Proposal of a Common Gate Method Suitable for Multi Sensing Using Diamond Electrolyte Solution Gate FETs

In this study, we report the pH sensitivity utilizing common-gate Diamond Electrolyte Solution Gate FETs (SGFET)[1].<br/>Since the introduction of SGFETs , the development of multi-ion sensors including pH sensor has been a great interest. Most of the multi-ion monitoring systems utilizing the FETs employ common-source FETs . However, the systems with the common-source FETs have the following drawbacks. In order to operate the common-source multiple sensors, it is required to immerse multiple reference gate electrodes in sample solution and to apply a different gate voltage (V_GS) for each FET. Consequently, simultaneous sensing of various ions at various points is not practically performed. Moreover, Ag/AgCl glass electrodes, which are commonly used as the gate electrodes, are difficult to miniaturize, resulting in a larger sensing system with more glass electrodes for detection of a wider variety of ions at many locations . Therefore, multi-sensing with a plurality of FETs at a common gate is desirable. Here, we propose biosensing measurement using a common gate FET that can be sensed by sharing one gate electrode with a plurality of FETs.In general, common-gate FETs are often used as cascade amplifiers and contribute to high output impedance in electronic circuits. The gate of the common-gate FETs is grounded, that is, gate bias is not applied. Instead, source voltage<i> V</i>_SG and drain voltage <i>V</i>_DG are applied and control drain current <i>I</i>_D. Sensing is also performed by changing only the source voltage <i>V</i>_SG and the drain voltage <i>V</i>_DG. Therefore, even when a plurality of FETs is used for common gate, only one gate electrode is completed.<br/>In this study, we have investigated whether multi-sensing is actually possible using common-gate SGFETs composed of one gate and plural source drain (SD) probesand the pH sensitivity of SD probes sis independently measured.<br/>In this experiment, we utilized two n-channel SGFETs (SGFET 1, SGFET 2) with different threshold voltages. The<i> I</i> –<i> V</i> characteristics of the common-gate SGFETs were measured in the following two ways: 1) measuring each SGFET independently and 2) measuring two both SGFETs operated simultaneously with a common gate electrode.<br/>A Carmody buffer solution was used for each pH solution (pH 2 - 12), and a gate electrode (Ag/AgCl) and SGFET were immersed in the Carmody buffer solution. The <i>I</i>_D -<i> V</i>_SG characteristics of the SGFET were measured in the range of <i>V</i>_SG = 0 to -2V at <i>V</i>_DG = 0 V from pH 2 - 12. From <i>I</i>_D -<i> V</i>_SG characteristics, the <i>V</i>_SG - pH characteristics were obtained from the source voltage <i>V</i>_SG at a certain drain current value <i>I</i>_D, and the pH sensitivity was calcurated from the slope of the <i>V</i>_SG - pH characteristics. At common gate, the n-channel SGFET showed high pH sensitivity comparable to the Nernst response (About 59 mV/pH) with -49.4 mV/pH and -48.2 mV/pH. Next, The pH sensitivity of the two SGFETs obtained by multisensing is shown in comparison with the previous results. The pH sensitivity of SGFET1 was -49.4 mV/pH, and the pH sensitivity of SGFET1 in multisensing was -49.7 mV/pH. The pH sensitivity of SGFET2 was -48.2 mV/pH, and the pH sensitivity of SGFET2 in multisensing was -48.0 mV/pH.<br/>It can be seen that the characteristics hardly change when measured with one SGFET and when a voltage is applied to two SGFETs simultaneously and multi-sensing is performed. The <i>I</i>_D -<i> V</i>_SG characteristics of two SGFETs has measured at the same time. From this result, we have shown how simultaneous applying gate voltage to two SGFETs affects the individual characteristics. We confirmed that multi-sensing is possible from the experimental results that , multiple ions can be sensed simultaneously by using FETs having different ion-sensitive films.<br/> <br/>[1]K. Igarasi, H.Kawarada et al, Proceedings of the 63rd Annual Meeting of the Japan Society of Applied Physics 19p-H103-16(2016).