Journal of Low Power Electronics and Applications Article Temperature Compensation Circuit for ISFET Sensor Ahmed Gaddour 1,2,* , Wael Dghais 2,3, Belgacem Hamdi 2,3 and Mounir Ben Ali 3,4 1 National Engineering School of Monastir (ENIM), University of Monastir, Monastir 5000, Tunisia 2 Electronics and Microelectronics Laboratory, LR99ES30, Faculty of Sciences of Monastir, University of Monastir, Monastir 5000, Tunisia; [email protected] (W.D.); [email protected] (B.H.) 3 Higher Institute of Applied Sciences and Technology of Sousse (ISSATSo), University of Sousse, Sousse 4003, Tunisia; [email protected] 4 Nanomaterials, Microsystems for Health, Environment and Energy Laboratory, LR16CRMN01, Centre for Research on Microelectronics and Nanotechnology, Sousse 4034, Tunisia * Correspondence: [email protected]; Tel.: +216-50998008 Received: 3 November 2019; Accepted: 21 December 2019; Published: 4 January 2020 Abstract: PH measurements are widely used in agriculture, biomedical engineering, the food industry, environmental studies, etc. Several healthcare and biomedical research studies have reported that all aqueous samples have their pH tested at some point in their lifecycle for evaluation of the diagnosis of diseases or susceptibility, wound healing, cellular internalization, etc. The ion-sensitive field effect transistor (ISFET) is capable of pH measurements. Such use of the ISFET has become popular, as it allows sensing, preprocessing, and computational circuitry to be encapsulated on a single chip, enabling miniaturization and portability. However, the extracted data from the sensor have been affected by the variation of the temperature. This paper presents a new integrated circuit that can enhance the immunity of ion-sensitive field effect transistors (ISFET) against the temperature. To achieve this purpose, the considered ISFET macro model is analyzed and validated with experimental data. Moreover, we investigate the temperature dependency on the voltage-current (I-V). Accordingly, an improved conditioning circuit is designed in order to reduce the temperature sensitivity on the measured pH values of the ISFET sensor. The numerical validation results show that the developed solution accurately compensates the temperature variation on the measured pH values at low power consumption. Keywords: ISFET sensor; conditioning circuit; temperature compensation; overall sensitivity 1. Introduction The ion-sensitive field-effect transistors (ISFETs) sensor has played a major role in enabling the fabrication of fully integrated CMOS-based chemical sensing systems. This has allowed several new application areas, with the most promising fields being ion imaging and full genome sequencing. However, the data from this sensor is strongly affected by the variation of the temperature. ISFET is a chemical sensitive microsensor that is based on the field effect transistor, which was invented by Bergveld in the 1970s [1]. The detection principle (shown in Figure1) of the ISFET sensor mechanism is centered on the charge adsorption at the solid ion interface between the electrolyte and the layer that contains the hydroxyl groups [2]. The assembly of hydroxyls can give or accept a proton. As a result of this process, a double layer capacity is generated with a potential variation that in its turn has an impact on the threshold voltage of the transistor according to the H+ ion concentration (i.e., pH) [2]. Moreover, ISFET is a compact sensor that enables a fast pH measurement. J. Low Power Electron. Appl. 2020, 10, 2; doi:10.3390/jlpea10010002 www.mdpi.com/journal/jlpea J. Low Power Electron. Appl. 2020, 10, 2 2 of 16 J. Low Power Electron. Appl. 2020, 10, x FOR PEER REVIEW 2 of 16 H+ Electrolyte H+ - - X + X H + Interfering H+ - H X- X Ion X- X- + X- H+ H H+ H+ + X- - H Ion H+ X + H + + + H+ H H H ISFET Array Vt Variation Figure 1. 3D representation of a 6 8 ion-sensitive field effect transistor (ISFET) array. Figure 1. 3D representation of a 6× × 8 ion-sensitive field effect transistor (ISFET) array. However,However, this this type type of of sensor sensor is is strongly strongly a ffaffectedected by by the the temperature temperature variation. variation. 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