ISSN 10637397, Russian Microelectronics, 2014, Vol. 43, No. 7, pp. 511–515. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.V. Kuznetsov, O.V. Chuyko, 2013, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Elektronika, 2013, No. 3(101), pp. 53–59.
The value of the pH of a solution is an important
indicator of the physiological state of the biological
medium and the index of the proceeding chemical and
biochemical reactions. The development of pHsen
sors with a high sensitivity, reliability, and small size
makes it possible, on the one hand, to carry out studies
in extremely small volumes of a sample, on the other
hand, a miniaturization of a sensor opens an opportu
nity for studying the pH gradient distribution over an
active surface or in volume, e.g., in a bioplast . The
development of integrated matrices based on such
sensitive elements allows us to create various complex
systems based on the principle of controlling local
changes in the pH index, in order to carry out the
chemical and biochemical analysis of solutions .
Ionsensitive fieldeffect transistors (ISFETs) are
widely applied as sensitive elements of pHsensors, in
which, in contrast to conventional fieldeffect transis
tors, the solution to be analyzed is used as a gate and
the potential at this gate is determined by a reference
The fabrication of an ISFETtransistor is com
pletely compatible with the main processes of planar
technology for integrated circuits (ICs), and the devel
opment of biosensors is possible on its base by func
tionalization of the surface of the sensitive element by
various surfaceactive substances. This opens broad
prospects for the development of analytical micro
and nanosystems, e.g., for sequencing DNA .
Despite numerous publications on the use of nan
odimensional ISFETstructures as pHsensors [4, 5],
their advantages have not been substantiated in prac
tice and the methods of their design systematic opti
mization are absent.
In order to determine the optimal design of an
ISFETtransistor as a pHsensor, the dependence of
its characteristics on the design parameters was stud
ied using the methods of finitedifference mathemati
cal modeling by employing the design instruments of
Synopsys TCAD Sentaurus.
It is reasonable to define the target function of the
ISFETelement design as its maximum sensitivity.
The expression for the sensitivity
of a pHsensory
element, which is determined as a change in the cur
rent of transistor drain
, caused by a change in the
pH of an aqueous medium, can be partitioned into two
is the surface potential at the solution–gate
The second multiplier is the sensitivity of the sur
face potential to a change in pH of the solution, and it
is determined as 
is the parameter, which takes a value from 0 to
1 depending on the buffer properties of the surface,
which in turn, depends on the gate insulator material
and ionic force of the solution:
is the differential electric capacitance of the double
is the intrinsic buffer capacitance of
is the temperature of the medium;
the electron charge; and
is the Boltzmann constant.
Study of the Sensitivity of pHSensors Based
on Silicon MOSNanotransistors
E. V. Kuznetsov and O. V. Chuyko
NPK Technological Center, Moscow, Russia
Received July 4, 2012
—The influence of designtechnological parameters of ionsensitive fieldeffect transistor structures
(ISFETs) on their sensitivity to pH has been considered with the use of numerical modeling. An analysis of
various silicon ISFETtransistors has been carried out. It is shown thatsuspended nanowire structures with
an intrinsic type of conductivity with a minimum possible diameter and maximum capacitance of the gate
oxide have the greatest sensitivity.