source of the stress, the introduction of other metal such as WSiN at the bottom of the gate stack is expected to mitigate H sensitivity by separating the Ti-layer from the heterostructure.…”

Electrical degradation mechanisms of RF power GaAs PHEMTs

“…This change was found to be mostly recoverable with unbiased storage at room temperature [23] and was independent of the stressing environment (the shift was the same, whether in air or in nitrogen). Negative shifts in VT have been previously observed in GaAs HEMTs under stress, though a wide variety of very different mechanisms have been proposed [5,10,12,21,46]. Although a hypothesis for the VT shift seen in our devices was proposed in [23], it was later discovered that some assumptions made about the relative levels of impact ionization in the devices studied were incorrect.…”

“…1.3.2, other HEMT reliability studies have demonstrated shifts in threshold voltages due to hydrogen degradation [10,20,481 . We needed to know if such effects were present in our devices, so that we could separate any such effects from those due to electrical stressing alone.…”

“…This is a reliability concern because the formation of TiH creates tensile stress in the semiconductor heterostructure that shifts VT. Since this involves a piezoelectric effect, the sign of the VT shift depends on the length and the orientation of the gate [10]. The conditions in which hydrogen degradation is readily observed involve relatively high temperatures, and so its effects must also be tested for and isolated from thermal and/or environmental effects in analyzing effects of electrical stress.…”