Chapter VI Development of ISFET model using ZnO...

Chapter‐VI

Development of ISFET model using ZnO as gate and its

simulation

• Introduction

• Theory of ISFET

• Experimental work

• Results and discussion

• Conclusion

Development of ISFET model using ZnO as gate and its simulation

84

6.1 General:

The human bodies pH and electrolyte potassium ion (K+) are of utmost importance to

the functioning of vital organs. These two are crucial parameters to keep brain, heart as well

as kidney to function normally. To monitor these parameters, researchers have been

attempting to derive materials, devices and sensors [85, 86]. The various design sensors have

been used for in vivo and in vitro sensing. Potentiometric ion sensors based on ion-sensitive

field effect transistor (ISFET) are based on a combination of “transistor technology” and

“chemical selective membrane technology” these sensors are attractive due to miniaturization

benefit, high sensitivity, robustness, fast response time, low output impedance, multi sensing

implementations, compatibility with integrated circuit technology, and suitability for large-

scale production at a low unit cost.

As bare silicon dioxide, silicon nitride gate ISFET is a pH sensor [124, 125] for

detecting a particular ion, these gate are to be modified with a sensing membrane containing

an ionophore. These membranes select the specific ion in presence of other ions in the

solution. The ionophores are molecules which bind to a particular ion and pass other ions

across the membrane. An ISFET in which the gate insulator is covered with an ion-selective

membrane is known as a Membrane FET or MEMFET[96, 126].

Generally, the electrode of electrochemical sensor has been used as a source or sink

of electrons as it has low resistivity. This paradigm has changed, largely due to the interest

shown by electrochemists in the field of metal oxide semiconductors. Design of a high

sensitive, reproducible and long last electrode has a great demand. Recently chemical sensing

based on ZnO material has attracted researchers to carry out fundamental studies of the

semiconductor-electrolyte interface for biosensor applications. Apart from this ZnO has also

remarkable properties like non-toxicity, bio-safety, excellent biological compatibility, high-

electron transfer rates, enhanced analytical performance, increased sensitivity, easy

fabrication and low cost [43]. Due to such advantages, stable and reproducible signals with

respect to analyte concentration changes are expected to be obtained [96].


85

In this Chapter, an attempt has been made to design, model and simulate a ZnO- ion-

sensitive field -effect transistor (ISFET) for bio sensing application particularly to sense

potassium ion (K+).

Firstly, the sol-gel and spin-coating method is used to fabricate the proposed ion-

sensitive gate. This is a low cost method compared to the other fabrication methodologies.

The ZnO is coated on a glass substrate by this method. This coating was characterized to

study its electrical properties. The ZnO electrical parameters of this ZnO coated device was

used to design a ZnO gate ISFET. The characteristic of this ISFET is studied using the

simulation tool. Prior to fabrication of bio sensor device, design and simulation are

extensively needed to avoid wastage of expensive time and cost. The device is modeled as

ZnO gate ISFET in a simulation environment tool using SILVACO TCAD. The I- V and C-

V characteristics of the ZnO gate ISFET have been studied.

6.2 Theory of ISFET:

An ISFET is a device which can be used to measure the concentration of ion in a

solution. With the change in the ion-concentration, the current through the transistor change.

The solution, ion selective membrane and the reference electrode behaves as the gate of this

transistor. The ISFETs are realized by modifying the metal gate electrode of a Metal Oxide

Field Effect Transistor (MOSFET). Therefore, the metal gate acts as a remote gate. The ZnO

based gate while exposed to an ionic solution (electrolyte) generates an emf and hence

modulates the threshold voltage (VT), of the transistor. Figure 6.1(a) shows schematic

diagram of a MOSFET where the gate generally SiO2. Figure 6.1(b) shows the schematic

diagram of ISFET where the insulating layer can be SiO2, Si3N4, Al2O3, Ta2O5 etc. In ISFET

the threshold voltage depends on the interaction of the insulating material with the ions in the

electrolyte. This interaction produces an ion sheath which rises the voltage between substrate

and the oxide layer. These rises in voltage increase the conductivity of the underlying

channel and hence the current flow through channel increases.


86

(a) (b)

Figure 6.1: (a) Schematic diagram of a MOSFET. (b) Schematic diagram of an

ISFET.

During normal operation, ISFETs are biased in non-saturated mode, since any change in

ion concentration in the solution is assumed to modulate the threshold voltage which, in this

mode of operation, exhibits a linear relation with drain current. Drain current of a MOSFET,

in non-saturated mode, can be expressed as:

2

21 ) ( DSDSTGSoxDS VVVV

LWCI (1)

Where, Cox’ is the oxide capacitance per unit area, W and L are the channel width and

length, respectively, μ is the effective surface mobility, and VGS, VDS, and VT are the gate-to-

source, drain-to-source, and threshold voltage, respectively. Threshold voltage of the

MOSFET is expressed as [127]:

ox

BFB

ox

oxFBT C

QCQ

qVV

2 (2)

푉 =M − Si푞 −

푄 + 푄퐶 (3)


87

The flat band voltage, VFB is composed of the metal-semiconductor work function

difference M and Si and any oxide charge/surface state per unit area introduced during the

process. BQ is the depletion charges per unit area and the FB is the Fermi potential of the

silicon substrate. Qss the surface state density at the silicon surface and QOx the fixed oxide

charge. COx is gate capacitance. The ISFET also follows the similar relationship with two

additional terms incorporated into the threshold voltage. The interface potential at the gate

oxide-electrolyte interface is determined by the surface dipole potential of the solution 휒 ,

and the surface potential 휓 , The reference electrode potential Eref and the interfacial

potential at the electrolyte-insulator interface 휓 + 휒 is added to the VFB of the

conventional MOSFET as follow.

Ag is considered as the reference electrode and is used to bias the ISFETs.

푉 = 퐸 − 휓 + 휒 −M − Si푞 −

푄 + 푄퐶 (4)

In semiconductor / liquid (electrolyte solution) interface the electrical field is not

equal to zero i.e. at n type-ZnO/KOH interface. To maintain the equilibrium condition the

excess charge in the semiconductor (|푒|) has to be balanced with equal magnitude and

opposite signed charge q in solution which is expressed as.

|푒| + 푞 = 0 (5)

In this study, KOH is used as electrolyte solution; ZnO is used as ion sensitive gate,

Silver (Ag) as reference electrode.


88

6.3 Experimental Work:

In the experimental section, the fabrication process of ZnO film on glass and silicon

substrate is briefly discussed. ZnO film which is used as gate oxide electrode of ISFET, are

used to investigate and evaluate the effectiveness of the sensing performance of K+ ions from

KOH solution. The ZnO film were prepared by sol gel spin coated method [128].

6.3.1 Spin Coating Technique:

In the spin coating process, the substrate spins around an axis which should be

perpendicular to the coating area. This spin coating process is developed using WT-S4K

instrument. In this case, the glass substrate is fitted on center pad with vacuum clamped. A

drop of the solution / gel is placed over the center pad and spinned at 5000 rpm for

180seconds.Then the annealing of ZnO film was done at 400oC for 4 hours to recrystallization.

A thickness of 72μm ZnO film has been coated over a glass substrate using this spin coater.

6.4 Result and Discussion:

6.4.1 Electrical characterization:

6.4.1.1 I-V Characteristic:

The I-V graph for ZnO film coated on glass substrate at room temperature was determined by

using four probe methods which is shown in Figure 6.3 for glass substrate and Figure 6.4 for

Silicon substrate. The graph is the measuring result using Electrometer [NI PXI-1042 (with

data acquisition card NI PXI 4072 and related software).


89

Figure 6.3: I-V Characteristics of ZnO coated on Glass Substrate

Figure 6.4: I-V Characteristics of ZnO coated on Silicon Substrate


90

6.4.1.2 C-V Characteristic:

The capacitance per unit area of the gate-induced depletion region at the onset of strong

inversion is presented as the C-V characteristics curve in Figure 6.5 for glass substrate and

Figure 6.6 for Silicon substrate. The graph is the measuring result using LCR meter [NI PXI-

1042 (with data acquisition card NI PXI 4072 and related software)]. As the presence of

oxygen vacancy in ZnO decreases the resistance in prepared ZnO, it decreases the

capacitance which is conformed from the C-V curve.

Figure 6.5: C-V Characteristics of ZnO coated on Glass Substrate


91

Figure 6.6: C-V Characteristics of ZnO coated on Slicon Substrate

6.5 Modeling and Simulation of ZnO-ISFET:

ISFET can be regarded as a MOSFET whose gate connection is replaced by the metal

connection of a reference electrode, which is immersed in the electrolyte to be analyzed. The

electrolyte includes the ions of interest and forms the conducting medium between the

reference electrode and the membrane/gate-insulator stack [129]. Effect of electrolyte a ZnO

is presented in chapter-III. This shows that by increasing the concentration of KOH in the

solution increases the voltage. This voltage can after the threshold voltage of the proposed

ISFET as ZnO is used as the gate oxide and KOH is used as the electrolyte. The voltage

generated in the ZnO film for pH value of 4 to 8 is in the range of 0.8 to 1.6 V [130]. Silvaco

is a TCAD tool where different materials can be arranged for virtual fabrication and

simulation of a device. Accordingly, the proposed ISFET using ZnO as gate oxide is

designed virtually in Silvaco TCAD tool using an equivalent MOSFET model.


92

Figure 6.7: ISFET Equivalent model as MOSFET

The structure is a resemblance of a practical ISFET. The ISFET equivalent model as

MOSFET is shown in Figure 6.7. The electrical characterization has been studied by the

simulation using a thin layer (72µm) of ZnO over silicon substrate and a comparative study

has been performed with the experimental observation. The simulation was performed by

varying the gate voltage from 0 to 3.4 V and the drain current increases linearly up to 2.2 V

and then saturate.

6.5.1 I-V Characteristic:

The I-V characteristic of ZnO in Figure 6.8 shows that the ZnO is slightly conducting

as it is fabricated in room temperature. This is due to the presence of oxygen vacancies for

which the resistance is less in the prepared ZnO film. The activation energy of ZnO is reported

to be 60meV. That is the reason why oxygen vacancy is the idealistic defect observed in ZnO

at room temperature.


93

In the simulation experiment, the gate voltage is assumed to vary as the electrode-

electrolyte interface voltage increases. For gate voltage variation 0.8 to 1.6V, the drain current

linearly and hence it is suitable for pH sensing application.

Figure 6.8: Output characteristics of ISFET

6.5.2 C-V Characteristic:

Figure 6.9: C-V characteristics of ISFET


94

Figure 6.9 shows the simulation graph of the proposed C-V characteristics of ZnO-

ISFET (equivalent to MOSFET). From results of both C-V analyses are nearly similar to FET

and simulation study of ZnO film gate oxide shows better than experimental ZnO film on

glass substrate. Because presence of defects provides more free conduction electrons for

which the capacitance is decreased by 109 Farad in comparison to simulation result. The

simulation done by SILVACO based on high purity of ZnO film gate oxide have zero defect.

However ZnO fabricated at room temperature always present in a non-stochiometry ratio of

Zn and O. Oxygen vacancy is a common point defect in ZnO at room temperature [131].

Hence the experimental observation is obvious to reduce capacitive value which is well

matched to the result of I-V graph.


95

6.6 Conclusion:

In this study, the ZnO film was fabricated using sol-gel and spin coating technique. Its

electrical characteristic showed that is semiconducting. The ZnO is also sensitive to KOH

solution and the voltage is generated by increasing KOH concentration in the solution. This

provided an idea that this ZnO can be used as the gate of the ISFET. Hence a simulation

study was carried out by providing equivalent voltage (as it would have obtained in ZnO-

electrolyte interface) to the gate. The drain current was increased linearly. Therefore the

proposed ZnO film can be used as a suitable membrane in an ISFET for sensing K+ ions in

any solution such as blood.

Chapter VI Development of ISFET model using ZnO...

Documents

Transcript of Chapter VI Development of ISFET model using ZnO...