Journal of Sensor Technology, 2013, 3, 57-62
http://dx.doi.org/10.4236/jst.2013.33010 Published Online September 2013 (http://www.scirp.org/journal/jst)
An Integrated ISFET pH Microsensor on
a CMOS Standard Process
Francisco López-Huerta1*, Rosa María Woo-Garcia2, Miguel Lara-Castro1,
Johan Jair Est ra da -L óp ez3, Agustín Leobardo Herrera-May1
1Research Center for Micro and Nanotechnology, University of Veracruz, Veracruz, Mexico
2Faculty of Electronics Sciences, Meritorious Autonomous University of Puebla, Puebla, Mexico
3Faculty of Mathematics, Autonomous University of Yucatán, Mérida, Mexico
Received June 7, 2013; revised July 7, 2013; accepted July 15, 2013
Copyright © 2013 Francisco López-Huerta et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
We present the design and integration of a nine-pH microsensor array on a single silicon substrate with its own signal
readout circuit, integrated in a 0.6-µm commercial standard complementary metal oxide semiconductor (CMOS) proc-
ess. An ion sensitive field effect transistor (ISFET) has been used as pH microsensor and an instrumentation amplifier
as the read-out circuit. The ISFET structure is conformed by the channel length and ratio of MOS transistor, gate ex-
tended and the selective membrane, for which silico n nitride (Si3N4) is employed as an ion selective element. The com-
plete design includes shielding around the pH microsensor and the readout circuit to avoid leakage of current to the
substrate. The readout circuit is composed by three operational amplifiers and resistances that form the instrumentation
amplifier, with a ±2.5 V bias has a 50 dB gain, power supply rejection ratio (PSSR) of 120 dB and common mode re-
jection ratio (CMRR) of 127 dB. The complete system is integrated in a 1.12 mm2 silicon area; it presents a 59 mV/pH
linearity, within a concen tration range of 2 to 12 of pH level, making it a good alternative for biological or medical ap-
plications.
Keywords: CMOS; Instrumentation Amplifier; ISFET; pH
1. Introduction
The ion sensitive field effect transistor (ISFET) has been
used as pH sensor during the last years; also, they can
detect chemical and biological phenomena or even can
be used for biosensing [1-8]. Actually, the research, de-
sign, development and application of ISFET-based sen-
sors have displaced to the ion sensitive electrodes (ISE),
pH indicator strips and optical sensors [9,10] due to their
great advantages such as mass- production low cost, light
weight, small size, fast response, low output impedance,
high speed signal, distributed sensing, multiplexing pos-
sibilities and temperature compensation; also, it allows
that not only the readout circuits but also their control
system can be integrated on the same integrated circuit
(IC) in standard consumer CMOS processes [11-14], of-
fering new tendencies for solid state sensors.
ISFET based electrochemical pH sensors have found
applications in many fields such as environmental moni-
toring, agriculture, medicine, biological sensing and me-
dical diagnosis [13-18]. Variations that exist in the IS-
FET selectivity depend on the selective membrane em-
ployed, the most used materials are: Al2O3, Si3N4, SiO2,
TaO5, ZrO2, SnO2/Al; also, other kind of materials like
polymers, metallic oxides and organic/inorganic materi-
als [19-27] are used for this purpose. These materials are
widely used due to its high selectivity with the hydrogen
ions. ISFET structure is different to the conventional
FET structure due to an exterior exposed window that is
left inside the sensor structure where the sensitive mate-
rial will be deposited to form the selective membrane of
the hydrogen ions or others [28,29].
Particularly, the integration of sensors and electronics
on the same substrate potentiate the benefits of the de-
vices, improving function alities like signal amplification,
calibration and better signal to noise ratio. All these ad-
vantages drive to call the attention for the studying and
producing of ISFETs to apply them in many knowledge
areas. The basic requirements that a sensor must accom-
plish are 1) the superficial dielectric located in the IS-
FET gate region must have a sensibility as great as pos-
*Corresponding author.
C
opyright © 2013 SciRes. JST

F. LÓPEZ-HUERTA ET AL. 58
sible to detect the pH levels of the analyzed solution with
rapidness, efficiency and reliability; 2) the selected
dielectric material must present an amount of drift cur-
rent and hysteresis as minimum as possible during meas-
urements, which is important if the ISFETs are used to
measure pH levels for long periods of time. This last
condition makes them very useful in biomedical field
[18,27,30]. During the last years, the design and research
of ISFETs and readout circuits have been developed in
different ways, some authors do the research about the
sensor and the others use the standard consumer CMOS
technology, where the pH sensors and readout circuits
are on the same substrate, including a reference electrode
that is connected externally [31,32].
Considering the disadvantages previously presented,
we can say that it is a necess ity to do research rela ted with
microelectromechanical systems (MEMS) to improve the
pH microsensors’ characteristics. In this work, we pro-
pose the integration of 9 pH microsensors, implemented
with ISFET’s, all of them have their own readout circuit
associated. In addition, the reference electrode is inte-
grated in the same silice substrate, using a 0.6 m stan-
dard consumer CMOS process, wherewith very high
level of reproducibility and dimens ion control of the fab-
ricated devices will be obtained. Electrical properties of
Si3N4 will be used to form the selective membrane for
hydrogen ions.
This paper is organized as follows: in Section 2, we
describe both ISFET pH microsensor and readout circuit
formed by an instrumentation amplifier and analog am-
plifier. The results and experimental data are described in
Section 3. Finally, we present our conclusions and pro-
posed future research in Section 4.
2. Design of the ISFET pH Microsensor
2.1. ISFET Devices
We design the ISFET pH microsensor using a standard
0.6 µm triple metal, double poly layer CMOS ON Semi-
conductor process [33]. The ISFET’s structure is con-
formed by a transistor geometric ratio, extended gate and
the selective membrane (Figure 1), for which silicon
Figure 1. Schematic of the ISFET pH microsensor.
nitride (Si3N4) is employed as an ion selective element.
The complete design includes shielding around the IS-
FET pH microsensor and the readout circuit to avoid
leakage of current to the substrate.
The conventional MOSFET structure is different from
ISFET since the sensor structure is left exposed the
extension gate on which the sensitive material deposited
for forming the selective membrane of hydrogen ions.
The operation of the ISFET pH microsensor is similar to
that the conventional metal oxide semiconductor field
effect transistor (MOSFET) The Equation (1) for the
ISFET in saturation as a function of the pH [20]:


2
*1
2
neff

D
GT DS
CW
IVV
L

 V (1)
where n

is the mobility of th e electrons in the ch an ne l,
the capacitance effective per unit area Equation (2),
eff
C
W and L the width and length of the channel, respec-
tively, the gate voltage, the threshold voltage
of the ISFET Equation (3),
G
V*
T
V
D
S
V the drain to source volt-
age,

the channel modulation parameter.
ox M
eff ox M
CC
CCC
 (2)
where ox the capacitance per unit area of the gate in-
sulator and
C
M
C is the capacitan ce of the selectiv e mem-
brane.

*ln
TTi Re
RT
VV aV
nF
 f (3)
T
V
Tthe threshold voltage, the universal gas constant, the absolute temperature, the numbers of elec-
trons perm mole,
Rn
F
the Faraday constant, i the ac-
tivity of the ions and a
R
ef
V is the voltage of reference
electrode.
2.2. CMOS Integrated Readout Circuit for pH
Measurement
The instrumentation amplifier readout circuit for the IS-
FET pH microsensor was designed to have a high input
impedance, high common mode rejection ratio (CMRR)
and power supply rejection ratio (PSRR). Figure 2
shows the readout circuit used for the pH measurement.
The resistors R1-R4 were all 10 k and R was 20 k.
The ISFET as sensible element to detect the pH changes
can detect certain molecules in chemical solutions. The
chemical reaction changes the charge stores on the gate
and shifts the threshold voltage of the transistor. The
current variations due to different pH levels cause the
ISFET pH microsensor, behaves as a variable resistance
(depended a chemical solutions) connected between the
source (node A) and the drain (node B), obtaining dif-
ferent gains for the readout circuit. The emulated resis-
tance can be written by Equation (4):

F. LÓPEZ-HUERTA ET AL. 59

*
1
DDS neffG T
L
RIV CWVV


  (4)
In order to integrate both the ISFET pH microsensor
and the readout circuit, we used the geometric pattern
editor of L-Edit of Tanner® [34]. The readout circuit is
also formed by three analog amplifiers and each one of
them with a voltage gain of 1000, and Figure 3 shows
the schematic. The analog amplifier has a first stage inte-
grated by differential-pair transistors M1-M2 and a po-
larization through transistors M5 and M6. The biasing of
the circuit is made by means of a current mirror formed
by the resistor R1 and transistors M3-M4. In order to
improve the stability of this circuit, the compensation is
implemented by the transistors M7-M10 as well as a ca-
pacitor Cc [35]. The final stage consists of transistors
M11 and M12 that form a buffer. Table 1 shows the
channel length and width (aspect ratios) of all MOS tran -
sistors obtained for the proposed design in this work.
Our design for the measurement of pH ISFETs has 9
which present d ifferen t aspect ratio to vary the sensitiv ity
and dynamic range of detection of microsensors, Tabl e 2
shows the dimensions. Each pH microsensor consists of
Figure 2. Block schematic of our system implementation.
Figure 3. Schematic of the analog amplifier used in our
design.
the extended gate, the ISFET structure and the readout
circuit formed by the instrumentation amplifier (Figure
4). The union between the extended gate and ISFET was
made with a distance equivalent to all sensors, and each
element is aligned there by facilitating the etching on
metal or depositing the material to form the membrane
sensitive to hydrogen ions. Figure 5 shows a cross sec-
tion of the layers used in the CMOS process and the ion
sensitive membrane sen sitive. In this figure, metal layers
are ML1, ML2, ML3 and the contacts between are via1
and via2.
3. Results and Discussions
The electrical results of the ISFET pH microsensor simu-
lations were obtained fro m the analysis conducted with a
Table 1. Aspect ratio of the transistors for the proposed
design.
Devices W/L (µm)
M1-M2 7.2/1.2
M3-M4 16.8/1.2
M5-M7 4.8/1.2
M8 33.6/1.2
M9 24/1.2
M10 74/1.2
M11 360/1.2
M12 1197/1.2
Table 2. Aspect ratio of the ISFET pH microsensor array.
W/L (20) W/L (25) W/L (30)
24/1.2 30/1.2 36/1.2
60/3 60/2.4 54/1.8
96/8 90/3.6 108/3.6
Figure 4. Diagram of an ISFET pH microsensor.

F. LÓPEZ-HUERTA ET AL. 60
circuit simulator. A sinusoidal excitation signal of 200
mVpp at 1 kHz is supplied to the readout circuit and its
output signal is amplified more than 20 times (26 dB), as
shown in Figure 6.
The common mode rejection ratio (CMRR) simulated
in the readout circuit was 127 dB at 20 kHz, as shown in
Figure 7.
The Figure 8 shows the dynamic range for three IS-
FET pH microsensor, 20, 25 and 30, the sensibility as
function of the aspect ratio of the ISFET and the sensib le
material Si3N4. From this discussion it can be said that
using all microsensors a pH range from 2 to 12 could be
covered. The second observed situation is the variation
on the microsensor sensitivity for low pH values, the
slope of the curves changes, in the pH range of 1 - 3.
Such slope increase when the aspect ratio is 30, as shown
in Figure 9.
The pH microsensors’ response and dynamic range
Figure 5. Cross section of the layers employed for the IS-
FET pH microsensor.
Figure 6. Frequency response of the readout circuit of the
pH microsensor.
Figure 7. Common mode rejection ratio in the readout elec-
trical circuit of the pH microsensor.
Figure 8. Response curves of the pH microsensor.
Figure 9. Microsensor responses for different pH values.
were obtained simulating the performance of each of them
for different pH levels that vary from acidic to alkaline

F. LÓPEZ-HUERTA ET AL. 61
(2 - 12) through the electrical simulator.
The preliminary test of the electrical performance
characteristics will be obtained of the ISFET pH mi-
crosensor, using an arbitrary waveform generator, a digi-
tal oscilloscope and two power supplies. The ISFET’s pH
microsensos works with a ±2.5 volts DC and was pack-
aged with a DIP-24 (twenty-four dual in-line package).
The voltage applied to reference electrode is set to 2
volts, a sinusoidal excitation signal of 200 mV peak to
peak at 1 kHz is supplied to the amplifier and its output
signal is amplified as a function of the pH level. In order
to measure the cutoff frequency of the amplifier, a fre-
quency sweep for its input signal from 0.5 Hz to 1.5
MHz will be applied. The pH level is controlled using
KOH and HNO3 to increase and decrease it, respectively
[10]. Figure 10 shows a schematic with the main com-
ponents of a signal conditioning system that could be
used for pH microsensor. Figure 11 shows the micro-
photograph of the chip, which was fabricated by MOSIS
[36] and the distribution for ISFET, extended gate and
readout circuit.
Figure 10. Schematic of the signal condition system for the
ISFET pH microsensor.
Figure 11. Microphotograph of the ISFET pH microsensor.
Selective parts are numbered. 1. Readout circuit; 2. ISFET;
3. Extended Gate.
4. Conclusion
The design of an ISFET pH microsensor array based on
CMOS process using ON Semiconductor technology was
presented. The microsensor array includes the following
elements: the ISFET (with different area), extended gate
(100 × 100 m) and readout circuit (213 × 272.4 m).
This array was designed to detect different pH level val-
ues with a simple signal conditions system. It presented a
linear response for pH levels between 2 pH and 12 pH,
and resolution around the 60 mV/pH. Future work will
include the electrical characterization of the pH micro-
sensor array under different pH levels and the depositio n
of some materials on s urface extended gate.
5. Acknowledgements
Authors wish to thank the facilities provided by MOSIS
Research Program and CONACYT through grant 48757.
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