APPLICATION OF ION SELECTIVE FIELD EFFECT TRANSISTOR FOR HYDROGEN ION MEASUREMENT SYSTEM

Ibrahim Elkhier Hussien Arfeen1, Abdul Rasoul Jabar Kizar Alzubaidi2 1 Sudanese Standards and Metrology Organization, Port Sudan 2 Electronic Dept- Engineering College-Sudan University for science and Technology

Received December 2010, accepted after revision June 2011

ُمـ ْســتَ ْخـلـَص َهذف هذا انبحث إنً دراصت تزانزصتىر تأثُز انًدبل كًحضبس كًُُبئٍ َضتخذو نًزاقبت األس انهُذروخُنٍ فٍ يدبل انكًُُبء انتحهُهُت و انبُئُت. تعتًذ حضبصُت حضبس تزانزصتىر تأثُز انًدبل عهٍ نىع انًبدة اإلنكتزونُت انًضتخذيت كًدش فٍ بىابت انتزانزصتىر. وَضتخذو فٍ يىاضع انقُبس انتٍ َصعب فُهب اصتخذاو انًحضبس انزخبخٍ انقببم نهكضز. تضتخذو دائزة إنكتزونُت نزبط يحضبس تزانزصتىر تأثُز انًدبل تًكنه ين انعًم بصىرة فعبنت فٍ انًذي ين األس انهُذروخُنٍ 4 إنً 44. وظُفت انذائزة اإلنكتزونُت هي تحىَم تُبر دخم انًحضبس انذٌ ًَثم قُى األس انهُذروخنٍ إنً خهذ خزج يتنبصب. تى اختببر انتصًُى بىاصطت يحبنُم قُبصُت وخًضت أنىاع ين انًحبنُم انًبئُت وتى انحصىل عهً اننتبئح انًطهىبت. تى رصى انعالقت بُن اننتبئح انًتحصم عهُهب بىاصطت انذائزة اإلنكتزونُت وقُى األس انهُذروخُنٍ نبعض انًحبنُم انًبئُت.

ABSTRACT This paper aims to study field effect transistors as chemical sensor used for monitoring hydrogen ion concentration (H+) measurement in chemical analysis and environment fields, to detect the pH of the subjected solution. The Ion-Selective Field Effect Transistor (ISFET) sensor sensitivity depends mainly on the choice of the gate dielectric material (Silicon Nitride Membrane). The Ion-Selective Field Effect Transistor is (ISFET) used in places where fragile glass electrodes will be damaged. The interface circuit supplies the sensor with a constant current and voltage. The pH value is identified from the output voltage of the circuit. From the results, the system can be used for measuring the pH values in the range of pH4 to pH14. The function of the electronic circuit is to convert input current from sensor (ISFET) which represents pH value to a proportional output voltage. Design has been tested by a standard buffer solution and five kinds of aqueous solution and obtained the desired results. The relationship between the results obtained by electronic circuit and value of hydrogen ion for some aqueous solution has been plotted.

Keywords: Transistor, Chemical Sensor, Ion-Selective Field Effect Transistor, pH, Electronic Circuit.

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` APPLICATION OF ION SELECTIVE FIELD EFFECT TRANSISTOR FOR HYDROGEN ION MEASUREMENT SYSTEM 1. INTRODUCTION for a conducting channel is called the threshold voltage (VTH). With these properties, the FET In the field of chemical analysis, electrochemical can be configured as a biosensor by modifying techniques based on various phenomena the gate terminal with molecular receptors or occurring within an electrochemical cell are ion selective membranes for the analyses of very much favored for application in laboratory interest. The binding of a charged bimolecular and industry. The concentration of a result in depletion or accumulation of carriers component in solution is measured in terms of caused by change of electric charges on the the effect it has on the electrical properties of gate terminal. the cell. The effect of the component on an electrode introduced into the solution forms 1.2 Hydrogen Ion Measurement (pH) the basis of the electrochemical transducer of The measurement of the hydrogen ion activity the active type and the emf developed between in a liquid solution. It is one of the most the two electrodes of the cell constituted for common forms of analytical measurement in this purpose is used to signify the concentration industry. The pH has a great effect on the level. outcome of various chemical processes. Food Electrochemical transducers enable processing, water treatment, pharmaceutical presentation of these ionic potentials into a production, steam generation (thermal power suitable form so that the output signals are plants), and alcohol manufacturing are just properly related to the basic biochemical some of the industries which make extensive phenomena. In to some electrode systems for use of pH measurement (and control). the measurement of certain chemical Hydrogen ion is also a significant factor in the properties of solutions with which the corrosion of metal pipes and vessels carrying electrodes come in contact are described. aqueous (water based) solutions. Thus pH 1.1 Semiconductor Transducer measurement and control are important in the life extension of those capital investments. In general, a field-effect transistor (FET) consists of three terminals; the source, drain, and gate. 1.2.1 Chemistry of Hydrogen Ion The voltage between the source and drain of Hydrogen ion activity in aqueous (water-based) the FET regulates the current flow in the gate solutions is a very important parameter for a voltage. Specifically, the current-control wide variety of industrial processes. Hydrogen mechanism is based on an electric field ions are always measured on a logarithmic generated by the voltage applied to the gate. scale, and referred to as pH. The current is also conducted by only one type + of carrier (electrons or holes) depending on the Free hydrogen ions (H ) are rare in a liquid type of FET (n-channel or p-channel). A positive solution, and are more often found attached to voltage applied to the gate causes positive whole water molecules to form a positive ion + charges (free holes) to be repelled from the called hydronium (H3O ). region of the substrate under the gate. These Professionals usually refer to these positive ions positive charges are pushed downward into the simply as “hydrogen” even though the truth is a substrate, leaving behind a carrier-depletion bit more complicated. region. Hydrogen ion is mathematically defined as the The depletion region is populated by the bound negative common logarithm of hydrogen ion negative charge associated with the acceptor activity in a solution. Hydrogen ion activity is atoms. These charges are “uncovered” because expressed as a molarity (number of moles of the neutralizing holes have been pushed active ions per liter of solution), with “pH” downward into the substrate. being the unit of measurement for the The gate voltage applied to accumulate a logarithmic result: sufficient number of electrons in the channel pH = −log*H++ .…………….... (1)

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For example, an aqueous solution with an active as a result, each whole ph value below 7 is hydrogen concentration of 0.00044 M has a ten times more acidic than the next higher pH value of 3.36 pH. value for example, pH4 is ten times more acidic than pH 5 and 100 times (10 times Water is a covalent compound, and so there is 10) more acidic than pH 6. The same holds little separation of water molecules in liquid true for pH values above 7, each of which form. Most of the water molecules remain as is ten times more alkaline than the next whole molecules (H2O) while a very small lower whole value. For example, pH 10 is percentage ionizes into positive hydrogen ions + − ten times more alkaline than pH 9 and 100 (H ) and negative hydroxyl ions (OH ). The times (10 times10) more alkaline than pH mathematical product of hydrogen and 8. hydroxyl ion molarity in water is known as the ionization constant (Kw), and its value varies Pure water is neutral. But when chemicals with temperature. are mixed with water, the mixture can become either acidic or basic. Examples of At 25 degrees Celsius (room temperature), the −44 acidic substances are vinegar and lemon value of Kw is 1.0 × 10 units. Since each one juice. Lye, milk of magnesia, and ammonia of the water molecules that does ionize in this are examples of basic substances. See absolutely pure water sample separates into + table (1) exactly one hydrogen ion (H ) and one hydroxyl ion (OH−), the molarities of hydrogen and 1.4 Hydrogen Ion Electrodes hydroxyl ions must be equal to each other. Most often used pH electrodes are glass Means that pure water is neutral, and that the electrodes shown figure 1. Typical model is molarity of hydrogen ions is equal to the square made of glass tube ended with small glass root of Kw: bubble. Inside of the electrode is usually filled [H+] = (Kw)1/2 = (1.0 × 10−44)1/2 = 1.0 × 10−7 M with buffered solution of chlorides in which silver wire covered with silver chloride is Since we know pH is defined as the negative immersed. pH of internal solution varies - for logarithm of hydrogen ion activity, and we can example it can be 1.0 (0.1M HCl) or 7.0 be assured all hydrogen ions present in the (different buffers used by different producers) solution will be “active” since there are no other positive ions to interfere with them, the Active part of the electrode is the glass bubble. pH value for water at 25 degrees Celsius is: While tube has strong and thick walls, bubble is Hydrogen ion of pure water at 25oC = −log (4.0 made to be as thin as possible. Surface of the × 10−7 M) = 7.0 pH glass is protonated by both internal and external solution till equilibrium is achieved. As the temperature of pure water sample changes, the ionization constant changes as Both sides of the glass are charged by the well. Increasing temperature causes more of adsorbed protons, this charge is responsible for the water molecules to ionize, resulting in a potential difference. This potential in turn is larger Kw value. This means that while any pure described by the Nernst equation and is directly water sample is neutral (an equal number of proportional to the pH difference between positive hydrogen ions and negative hydroxyl solutions on both sides of the glass. ions), the pH value does change with temperature, and is only equal to 7.0 pH at one 1.4.1 Field Effect Transistors as Hydrogen Ion particular temperature: 25oC. Sensor 1.2.2 Hydrogen Ion Scale The basic principle of the Ion Selective Field The scale ranges from 0 to 14. A pH of 7 is Effect Transistors (ISFET) working is the control neutral. A pH less than 7 is acid a pH of the current flowing between two greater than 7 is basic. It's logarithmic and semiconductor elements (drain and source) by

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` APPLICATION OF ION SELECTIVE FIELD EFFECT TRANSISTOR FOR HYDROGEN ION MEASUREMENT SYSTEM Table 1: Hydrogen Ion Scale

pH Scale Concentration of hydrogen Example of Solution at this pH

pH=0 10000000 Battery Acid pH=1 1000000 Hydrochloric Acid secreted by Stomach pH=2 100000 Lemon Juice pH=3 10000 Grapefruit, Orange Juice pH=4 1000 Tomato Juice pH=5 100 Soft drinking Water/ Black Coffee pH=6 10 Urine/ Saliva pH=7 1 Pure Water pH=8 1/10 Sea Water pH=9 1/100 Baking Soda pH=10 1/1000 Great Salt Lake / Milk of Magnesia pH=11 1/10000 Ammonia Solution pH=12 1/100000 Soapy Water pH=13 1/1000000 Bleaches Oven Cleaner pH=14 1/10000000 Liquid Drain Cleaner

current flowing through the transistor. This current is a signal that can be measured to check the pH value. Figure 2 shows the schematic of MOSFET compared to ISFET. Generally, there are two types of planar FET- based biosensors, according to their structure; insulated gate field effect transistors (IGFET) and ISFET. In the case of IGFET, particularly MOSFET (metal-oxide-semiconductor field- effect transistor), the gate terminal is electrically isolated from the source and drain terminals. ISFET is similar to IGFET, but in the ISFET, the metal gate is replaced by an ion- Figure 1: Combination Glass Electrode selective membrane, electrolyte and a electrostatic field, generated by the protonated reference electrode (Figure 1). In the case of an oxide gate. Protonation of the gate is in a way ISFET biosensor, the amount of the current flow identical to the process taking place in glass pH will be not only determined by the charges of electrode, just the methodology used to bimolecular interacting on the gate dielectric, measure protonation degree is different. but also sensitive to pH, different ions, products Instead of measuring potential difference on of enzyme reactions, etc. An attractive feature two sides of the glass, measuring the current of such FETs is that it is possible to detect flowing through the transistor. The lower the bimolecular interactions in a label free manner pH, the more protonated and charged gate is through a direct change in conductance or a which changes its electric field changing in turn related electrical property.

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devices are known as the Chemically Modified Field Effect Transistors (CHEMFETs).

The equation (2) governs the key pH variation sensing detection mechanisms, one is to maintain and operate the ISFET device at its liner region and the other is to keep the drain current and drain source voltage constant. These mechanisms lead to the gate source voltage variation of ISFET’s threshold voltage (A) directly proportional to the variation of the pH value. The transconductance (KP) of the ISFET is defined as the derivative of ID with respect to

VGS. A larger KP provides a better sensitivity to the potential changes within the solution. I =K [(V ─V *) V ─V2 /2+ *4+λV ] (2) D P GS T DS DS DS Where (B) * * VT = ISFET’s threshold voltage VT = VT + Ф Figure 2: (A) MOSFET has agate dielectric Ф= interface potential between sensing deposited on top of insulator (Silicon Nitride) membrane and buffer solution membrane. (B) ISFET does not have gate KP= transconductance factor directly deposited on top of dielectric λ =channel length modulation factor VT = MOSFET’S threshold voltage

2. Material and Method 2.2 Ion Selective Field Effect Transistor Design Circuit 2.1 Ion Selective Field Effect Transistor Structure The sensing circuit of figure 4 detects the hydrogen ion concentration of the solution with An ISFET structure shown in figure 3 is similar to feature of constant voltage or constant current MOS transistor but using an exposed gate operation mode and floating reference insulator or membrane a selected ion electrode with this configuration, the potential concentration in electrolyte. The ISFET is difference between the gate sensing membrane transistor element, which has the feature of low and the reference electrode (Ref) determined out put impedance of MOSFET and operation of by the ion concentration of the solution. ion selective electrode. The ISFETs are usually operated under the The device operation of an ISFET can be constant drain current mode, which means that deduced from both of the pH dependent the change of the drain current due to the characteristic and the MOSFET behavior. When change of the ion concentration in the the transistor gate is coated with some ion electrolyte is compensated for by the selective membrane and the so called adjustment of the reference electrode potential PolyHEMA layer stabilizing the operation of the (the gate voltage). sensor, the ISFETs can be used for the selective detection of various species in the surrounding Therefore, the ISFET sensitivity is usually electrolyte, other than the hydrogen ions. Such expressed as the gate voltage change per a decade of the hydrogen ion concentration pH.

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` APPLICATION OF ION SELECTIVE FIELD EFFECT TRANSISTOR FOR HYDROGEN ION MEASUREMENT SYSTEM A simple current to voltage converter (amplifier which in turn modifies the transistor threshold LF444) is shown in Figure 4. Its behavior can be voltage. In this way, the hydrogen ion easily understood using the previously concentration exercises an electrostatic control explained nodal analysis. The input current (ID) on the drain source current. An op amp (LF444) flowing into the inverting input is equal to the can be used to produce a voltage proportional current flowing out of it through the feedback to a given current variable resistor 4kΩ can be adjusted to set 3. RESULT AND DISCUSSION point. This current creates a potential drop across R, given as: The theme of this paper can have special scope of research. The Ion Selective Field Effect V = I × R ……………. (3) D Transistor sensor used for monitoring Hydrogen Ion measurement in chemical analysis and environment fields. To measure hydrogen ion concentration correctly the sensor and current to voltage converter circuit set need to be calibrated with buffer solutions. A buffer solution is a solution of special composition, which is able to resist change in pH that occur if small range in pH. These different standard solutions were intended to cover the range for detection of Figure 4: Ion Selective Field Effect Transistor acidity or alkalinity value through out Design Circuit measuring range. At the operation point the The voltage at inverting input is equal to that circuit supplies +5V and the output voltage applied at the non inverting input because the were recorded for the period of three minutes circuit has negative feedback; that is, the with the following steps: inverting input is fixed at VREF. This "virtual 1- The sensor is immersed in buffer solution pH 4. ground" at the inverting. The output voltage at 2 - The sensor is immersed in buffer solution pH 7. VREF with zero input current and it decreases 3- The sensor is immersed in buffer solution pH proportionally for an increasing ID. The 10. following equation results: 4- The sensor is immersed in different solution

VOUT = VREF - (ID × R) ………… (4) Figure 5 illustrates result and calibration for the 2.2.1 Operation Principle ion selective field effect transistor circuit. Essentially, the ISFET construction is the same 3.1 The Result of Measuring Aqueous as the one of an ordinary MOSFET, except for Solution the fact that the standard metal polysilicon Table (2) explains the result of aqueous dielectric gate is replaced by a more complex solution measurement. Performed this is structure sensitive to hydrogen ion measurement on different samples of concentration. Namely, the gate structure, water from different sources to know the presented in figure 4, consists of a reference value of hydrogen ion concentration. electrode and a dielectric between which an The review of major research work in the area electrolyte is flowing. The ion concentration in of casting design reveals that the systems the electrolyte influences the gate potential, developed are generally not CAD-based design.

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Figure 5: Result of Design Circuit

Table 2: Result of Aqueous Solution

Aqueous Solution Design Circuit Output pH Equivalent

Standard Solution pH4 1100mV pH= 4

Standard Solution pH7 1250mV pH= 7

Standard Solution pH10 1400mV pH= 10

Distilled Water 1257mV pH= 7

Drink Water 1267mV pH= 7.3 Sea water (Red Sea) 1305mV pH= 8 Sodium Hydroxide 1504mV pH= 12 Boric Acid 1085mV pH= 3.8

4. CONCLUSION However, ISFET electrode can't be used with standard pH meters (unless it is connected The ISFET sensitivity depends mainly on the through special interface) and the pH choice of the gate dielectric material. The most measurements are generally less precise when commonly used materials are silicon and metal compared to glass electrode. oxides or nitrides. Although among these materials especially high sensitivity to the The application of the sensors based on field hydrogen ion concentration exhibits the effect transistor is especially suitable in the aluminum oxide. construction of a portable analyzer for water quality monitoring, providing in situ water ISFET electrodes can be very small when analysis without the necessity of sampling and compared to the bulky glass bubble of the sample preparation. These systems are standard glass electrode. They are also much dedicated to environmental pollution survey of sturdier, so they can be easily used in places wastewater and natural water resources. where fragile glass electrodes will not survive.

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` APPLICATION OF ION SELECTIVE FIELD EFFECT TRANSISTOR FOR HYDROGEN ION MEASUREMENT SYSTEM ISFET sensors can be useful for biomedicine and 5. Petruzzellis Tom, Electronic Sensors, Mc clinical diagnosis. Graw-Hill Companies, 2006 6. W. Bolton, Measurement and Reference Instrumentation Systems, Newnes, First 1. David .V.S, Transducers and Edition, 1996. Instrumentation, Prentice-Hall, Third 7. Chang.Soolee,Sang .Kyu .Kim, Moonil.Kim. Printing, 1999. Ion Sensitive Effect Transistor for Biological 2. Thomas Floyd, Electronic Device, Pearson Sensing Published 7 September 2009. Eduction, Sixth Edition, 2006 8. Shen-Ren. Chang, Hsin Chen. A CMOS- 3. KHANDPUR.R.S., Hand Book of Analytical Compatile, Low Nose ISFET Based on High Instruments, Tata McGraw-Hall, Fifteenth Efficiency Ion- Modulated Lateral Bipolar Reprint, 2004. Conduction, Published October 2009. 4. Mary. C,Gregory A,Jerald R, Laboratory Instrumentation, Van Nostrand Reinhold, Fourth Edition,1994.

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