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
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