ISSS International Conference on Smart Materials, Structures and Systems July 5-7, 2017, Bangalore, India MODELING AND SIMULATION OF ISFET USING TCAD TOOL FOR VARIOUS SENSING FILMS Neel Choksi a* , Dewanshu Sewake a , Soumendu Sinha b, c , Ravindra Mukhiya b, c , Rishi Sharma b, c a Birla Institute of Technology and Science, Pilani, Rajasthan 333 031, India b CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani, Rajasthan 333 031, India c Academy of Scientific and Innovative Research (AcSIR), New Delhi-110001, India *Email: neelchoksi2809@gmail.com Abstract: Ion-Sensitive Field-Effect Transistor (ISFET) is a popular platform for chemical/biochemical sensing. The simulation of the typical Electrolyte-Insulator-Semiconductor structure using commercial TCAD simulators requires mapping of the chemical reactions taking place at the electrolyte-insulator interface to the Fermi-Dirac equations. This paper presents the modelling of ISFET pH sensor using Silvaco ® TCAD tool and an analysis of the effect of using different sensing films on the performance of the device has been performed. It is observed that the pH sensitivity of ISFET varies significantly with the sensing film deposited on the gate as well as the thickness of the sensing film. We provide a comparative study for Silicon Nitride, Aluminum Oxide and Silicon Dioxide used as sensing films. Output and transfer characteristics are obtained for these films for various pH values. It is observed that aluminum oxide gives better performance than silicon oxide and silicon nitride. The fabrication process of the device with aluminum oxide as the sensing film has been discussed and the effect of thickness of the film on device performance has been shown. Keywords: Electrolyte Model, ISFET, MOSFET, Silvaco, TCAD. 1. INTRODUCTION Traditional pH sensing mechanisms use glass electrodes that are vulnerable to handling issues and inability to operate at high temperatures etc. These glass electrodes are not suitable for measuring the pH in a nano/microsystems because of difficulties in its miniaturisation. The concept of ISFET (Ion-Sensitive Field-Effect Transistor) was introduced by Dr. P. Bergveld in 1970 which can be microfabricated [1]. It overcomes the difficulties in miniaturisation as in the case of glass electrodes and provides a better medium for pH sensing because of its solid state nature, low production cost, low energy consumption and the possibility of lab-on-a-chip integration [2]. Moreover, this platform can also be utilised for chemical/biochemical sensing with proper functionalization of the gate area [3]. Biosensors based on ISFET technology has its applications in the monitoring of blood, environment, analysis of biological and chemical fluids and laboratory detection [4] [5]. This device is compatible with the existing CMOS technology, which is attractive for batch production of sensors along with signal conditioning circuit. ISFET is based on the concept of MOSFET (Metal Oxide Field-Effect Transistor). In the structure of ISFET, a pH sensing film replaces the metal gate of MOSFET. This film is in direct contact with the electrolyte and is used for detecting the activity of ions in the analyte. The current flowing through the channel of the transistor is modulated by the ion concentration in the electrolyte [6]. The sensitivity of the device depends upon the gate area dimensions as well as the sensing film. The sensing layer of the ISFET is an area of interest for the researchers and they have examined different types of insulators such as Aluminum Oxide (Al2O3), Silicon Dioxide (SiO2), Tantalum Pentoxide (Ta2O5) and Silicon Nitride (Si3N4) and various other dielectrics as sensing films [3]. In this paper, we provide a comparison of the effect of using different sensing films, namely, silicon dioxide (SiO2), aluminum oxide (Al2O3) and silicon nitride (Si3N4) on the sensitivity of ISFET based pH sensor. We also show the effect of thickness of sensing film on the device performance. The simulations are carried out using the platform provided by Silvaco TCAD. Commercially available TCAD tools cannot model the chemical reactions taking place at the electrolyte-insulator interface. The typical Electrolyte- Insulator-Semiconductor structure has been modeled in this work by mapping the behaviour of cations and anions in the electrolyte region governed by Poisson-Boltzmann equation to holes and electrons as described by Fermi-Dirac distribution. In the next section, we discuss the electrolyte model developed to simulate ISFET using Silvaco Athena TM . The next section discusses the fabrication process of the device. Further, the transfer and output characteristics of ISFET using different sensing films has been discussed and the effect of film thickness on device performance has been shown. 2. MODEL FORMULATION The drain current for both MOSFET and ISFET are given by same expressions in non-saturated region shown by equation (1) [1]