Artigo Seminatec final
Transcrição
Artigo Seminatec final
New technologies of measuring pH based on MOS transistors L.G. Dominguesb, A. L. Siarkowskia,b, B. S. Rodriguesa,b and N.I. Morimotoa a LSI, University of São Paulo, Av. Prof. Luciano Gualberto, trav. 3 n°158, São Paulo, Brazil. b Nove de Julho University, Av. Dr. Adolpho Pinto, 109 , São Paulo, Brazil. e-mail: [email protected] 1. Abstract The objective of this work is comparing methods to determine the pH in aqueous solutions using solid state devices (ISFET, SGFET), demonstrating the operating principles, results and others advantages about both devices. The sensibility is the main advantage of SGFET in comparison of ISFET and the possibility to create miniaturized systems performing continuous measurements. The principle of the ISFET detection is a selective membrane of H+ that will accumulating charges on the surface of the sensor creating an electric field that modulate the current channel. The potential on transistor’s surface is measured in relation of a common reference electrode Ag/AgCl. In consequence of this principle the threshold voltage of the ISFET is given by: (1) 2. Introduction 3. Technologies of pH measurements The ISFET architecture is limited by the Nernst’s theory. The figure 2 shows the potential generated by the device in relation of pH solution (mV/pH): 0 Potential (mV) The pH or Hydrogenic potential is a measure unit created in 1909 by Danish chemist Sörensen to indicate the level of acidity or alkalinity following a numeric scale 0 to 14, through the ions activity in any substance that varies in relation of hydrogen ions concentration (H+) and hydroxyl ions (OH-). The pH control is important in several areas as agriculture where the soil needs maintaining the equilibrium for the good nutrients absorption by plants, regarding flower color, control of physiological factors in human blood, food industries to obtain quality, pharmaceutical industries with respect to fabrication and stability of its products. -100 -200 -300 -400 -500 -600 0 2 4 6 8 10 pH A. ISFET Fig.2. Potential ISFET device (mV/pH) [2]. The ISFET (Ion-Sensitive Field Effect Transistor) is a chemical sensor based on MOSFET (Metal-OxideSemiconductor Field Effect Transistor) technology. However, it has a difference about the gate contact, invented by Bergveld in 1970 [1]. Figure 1 shows the scheme of the device: The ISFET device offers many advantages such as low cost, mass production, durability, low output impedance, short response time and allows integration of circuitry, can be implemented on compact probes, however it has a limited sensitivity of 59,6 mV/pH. B. SGFET The SGFET (Suspended Gate Field Effect Transistor), is a high sensitive sensor for detection low pH variances. Even though it follows the MOSFET model, differentiated by the suspended gate over the channel according to figures 3 and 4: [3][4]. Fig.1. Scheme of the ISFET device [2] possibility to develop new low cost microsystems and compatibility with CMOS technology [5] [6]. 4. Conclusions Thereby, the solution is defined as acidic or basic through the result of pH in a numeric scale 0 to 14. The necessity to miniaturize devices that measure pH, increase the reliability results and decrease the costs appeared solid state devices ISFET and SGFET. The ISFET uses as gate the reference electrode that is limited by the Nernst potential. Differently, the SGFET has high sensibility 10 times over the Nernst potential due to suspended gate that create the GAP region to determine the pH through the change charges between the gate and gate dielectric. Fig.3. Scheme of the SGFET device [5] Acknowledgments Authors would like to thanks Tayeb MohammedBrahim, Olivier De Sagazan, Samuel Crand (University of Rennes 1), Patrice Lancelot and Joseph Tregret from MHS (Nantes-France) for their help and their manufacturing advices in the project and FAPESP, CAPES/CNPq and Namitec for the financial support References Fig.4. SGFET device [5] In the SGFET architecture the principle of ph-meter measures is based in the density of charges in the Gap area. The concentration of H+ or OH- in the GAP(empty space between the suspended gate and gate dielectric, with 500nm) will influence the threshold voltage (VT) of the SGFET. (2)[4] Figure 5 shows the potential generated by the device in relation of pH solution (mV/pH): Fig.5. Potential SGFET device (mV/pH) The linear shift presented in figure 5 led us to determine a sensitivity of 650 mV/pH, sensitivity 10 times more than Nernst potential [4] and consequently more than ISFET sensors. Other advantage is the [1] A Al-Ahdal, C Toumazou. 2012. ISFET – Based Chemical Switch. IEEE Sensors Journal, Vol. 12, Nº. 5, Maio 2012. [2] R. Scaff. 2008. Caracterização elétrica de dispositivos tipo ISFET com estrutura Si/SiO2/Si3N4 para medição de pH utilizando pseudoeletrodos de Pt, Ag e Au. Dissertação apresentada à Escola Politécnica da Universidade de São Paulo para obtenção do título de Mestre em Engenharia Elétrica.2008. [3] B. da Silva Rodrigues, O. De Sagazan, S. Crand, T. Mohammed-Brahim, N.I. Morimoto. 2011. pH meter based in Suspended Gate Field Effect Transistors to application in monitoring of water quality. GM-IETR UMR-CNRS 6164, Universite RENNES I, FRANCE e LSI, University of São Paulo, São Paulo, Brazil. [4] B. da Silva Rodrigues, O. De Sagazan, S. Crand, T. Mohammed-Brahim, N.I. Morimoto. 2012. Uso de transistores de porta suspensa para medidas de pH. Ibersensors 2012. GMIETR UMR-CNRS 6164, Universite RENNES I, FRANCE e LSI, University of São Paulo, São Paulo, Brazil. [5] B. da Silva Rodrigues. 2011. Projeto e implementação de um sistema matricial para medição de pH baseado em transistores de porta suspensa (SGFET).Tese de doutorado apresentada à Escola Politécnica da Universidade de São Paulo e a Universidade de Rennes I - França para obtenção do titulo de doutor em Engenharia.2011. [6] A. Kherrat, F. Le Bihan, E. Jacques, M. Thomas, O. De Sagazan, S. Crand, T. Mohammed-Brahim. 2011. pH sensing from frequency response of SGFET. IETR, University of Rennes I, Rennes, France.