Study on pHpzc and surface potential of tin oxide gate ISFET
Introduction
Since P. Bergveld [1]first employed the field-effect transistor for neurophysiological measurements in 1970, Ion Sensitive Field Effect Transistors (ISFETs) have developed into a new type of chemical-sensing electrode. Many theoretical and experimental studies have been published describing the behavior of this chemical-sensing electronic device [2].
Several inorganic materials such as Si3N4, Al2O3 and Ta2O5 have been used for pH-ISFET applications. Tin oxide, prepared using thermal evaporation or sputtering, as a pH-sensitive material for ISFET applications was first studied in our laboratory 3, 4, 5. The characteristics (e.g. sensitivity, drift, hysteresis, response time and temperature effect) of the amorphous tin oxide prepared using thermal evaporation have been studied [6].
Even though this device was discovered over 27 years ago, there are few fundamental studies on the ISFET. For example, in ISFET's applications, it is important to understand the behavior of pHpzc and surface potential at the inorganic material–electrolyte interface. Therefore, more investigations on these issues can give us a better understanding of mechanisms of the ISFET.
Up to now, the conventional methods 7, 8used to obtain the pHpzc and surface potential are streaming potential, electrophoretic mobility and light-addressable potentiometric sensors (LAPSs). In the latest publication [9], A.A. Poghossian used a new method for determination of pHpzc of SiO2, Ta2O5 and Al2O3 from the capacitance–voltage characteristics of metal insulator semiconductor (MIS) and electrolyte insulator semiconductor (EIS) structures. The results obtained by A.A. Poghossian agree well with the results obtained by the streaming potential method. Although he provided a convenient method compared to the conventional methods, one thing must be noted. Using the above methods to investigate the pHpzc surface potential is based on one assumption. That assumption is that charges within the insulator or at the semiconductor–insulator interface are not considered [2]. As in standard MOS theory [10], these charges can be taken into account using flat band voltage. However, in practical situations, the ISFET's operation is not at flat band condition. The ISFET always operates at linear region or saturation region. This implies that the assumption cannot be used when the ISFET operates far from the flat band condition. This effect will cause a difference in pHpzc and surface potential extracted using different operating conditions.
In this paper, the pHpzc and surface potential of tin oxide–electrolyte interface when ISFET operates under a flat band condition and linear region are investigated. Dual FETs configuration: Al–SnO2–Si3N4–SiO2 gate MOSFET and SnO2–Si3N4–SiO2 gate ISFET were fabricated for extraction of the pHpzc and surface potential of a tin oxide–electrolyte interface. C–V measurement was used to obtain the flat band voltages of the MOSFET and ISFET. Based on the C–V curves at a flat band condition, the surface potential of a tin oxide–electrolyte was investigated and the pHpzc was evaluated equal to 5.6. On the other hand, the surface potential of tin oxide–electrolyte was investigated and the pHpzc is evaluated equal to 6 at the linear region using I–V curves of Al–SnO2–Si3N4–SiO2 gate MOSFET and SnO2–Si3N4–SiO2 gate ISFET. Furthermore, according to a series of theoretical simulations, we can conclude that the surface parameter ΔpK is the dominated factor of pH response in ISFET.
Section snippets
Experimental procedures
Four types of structures were fabricated for this study.Type 1 Al–SiO2–Si MIS diode Type 2 Electrolyte–SiO2–Si EIS diode Type 3 Al–SnO2–Si3N4–SiO2 gate MOSFET Type 4 SnO2–Si3N4–SiO2 gate ISFET
After Si3N4 gate ISFETs were completed, we used an additional metal mask to deposit aluminum and tin oxide in a thermal evaporation system (tin oxide powder: 99.9%) with a substrate temperature of 150°C. The thickness of these films were about 5000–6500 and 1500–2000 Å, respectively. Additionally, the thickness of
Approach methods
The simple structure of the ISFET is shown in Fig. 1. In Fig. 1, we see that the ISFET is a chemical sensor based on MOSFET, except that a solution gate and reference electrode are used instead of a metal gate. Furthermore, because the site bind model is widely used for describing ISFET's operation. In this study, we use MOSFET's theory and the site-binding model [11]for the extraction of pHpzc and surface potential.
The flat band voltage of MOSFET and ISFET are as list, respectively [11]
Conclusions
A study of the pHpzc and surface potential of tin oxide gate ISFETs using dual FETs configuration: Al–SnO2–Si3N4–SiO2 gate MOSFET and SnO2–Si3N4–SiO2 gate ISFET was presented. The conclusions can be summarized as follows:
- 1.
Eref − ΦM/q value is investigated using SiO2–Si EIS and Al–SiO2–Si MIS, the value 0.705 V agrees well with the theoretical value.
- 2.
Based on the C–V characteristics of Al–SnO2–Si3N4–SiO2 gate MOSFET and SnO2–Si3N4–SiO2 gate ISFET at flat band voltage, the surface potential can be
Nomenclature
VFB flat band voltage ΦM aluminum electron work function Φsi silicon electron work function q electron charge Qox charges located in the oxide Qss surface and interface states Cox oxide capacitance Eref potential of the reference electrode Ψ surface potential between sensitive layer and electrolyte interface χsol surface dipole potential of the solution k Boltzmann constant T absolute temperature (K) β sensitivity factor pHpzc pH at point zero charge Ka, Kb equilibrium constant CDL double layer capacitance Ns total number of
Acknowledgements
This study is supported by the National Science Council of Taiwan under contract NSC87-2218-E-033-007.
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