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Armenian Journal of Physics, 2012, vol. 5, issue 4, pp. 194-202
SURFACE POTENTIAL BEHAVIOR
IN ISFET BASED BIO-(CHEMICAL) SENSORS
A. V. SURMALYAN
Department of Semiconductor Physics & Microelectronics, Yerevan State University, Yerevan, Armenia
E-mail:[email protected]
Received 3 December, 2012
1. Introduction
For the most recent of 30 years, the developments of ion-selective field-effect transistors
(ISFET) are very furious and create pride [1]. The importance of work in this field issued in last
years because of increased ISFETs applications. Many works have been recently done to
characterize ISFET based on MOS technology [2-4]. Among these devices, proton-sensitive
ISFETs are the more deeply analyzed. ISFET have a very fast response time, high sensitivity, small
size, robustness and the potential for on-chip circuit integration. Because of the advantages, the
ISFET can be widely used in many areas especially in biomedical areas such as medical
diagnostics, monitoring clinical or environmental samples, fermentation and bioprocess control and
testing pharmaceutical or food products [5]. An ISFET-based penicillin sensor [6], ISFET-based
zeta potential analyzer (protein detection) [7], urea detection [8] and ISFET glucose sensor [9] are
the several examples of ISFET application in medical area.
The ISFET sensitivity depends mainly on the choice of the gate dielectric material(s). The
most commonly used materials are silicon and metal oxides or nitrides (SiO2, Si3N4, Ta2O5, Al2O3
and TiO2). When the transistor gate is coated with some ion selective membrane, the ISFETs can be
used for the selective detection of the various species in the surrounding electrolyte, other than the
hydrogen ions. Such devices are known as the CHEmically Modified Field Effect Transistors
(CHEMFETs) [4].
In this work, results of analytical investigation of semiconductor surface potential dependence
on the hydrogen ion concentration and parameters of gate layer(s) in case of complex gate dielectric
materials for ISFET based bio-sensors are presented.
2. ISFET Structure and Principle of Operation
As it is known the ISFET is in fact nothing else than a MOSFET with the gate connection
separated from the chip in the form of a reference electrode inserted in an aqueous solution which is
in contact with the gate oxide. Namely, the gate structure, presented in Fig.1, consists of a reference
electrode and an insulator layer between which an electrolyte is flowing. In Fig.1 S is the gate
potential, d is the potential of diffusion layer in the electrolyte solution, ox is the potential of
ISFET Based Bio-(Chemical) Sensor || Armenian Journal of Physics, 2012, vol. 5, issue 4
oxide layer and s is the semiconductor surface potential. The ion concentration in the electrolyte
influences the gate potential, which in turn modifies the lateral transistor threshold voltage. In this
way, the hydrogen ion concentration exercises an electrostatic control on the drain current mode,
which means that the change of the drain current due to the change of the ion concentration in the
electrolyte is compensated for by the adjustment of the RE potential (the gate voltage) [4-6].
Therefore, the ISFET sensitivity is usually expressed as the gate voltage change per a decade of the
hydrogen ion concentration pH, i.e. the change of the H s concentration by 10 times. Note that pH


denotes  log  H s  , e.g. if the value of the pH is equal to 3, the concentration of the hydrogen
ions amounts to 103 mole per liter [4].
All considerations are approving to n-channel ISFET structure.
G
(RE)
S
D
Potential
Al
n+
Depletion Layer
g
d
 ox
Electrolyte
Al
n+
Insulator
S
p-Silicon
Back contact (Al)
Fig.1. ISFET structure.
3. The Solid-Liquid Interface Model
When a silicon oxide surface is in contact with an electrolyte solution, surface hydroxyl
groups (SiOH) are built up at the oxide–electrolyte interface (Fig.2). These hydroxyl groups are
charged depending on the pH-value of the solution. The pH-value at which the surface is neutral, is
called the point of zero charge (pHpzc) (pHpzc 2.2 for SiO2) [10]. At a pH lower than pHpzc, the
oxide surface is positively charged and at a pH higher than pHpzc the surface is negatively charged.
Consequently, at physiological pH value ( pH  7 ), the net surface charge of silicon oxide is
negative.
In this section, the relationships between the surface potential S and the pH-value of the
solution, the surface charge density  and the concentration of ions in the electrolyte are
considered [10,11].
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A.V. Surmalyan || Armenian Journal of Physics, 2012, vol. 5, issue 4
Fig.2. Schematic illustrates the acid-base equilibrium (chemical reactions) on silicon oxide surface.
The theoretical studies of the phenomena occurring at the solid–liquid interface in the ISFET
sensors (in the case considered here, it is the interface between the gate dielectric and the
electrolyte) had been undertaken by many authors. Usually, the ISFET operation is explained by
so-called site-binding theory, which relates the interface potential to the concentration of the
hydrogen ions in the analyzed solution [12,13]. According to this theory, the ions present in the
solution react with positively or negatively charged active sites at the dielectric surface creating
hydrogen-active site pairs and consequently changing the total value of the active site charge at the
insulator surface. This, in turn, influences the transistor channel current through the variation of the
threshold voltage. Moreover, the active sites might react not only with the hydrogen ions but also
with other ions present in the measured solution, the so-called disturbing ions. All these chemical
reactions occurring at the phase boundary are reversible and described by the dissociation constant
k , which is temperature dependent as well [14].
Fig.3. Charge and potential distribution in the double layer at solid–liquid interface.
196
ISFET Based Bio-(Chemical) Sensor || Armenian Journal of Physics, 2012, vol. 5, issue 4
Because of the binding of the ions with the active sites, the gradient of ion concentration is
created in the electrolyte and, according to the Guy–Chapman–Stern theory [2], the so-called
double layer is established at the dielectric–electrolyte border as it is shown in Fig.3. The
double-layer consists of the diffuse layer and the Helmholtz layer. The Helmholtz layer comprises
the layer of adsorbed hydrogen ions and the common plane of adsorbed anions and cations [15].
The electrical representation of the double layer is also shown in Fig.3. The indices D, AK , S , C
and ins refer to the diffusion layer, the common plane of disturbing anions or cations, the insulator
surface, the transistor channel and the gate insulator, respectively. Based on the theory, assuming
that the number of active sites on the surface of the insulator is constant, the system of nonlinear
equations, presented further on, describes the relation between all the considered quantities.
More details on the theory of charge and potential distribution in the double layer and the
derivation of the equations can be found in [2,4,16,17].
Yates and coauthors developed a site-binding model to describe the charging mechanism of
the oxide surface [14]. According to this model, the charging of the oxide surface is the result of a
thermodynamic equilibrium reaction between the surface hydroxyl groups (SiOH) and the H ions
of the bulk electrolyte solution. The reactions are:
SiOH 2  SiOH  H  ,
SiOH  SiO   H  ,
K 
SiOH H  
SiOH 2 
,
SiO    H  
K  
.
SiOH 
This chemical reaction shows that an originally neutral surface hydroxyl group can bind a proton
from the bulk solution to become a positive surface charge with the dissociation constant K a , as
well as to donate a proton to the solution, leaving a negative charge on the oxide surface with the
dissociation constant K b [10,11].
4. Semiconductor surface potential behavior
4.1. One insulator material
Usually, the ISFET operation is explained by Site-Binding Theory (SBT), which relates the
interface potential to pH in the analyzed solution [14]. It is the amphoteric nature of the oxide
groups at the interface, in case of SiO2 these are SiOH groups, which causes the variation of the
oxide surface charge at varying pH. The neutral surface hydroxyl site can either bind ( SiOH 2 ) or
release (SiO) a proton depending on the solution pH. Because of the binding of the ions with the
active sites, the gradient of ion concentration is created in the electrolyte and, according to the
197
A.V. Surmalyan || Armenian Journal of Physics, 2012, vol. 5, issue 4
Gouy–Chapman–Stern theory the double layer is established at the insulator’electrolyte interface
[2-4]. By utilizing the Gouy–Chapman–Stern theory [15], it can be shown that
 g  d 
 C    0  
2kT
sinh 1  h d
.
q
 0  w kTC0 
(1)
Here  g and d are the gate potential and electrolyte diffusion layer potential, Ch  d  0    d is
the charge density in the diffuse layer, Ch and C0 are the Helmholtz layer capacitance per unit area
and the solution concentration [4], respectively, and  w is the water relative permittivity,  0 is the
vacuum dielectric permittivity. The condition of charge neutrality for the investigated structure
(see Fig.1) gives:
d  0  mos  0,
(2)
where 0 is the charge density of the surface sites, mos is the charge density in the semiconductor
given by
 mos
  q
 q   n  q
 q   
  2 0  s kTp0  s  1  exp   s    0   s  1  exp  s   
 kT   p0  kT
 kT   
 kT
12
(3)
 s is the semiconductor relative permittivity, n0 and p0 are the equilibrium concentrations of
electrons and holes, respectively [3].
The charge density is taken positive for s  0 and negative for s  0. Otherwise 0 can be
written as [3]
2

 H s   K  K 
0 

qN s   H   2  K  H    K K
s 
 
 
 s 


 H s 
 N sil  
 N
 H    KN
 s  s 
N
 nit ,
 N
 s
(4)
where N s is the total number of available surface binding sites, N sil and N nit are the number of
silanol sites and primary amine sites per unit area, respectively; K i are the dissociation constants
for the chemical reaction at the insulator interface, and H s is the concentration of protons at the
insulator surface. For d we have
 qd


d   8kT  r  0  K AK
   H s  sinh  2kT




,

(5)

where  r is the electrolyte relative dielectric permittivity, K AK
is the cation concentration at the
common plane.
The solution of the above set of equations (1)–(5) lead to the computation of the dependence
of the semiconductor surface potential s on the hydrogen ion concentration (pH):
198
ISFET Based Bio-(Chemical) Sensor || Armenian Journal of Physics, 2012, vol. 5, issue 4
2kT
s 
q

 s 0 kT  n0  p0 

1
2


 H   
 H s   K  K 

 qN    s   qN
  H   2  K  H    K K  sil   H s   K N   nit
  s 
  
 

  s 
(6)



   H s   8kT 0 r  wC0  K AK
   H s   .
  q g 2kT  2kT  w0  K AK





Here  w is the water permittivity,  r is the electrolyte relative electric permittivity,  0 is the
vacuum electric permittivity, C0 is the solution concentration.
4.2. Two insulator materials
Many works has been done to demonstrate the feasibility of using ISFETs for measuring pH
and other ions in the electrolyte. In explaining interactions of electrolyte ions with oxide surfaces,
some researchers have emphasized the structural and the physical aspects of the distribution of
solutes. Others have stressed the specific interactions of solutes with oxide surfaces and solution.
However, no comprehensive model existed to illustrate surface potential behavior in ISFETs with
two insulator materials. As we have got an equation for dependence of the semiconductor surface
potential s on the hydrogen ion concentration (Eq.6), we can propose that existence of the second
dielectric layer can change of sensitivity of ISFET. By changing the dielectric material or dielectric
thickness we can obtain the surface potential characteristics capable of pH.
Additionally we have got an equation for the case of two different insulator dielectric
materials
2

 H s   K  K 
qN sil 
C1
s    E1d1  E2 d 2   d 
Ch
Ch   H   2  K  H    K K
s 
 
 
 s 

  qN nit
 Ch


 H s 

  H s   K N 
 

,


(7)
where C1 is the two dielectrics double-layer capacitance per unit area, E1 and E2 are electric field
intensities in the first and second dielectrics, correspondingly, and
C1   0 1 2  1d 2   2 d1  ,
(8)
where 1,2 and d1,2 are the dielectric relative permittivity and thickness of dielectric materials,
respectively,



.
d   g   2kT q  4  wC0 r  H s   K AK
(9)
As we see in Fig.4, in presence of second dielectric the range of sensitivity changes and
 s  pH  -sensitivity increased. For the same value of the surface potential the hydrogen ion
concentration is decreased. So, taking into account Eq.(8), we can assume that by changing the
dielectric thickness or dielectric material types we can obtain the surface potential characteristics
capable of pH.
199
A.V. Surmalyan || Armenian Journal of Physics, 2012, vol. 5, issue 4

 s V


SiO2












 



SiO2/Ta2O5
Fig.4. pH-dependence of semiconductor surface potential in case of one and two insulators.
 s V
 s V
 pH  -dependence in case of two insulator pairs (a) SiO2/Al2O3 and (b) SiO2/Si3N4 in case of one (1) and two
s
(2) dielectric materials.
Fig.5. 
Dependences in case of two insulator pairs SiO2/Al2O3 and SiO2/Si3N4 are shown in Fig.5. In
these cases also different characteristics of surface potential behavior and a little change of
sensitivity are present. As you can see in both cases the sensitivity range of structures compared to
one dielectric material case is nearly the same. So SiO2/Al2O3 and SiO2/Si3N4 pairs have the same
sensitivity range in theoretical results. Maybe they are much more different in experimental results.
Dependences in Figs.2–5 are plotted using following parameters for Si, insulators and
electrolyte at 0.7V gate potential and 300K [18-20]:  s  11.7,  w  78.3,  r  5, 0  8.85 1012 ,
 Si  1  cm ( p0  N A  1.3 1016 cm3, n0  1.73 104 cm3),   10 6 s, R  0,   500 cm1,
h  1.12
eV,
N sil  N nit  1012
cm2,
N a  2.6  1015
cm3,

K   102 cm3, K   10 6 cm, K N   0.0001 cm3, K AK
 0.01 cm3.
200
Ch  4.89  106
F
cm2,
ISFET Based Bio-(Chemical) Sensor || Armenian Journal of Physics, 2012, vol. 5, issue 4
5. Summary
Many works have been done to demonstrate the feasibility of using ion-sensitive field-effect
transistors for measuring pH and other ions in the electrolyte. However, no comprehensive model
existed to illustrate surface potential behavior in ISFETs with two insulator materials. As we have
got an equation for dependence of the semiconductor surface potential s on the hydrogen ion
concentration (Eq.6), we can propose that the existence of the second dielectric layer can change the
sensitivity of ISFET.
In this paper we suggest an ISFET model with a gate of double insulators. The model proved
to be appropriate for analyzing the ISFET behavior as a function of hydrogen ion concentration and
binding site density.
By utilizing the Gouy–Chapman–Stern theory and the condition of charge neutrality in builtup ISFET based bio-sensor we have got an equation which expresses dependence of insulator
surface potential on hydrogen ion concentration (pH) in the case of built-up double gate dielectric
materials. In presence of the second dielectric the range of sensitivity changes and pH-sensitivity is
increased.
Dependences in case of two insulator pairs SiO2/Al2O3 and SiO2/Si3N4 are showing that in
both cases the sensitivity range of structures compared to one dielectric material case is nearly the
same. But in case of SiO2/Ta2O5 pair in presence of the second dielectric the range of sensitivity
changes.
We can assume that by changing the dielectric thickness or dielectric material types we can
obtain the surface potential characteristics capable of pH.
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