Graphene is a promising material for chemical sensing applications and many studies have focused on incorporating graphene into \sgfet s sensors. The purpose of this work is to get a deeper understanding of the physics governing the surface interaction of graphene in \sgfet s with ions and charged molecules. With a clearer understanding of how these interactions register in the conductivity of graphene, it then may be possible to design the ultrasensitive sensors that are often predicted to be possible when using graphene. Epitaxial graphene (EG) and graphene produced by chemical vapor deposition (CVD) were used to fabricate \sgfet s that were tested under different ionic strength conditions and concentrations of charged proteins. To get a clearer picture of the electrostatic gating effect in ionic solutions, we analyzed our data combining two models: the electrical double layer model, which accounts for the distribution of ions inside the solution, and a ionization model that accounts for ionizable groups on the graphene surface. This gave us an insight into the influence of charged groups fixed to the surface on the gating effect which is fundamental to the performance of \sgfet s as sensors. Using our experimental data we were also able to estimate the density of charged impurities in two carrier density regimes. For high densities, we found a correlation between our estimated impurities and the surface charge that suggests that the ionizable groups act as impurities. For small carrier densities, we modeled the carriers using a self-consistent approximation (SCA). The impurities estimated from the SCA model do not seem to be related to the ionizable groups and so the origin of the conductivity for small density seems to be originated by the permanently charged impurities only. Our estimation of the charged impurities for our charged-protein adsorption experiments showed a relation between their values and the protein concentration. This shows that the proteins interact with the graphene as charged impurities. Overall, our experiments allowed us to gain a deeper understanding of the interaction of charged particles with graphene. The analysis performed in this work gives a guide for the development of graphene \sgfet s sensors by engineering the impurities at the surface to optimize the sensitivity. The design of receptors for specific sensing that do not require charged targets is possible with engineering the charge that the receptor presents to graphene when the analyte concentration changes.

Este trabalho tem como objetivos: a caracterização dos sensores SGFET (Suspended Gate Field Effect Transistors) para medidas de pH, o desenvolvimento de um protocolo de calibração destes sensores e o desenvolvimento de um sistema autônomo de medidas utilizando matrizes de sensores SGFET para controle do pH e monitoramento da qualidade de água potável. Diferentes arranjos experimentais e metodologias de testes foram analisados no decorrer do trabalho. Na caracterização do sensor de pH, foram obtidas sensibilidades de até S=449 mV/pH valor 7 vezes superior à sensibilidade máxima obtida em pHmetros de eletrodos combinados que baseiam-se no potencial Nernst (59,15 mV/pH), para medidas de Vgs e S=6 A/pH, para medidas da corrente Ids. Com base nesta caracterização, foram realizados testes para o monitoramento da qualidade de água potável durante um período de 135 horas. As medidas mostraram uma grande estabilidade e reprodutibilidade dos sensores SGFET. Nos testes utilizando o sistema autônomo de medidas, foram obtidas sensibilidades de até S=715 mV/pH. No primeiro capítulo será apresentada uma revisão bibliográfica das características, vantagens e desvantagens dos principais medidores de pH utilizados atualmente. No segundo capítulo apresentamos os princípios de funcionamento dos sensores SGFET utilizando como base transistores de efeito de campo do tipo metal óxido semicondutor (MOS-FET). No terceiro capítulo apresentamos um resumo das etapas de fabricação dos sensores SGFET, onde compararemos dois processos de fabricação. O primeiro processo é referente às etapas de fabricação do transistor SGFET realizadas no grupo de microeletrônica do Institut d\'Electronique et des Télécommunications de Rennes (IETR) na Universidade de Rennes 1 e o segundo processo é referente às etapas de fabricação de transistores SGFET realizada na empresa MHS situada na cidade de Nantes na França. Também introduzimos o conceito de matrizes sensores SGFET. No quarto capítulo serão apresentados os resultados das medidas experimentais dos sensores SGFET, tanto para o uso do dispositivo no controle do pH de soluções aquosas, como no monitoramento de água potável. Também apresentamos o detalhamento do protocolo de calibração desenvolvido para aumentar a reprodutibilidade das medidas. No quinto capítulo serão apresentados os elementos do sistema autônomo de medidas e os resultados iniciais das medidas experimentais realizadas utilizando esse sistema. Também serão apresentadas as vantagens e facilidades que o desenvolvimento desse sistema de medidas agregou aos resultados experimentais. Finalmente no Capítulo Seis as conclusões e no Capítulo 7 as sugestões de trabalhos futuros. / The objectives of this work are: the characterization of the and SGFET (Suspended Gate Field Effect Transistors) sensors applied to pH measurements, the development of a calibration protocol of these SGFET sensors and the development of an autonomous measurement system using SGFET sensor matrixes for pH control and drink water quality monitoring. In this work, many experimental arrangements and tests methodologies were analyzed. Sensitivities up to 449 mV/pH, measuring the variation of Vgs, and 6 A/pH, measuring the Ids current were obtained. For drink water quality monitoring, long time test (135 hours) was performed. The sensors showed a high stability and reproducibility. The autonomous measurements system also showed a high sensitivity (up to S = 715 mV/pH). The first chapter shows a review of the characteristics, advantages and disadvantages of the main pH meters used today. In the the second chapter, the SGFET sensor, based on MOS-FET transistor, operating principles will be present. The third chapter is the fabrication steps of SGFET sensors summary, where we compare two processes: the IETR (\"Institut d\'Electronique et de Télécommunications de Rennes\") process, developed at the University of Rennes 1, and the MHS (a private company located in Nantes-France) process. Also, we introduce the SGFET sensors matrixes. In the fourth chapter, it is present the experimental results obtained from the SGFET sensors measurements to pH control and drink water quality monitoring. Also, the details of the calibration protocol, developed to increase the measurements reproducibility, are present. In the fifth chapter, the details of the measurement autonomous system are described as well as the preliminary results are presented. Also, the advantages and the added value of this system gave to the experimental results are showed. Finally, in the sixth chapter, the conclusion of this work are presented and in the seventh chapter some future works are suggested.

Este trabalho tem como objetivos: a caracterização dos sensores SGFET (Suspended Gate Field Effect Transistors) para medidas de pH, o desenvolvimento de um protocolo de calibração destes sensores e o desenvolvimento de um sistema autônomo de medidas utilizando matrizes de sensores SGFET para controle do pH e monitoramento da qualidade de água potável. Diferentes arranjos experimentais e metodologias de testes foram analisados no decorrer do trabalho. Na caracterização do sensor de pH, foram obtidas sensibilidades de até S=449 mV/pH valor 7 vezes superior à sensibilidade máxima obtida em pHmetros de eletrodos combinados que baseiam-se no potencial Nernst (59,15 mV/pH), para medidas de Vgs e S=6 A/pH, para medidas da corrente Ids. Com base nesta caracterização, foram realizados testes para o monitoramento da qualidade de água potável durante um período de 135 horas. As medidas mostraram uma grande estabilidade e reprodutibilidade dos sensores SGFET. Nos testes utilizando o sistema autônomo de medidas, foram obtidas sensibilidades de até S=715 mV/pH. No primeiro capítulo será apresentada uma revisão bibliográfica das características, vantagens e desvantagens dos principais medidores de pH utilizados atualmente. No segundo capítulo apresentamos os princípios de funcionamento dos sensores SGFET utilizando como base transistores de efeito de campo do tipo metal óxido semicondutor (MOS-FET). No terceiro capítulo apresentamos um resumo das etapas de fabricação dos sensores SGFET, onde compararemos dois processos de fabricação. O primeiro processo é referente às etapas de fabricação do transistor SGFET realizadas no grupo de microeletrônica do Institut d\'Electronique et des Télécommunications de Rennes (IETR) na Universidade de Rennes 1 e o segundo processo é referente às etapas de fabricação de transistores SGFET realizada na empresa MHS situada na cidade de Nantes na França. Também introduzimos o conceito de matrizes sensores SGFET. No quarto capítulo serão apresentados os resultados das medidas experimentais dos sensores SGFET, tanto para o uso do dispositivo no controle do pH de soluções aquosas, como no monitoramento de água potável. Também apresentamos o detalhamento do protocolo de calibração desenvolvido para aumentar a reprodutibilidade das medidas. No quinto capítulo serão apresentados os elementos do sistema autônomo de medidas e os resultados iniciais das medidas experimentais realizadas utilizando esse sistema. Também serão apresentadas as vantagens e facilidades que o desenvolvimento desse sistema de medidas agregou aos resultados experimentais. Finalmente no Capítulo Seis as conclusões e no Capítulo 7 as sugestões de trabalhos futuros. / The objectives of this work are: the characterization of the and SGFET (Suspended Gate Field Effect Transistors) sensors applied to pH measurements, the development of a calibration protocol of these SGFET sensors and the development of an autonomous measurement system using SGFET sensor matrixes for pH control and drink water quality monitoring. In this work, many experimental arrangements and tests methodologies were analyzed. Sensitivities up to 449 mV/pH, measuring the variation of Vgs, and 6 A/pH, measuring the Ids current were obtained. For drink water quality monitoring, long time test (135 hours) was performed. The sensors showed a high stability and reproducibility. The autonomous measurements system also showed a high sensitivity (up to S = 715 mV/pH). The first chapter shows a review of the characteristics, advantages and disadvantages of the main pH meters used today. In the the second chapter, the SGFET sensor, based on MOS-FET transistor, operating principles will be present. The third chapter is the fabrication steps of SGFET sensors summary, where we compare two processes: the IETR (\"Institut d\'Electronique et de Télécommunications de Rennes\") process, developed at the University of Rennes 1, and the MHS (a private company located in Nantes-France) process. Also, we introduce the SGFET sensors matrixes. In the fourth chapter, it is present the experimental results obtained from the SGFET sensors measurements to pH control and drink water quality monitoring. Also, the details of the calibration protocol, developed to increase the measurements reproducibility, are present. In the fifth chapter, the details of the measurement autonomous system are described as well as the preliminary results are presented. Also, the advantages and the added value of this system gave to the experimental results are showed. Finally, in the sixth chapter, the conclusion of this work are presented and in the seventh chapter some future works are suggested.

MEMS microphones have been a research topic for the last two and half decades. The state-of-the-art comprises surface mount MEMS microphones in laptops, mobile phones and tablets, etc. The popularity and the commercial success of MEMS microphones is largely due to the steep cost reduction in manufacturing afforded by the mass scale production with microfabrication technology. The current MEMS microphones are de-signed along the lines of traditional microphones that use capacitive transduction with or without permanent charge (electret type microphones use permanent charge of their sensor element). These microphones offer high sensitivity, stability and reasonably at frequency response while reducing the overall size and energy consumption by exploiting MEMS technology. Conceptually, microphones are simple transducers that use a membrane or diaphragm as a mechanical structure which deflects elastically in response to the incident acoustic pressure. This dynamic deflection is converted into an electrical signal using an appropriate transduction technique. The most popular transduction technique used for this application is capacitive, where an elastic diaphragm forms one of the two parallel plates of a capacitor, the fixed substrate or the base plate being the other one. Thus, there are basically two main elements in a microphone { the elastic membrane as a mechanical element, and the transduction technique as the electrical element. In this thesis, we propose and study novel design for both these elements. In the mechanical element, we propose a simple topological change by introducing slits in the membrane along its periphery to enhance the mechanical sensitivity. This simple change, however, has significant impact on the microphone design, performance and its eventual cost. Introduction of slits in the membrane makes the geometry of the structural element non-trivial for response analysis. We devote considerable effort in devising appropriate modeling techniques for deriving lumped parameters that are then used for simulating the system response. For transduction, we propose and study an FET (Field Effect Transistor) coupled micro-phone design where the elastic diaphragm is used as the moving (suspended) gate of an FET and the gate deflection modulated drain current is used in the subthreshold regime of operation as the output signal of the microphone. This design is explored in detail with respect to various design parameters in order to enhance the electrical sensitivity. Both proposed changes in the microphone design are motivated by the possibilities that the microfabrication technology offers. In fact, the design proposed here requires further developments in MEMS technology for reliably creating gaps of 50-100 nm between the substrate and a large 2D structure of the order of a few hundred microns in diameter. In the First part of the thesis, we present detailed simulations of acoustic and squeeze lm domain to understand the effect slits could bring upon the behaviour of the device as a microphone. Since the geometry is nontrivial, we resort to Finite element simulations using commercial packages such as COMSOL Multiphysics and ANSYS in the structural, acoustic and Fluid-structure domains to analyze the behaviour of a microphone which has top plate with nontrivial geometry. On the simulated Finite element data, we conduct low and high frequency limit analysis to extract expressions for the lumped parameters. This technique is well known in acoustics. We borrow this technique of curve Fitting from the acoustics domain and apply it in modified form into the squeeze lm domain. The dynamic behaviour of the entire device is then simulated using the extracted parameters. This helps to simulate the microphone behaviour either as a receiver or as a transmitter. The designed device is fabricated using MEMSCAP PolyMUMPS process (a foundry Polysilicon surface micromachining process). We conduct vibrometer (electrostatic ex-citation) and acoustic characterization. We also study the feasibility of a microphone with slits and the issues involved. The effect of the two dissipation modes (acoustic and squeeze lm ) are quantified with the experimentally determined quality factor. The experimentally measured values are: Resonance is 488 kHz (experimentally determined), low frequency roll-off is 796 Hz (theoretical value) and is 780 Hz as obtained by electrical characterization. The first part of this thesis focusses on developing a comprehensive understanding of the effect of slits on the performance of a MEMS microphone. The presence of slits near the circumference of the clamped plate cause reduction in its rigidity. This leads to an increase in the sensitivity of the device. Slits also cause pressure equalization between the top and bottom of the diaphragm if the incoming sound is at relatively low frequencies. At this frequency, also known as the lower cutoff frequency, the microphone's response starts dropping. The presence of slits also changes the radiation impedance of the plate as well as the squeeze lm damping below the plate. The useful bandwidth of the microphone changes as a consequence. The cavity formed between the top plate and the bottom fixed substrate increases the stiffness of the device significantly due to compression of the trapped air. This effect is more pronounced here because unlike the existing capacitive MEMS microphones, there is no backchamber in the device fabricated here. In the second part of the thesis, we present a novel subthreshold biased FET based MEMS microphone. This biasing of the transistor in the subthreshold region (also called as the OFF-region) offers higher sensitivity as compared to the above threshold region (also called as the ON-region) biasing. This is due to the exponentially varying current with change in the bias voltage in the OFF-region as compared to the quadratic variation in the ON-region. Detailed simulations are done to predict the behaviour of the device. A lumped parameter model of the mechanical domain is coupled with the drain current equations to predict the device behaviour in response to the deflection of the moving gate. From the simulations, we predict that the proposed biasing offers a device sensitive to even sub-nanometer deflection of the flexible gate. As a proof of concept, we fabricate fixed-fixed beams which utilize CMOS-MEMS fabrication. The process involves six lithography steps which involve two CMOS and the remaining MEMS fabrication. The fabricated beams are mechanically characterized for resonance. Further, we carry out electrical characterization for I-V (current-voltage) characteristics. The second part of the thesis focusses on a novel biasing method which circumvents the need of signal conditioning circuitry needed in a capacitive based transduction due to inbuilt amplification. Extensive simulations with equivalent circuit has been carried out to determine the increased sensitivity and the role of various design variables.