Chapter VI Development of ISFET model. simulation

Transcription

1 Chapter VI Development of ISFET model using ZnO as gate and its simulation Introduction Theory of ISFET Experimental work Results and discussion Conclusion

2 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 largescale 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, highelectron 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]. 84

3 In this Chapter, an attempt has been made to design, model and simulate a ZnO- ionsensitive 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 ionsensitive 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 (V T ), of the transistor. Figure 6.1(a) shows schematic diagram of a MOSFET where the gate generally SiO 2. Figure 6.1(b) shows the schematic diagram of ISFET where the insulating layer can be SiO 2, Si 3 N 4, Al 2 O 3, Ta 2 O 5 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. 85

4 (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: W 1 2 I DS Cox ( VGS VT ) VDS VDS L 2 (1) Where, C ox is the oxide capacitance per unit area, W and L are the channel width and length, respectively, μ is the effective surface mobility, and V GS, V DS, and V T are the gate-tosource, drain-to-source, and threshold voltage, respectively. Threshold voltage of the MOSFET is expressed as [127]: V T V q FB Q C ox ox Q B 2 FB (2) Cox V = M Si Q + Q (3) q C 86

5 difference M The flat band voltage, V FB is composed of the metal-semiconductor work function and Si and any oxide charge/surface state per unit area introduced during the process. Q B is the depletion charges per unit area and the FB is the Fermi potential of the silicon substrate. Q ss the surface state density at the silicon surface and Q Ox the fixed oxide charge. C Ox 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 χ, and the surface potential ψ, The reference electrode potential E ref and the interfacial potential at the electrolyte-insulator interface ψ + χ is added to the V FB of the conventional MOSFET as follow. Ag is considered as the reference electrode and is used to bias the ISFETs. V = E ψ + χ M Si Q + Q (4) q C 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 ( e ) has to be balanced with equal magnitude and opposite signed charge q in solution which is expressed as. e + q = 0 (5) In this study, KOH is used as electrolyte solution; ZnO is used as ion sensitive gate, Silver (Ag) as reference electrode. 87

6 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] 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 400 o C 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: Electrical characterization: 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). 88

7 Figure 6.3: I-V Characteristics of ZnO coated on Glass Substrate Figure 6.4: I-V Characteristics of ZnO coated on Silicon Substrate 89

8 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 (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 90

9 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. 91

10 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 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. 92

11 In the simulation experiment, the gate voltage is assumed to vary as the electrodeelectrolyte 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 C-V Characteristic: Figure 6.9: C-V characteristics of ISFET 93

12 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 10 9 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. 94

13 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 ZnOelectrolyte 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. 95