Microelectronics Journal 37 (2006) 1105–1114 New ISFET interface circuit design with temperature compensation Wen-Yaw Chung a, , Yeong-Tsair Lin a , Dorota G. Pijanowska c , Chung-Huang Yang b , Ming-Chia Wang a , Alfred Krzyskow c , Wladyslaw Torbicz c a Department of Electronic Engineering, Chung Yuan Christian University, 32023 Chung-Li, Taiwan, ROC b Department of Electronic Engineering, Vanung University of Science and Technology, 32045 Chung-Li, Taiwan, ROC c Institute of Biocybernetics and Biomedical Engineering, Polish Academy of Sciences, ul. Trojdena 4, 02-109 Warsaw, Poland Received 24 December 2005; received in revised form 27 April 2006; accepted 12 May 2006 Available online 17 July 2006 Abstract An integrated and new interface circuit with temperature compensation has been developed to enhance the ISFET readout circuit stability. The bridge-type floating source circuit suitable for sensor array processing has been proposed to maintain reliable constant drain–source voltage and constant drain current (CVCC) conditions for measuring the threshold voltage variation of ISFET due to the corresponding hydrogen ion concentration in the buffer solution. The proposed circuitry applied to Si 3 N 4 and Al 2 O 3 -gate ISFETs demonstrate a variation of the drain current less than 0.1 mA and drain–source voltage less than 1 mV for the buffer solutions with the pH value changed from 2 to 12. In addition, the scaling circuitry with the V T temperature correction unit (extractor) and LABVIEW software are used to compensate the ISFET thermal characteristics. Experimental results show that the temperature dependence of the Si 3 N 4 -gate ISFET sensor improved from 8 mV/1C to less than 0.8 mV/1C. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ion sensitive field effect transistor (ISFET); pH sensor; Temperature compensation 1. Introduction Although the ion sensitive field effect transistor (ISFET) has been invented more than 30 years ago [1–2], the diversified applications and issues related to it become more important in the era of biological technology and microsensor-based lab-on-a-chip in recent years [3]. The major trends to utilize biosensors efficiently are sensor array processing and biochip system integration with CMOS fabrication technology [4] for various applications such as physiological parameter detection and homeland security monitoring [5]. A typical ISFET structure, shown in Fig. 1, is similar to a MOS transistor but uses a gate insulator exposed to liquid. At the electrolyte/gate di- electric interface, ion sensitive membranes, e.g. Si 3 N 4 or Al 2 O 3 thin film interacts with the hydrogen ions present in the electrolyte and transforms the pH change of the electrolyte into a corresponding shift in the threshold voltage of the ISFET. If the sensing membrane contains different biochemical receptors such as ionophores, en- zymes, or antibodies, important biochemical analytes can be detected [6,7]. These chemicals such as urea, glucose or acetylcholine indicate personal health condition in daily life. In order to obtain reliable output data, electronics for ISFET array structures with appropriate temperature compensation is required. It is well known that ISFETs are thermally unstable due to properties of the semicon- ductor structure and the sensing films [8,9]. Many interface circuits have been developed to obtain the response of ISFET sensors by maintaining constant drain–source voltage and constant drain current (CVCC) conditions [10–12]. Most of these circuits involve floating gate configurations [11,12] as shown in Fig. 2, which are not suitable for multi-sensors or sensor array applications because they cannot be utilized for measurements with a common reference electrode. Recently, Morgenshtein introduced a Wheatstone-bridge readout interface for ISFET/REFET applications [13]. The requirement of extra bias voltages to the gate of all transistors to maintain the ARTICLE IN PRESS www.elsevier.com/locate/mejo 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.05.001 Corresponding author. Tel.: 886 3 265 4615; fax: 886 3 265 4699. E-mail address: eldanny@cycu.edu.tw (W.-Y. Chung).