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 اﻷس انهُذروخُنٍ فٍ يدبل انكًُُبء انتحهُهُت و انبُئُت. تعتًذ حضبصُت حضبس تزانزصتىر تأثُز انًدبل عهٍ نىع انًبدة اﻹنكتزونُت 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 اﻷس انهُذروخُنٍ 4 إنً 44. وظُفت انذائزة اﻹنكتزونُت هي تحىَم تُبر دخم انًحضبس انذٌ ًَثم قُى اﻷس انهُذروخنٍ إنً خهذ خزج يتنبصب. تى اختببر انتصًُى بىاصطت يحبنُم قُبصُت وخًضت أنىاع ين انًحبنُم انًبئُت وتى انحصىل عهً اننتبئح انًطهىبت. تى رصى انعﻻقت بُن اننتبئح انًتحصم عهُهب بىاصطت انذائزة اﻹنكتزونُت وقُى اﻷس 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. Sudan Engineering Society Journal, September 2011, Volume 57; No.2 51 ` 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) 52 Sudan Engineering Society Journal, September 2011, Volume 57; No.2 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.