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

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

Abstract-We have experimentally investigated the hydrogen sensitivity of InP high-electron mobility transistors (HEMTs) with a WSiN-Ti-Pt-Au gate stack. We have found that exposure to hydrogen produces a shift in the threshold voltage of these devices that is one order of magnitude smaller than published data on conventional Ti-Pt-Au gate HEMTs. We have studied this markedly improved reliability through a set of quasi-two-dimensional mechanical and electrostatic simulations. These showed that there are two main causes for the improvement of the hydrogen sensitivity. First, the separation of the Ti-layer from the semiconductor by a thick WSiN layer significantly reduces the stress in the heterostructure underneath the gate. Additionally, the relatively thinner heterostructure used in this study and the presence of an InP etch-stop layer with a small piezoelectric constant underneath the gate reduces the amount of threshold voltage shift that is caused by the mechanical stress.Index Terms-High-electron mobility transistors (HEMTs), hydrogen, InP, piezoelectric effect, reliability.

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InAlAs/InGaAs metamorphic High Electron Mobility Transistors (HEMT) hold promise for power-millimeter wave applications. A major reliability concern in some of these devices is the degradation of the drain resistance that is observed when the device is electrically stressed for a long time at bias conditions necessary for power applications. The goal of this thesis was to find the physical origin of this reliability problem and to suggest solutions to it. State-of-the-art InAlAs/InGaAs metamorphic HEMTs, provided by our sponsor, Hewlett Packard, were stressed under different bias schemes. It was found that most figures of merit associated with the drain-side of the device degrade under severe bias stress. In particular, the drain resistance, RD, has been found to increase significantly. In order to understand the physical origin of this degradation, we have studied the degradation of simpler Transmission Line Model (TLM) structures. We have found that in TLMs and HEMTs there appear to be two different degradation modes, both associated with hot electrons. In the first degradation mechanisms, we postulate that hot electrons are trapped by defects at the interface between the GaAs etch-stopper and the AlInAs Schottky barrier layer, depleting the carrier concentration in the channel underneath. In the second mechanism hot electrons degrade the InGaAs ohmic contacts. No degradation mechanism associated with the metamorphic nature of the structure has been identified. AcknowledgementsI would like to thank Prof. Jesus del Alamo for giving me the chance to work on this interesting project. He has guided me through this research with a huge amount of patience, ideas and red ink, teaching me a methodology in research and reporting. He always tried to make time available for me, even if he was extremely busy. I would like to thank him for offering me the opportunity to be a teaching assistant for his very interesting class and for giving me a place at MIT from the time I arrived. This research has been funded by Hewlett-Packard. I would like to thank Don D'Avanzo for starting this project, appropriating the funding and giving me the opportunity to work as a SEED student in Santa Rosa. Larry Studebaker, thanks for the invaluable help and the samples during the year and the supervision this Summer. I am very happy that I have found such a great co-advisor. I had a great time in Santa Rosa and I learned a lot there, also thanks to Dan Scherrer, Fred Sughiwo and Bob Yeats. HP Labs also helped me a lot through this thesis. Hans Rohdin, thank you for answering so many questions, reading this thesis on time and sending me the samples that helped pull things together. Thanks to Arlene Wakita and Nick Moll for making these devices available to me.Roxann Blanchard, thanks for all the discussions we held and the questions you answered, even when you were busy with your own thesis. You have proven to be an infinite well of knowledge and I can only hope I have been able to soak up enough of it to continue this research.Tassan...

We have investigated the hydrogen sensitivity of InP HEMTs with a gate stack containing a thick Ti-layer (order of 1000 A). We have found that the hydrogen-induced piezoelectric effect in these devices is an order of magnitude smaller than conventional Ti/Pt/Au-gate HEMTs with a thin Ti layer (order of 250 A). This markedly improved reliability can be explained through the diffusion mechanism of H in Ti which limits hydrogenation of the Ti layer to a thin sheet at the top. Using Auger Electron Spechoscopy, we have confumed that under the studied conditions, TiH, is only formed in the top 250 A of the Ti-layer. In some devices located in the periphery of the wafer, we have observed a second hydrogen degradation mechanism that induces a large positive AV,. This appears to be related to an improperly fabricated gate recess. The use of a thick Ti layer in the gate stack allows for a simple and effective mitigation of the H-induced piezoelectric effect in InP and other 111-V HEMTs. 2

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We have experimentally investigated the hydrogen sensitivity of InP high-electron mobility transistors (HEMTs) with a WSiN-Ti-Pt-Au gate stack. We have found that exposure to hydrogen produces a shift in the threshold voltage of these devices that is one order of magnitude smaller than published data on conventional Ti-Pt-Au gate HEMTs. We have studied this markedly improved reliability through a set of quasi-two-dimensional mechanical and electrostatic simulations. These showed that there are two main causes for the improvement of the hydrogen sensitivity. First, the separation of the Ti-layer from the semiconductor by a thick WSiN layer significantly reduces the stress in the heterostructure underneath the gate. Additionally, the relatively thinner heterostructure used in this study and the presence of an InP etch-stop layer with a small piezoelectric constant underneath the gate reduces the amount of threshold voltage shift that is caused by the mechanical stress.