(Invited) ISFET-Based Sensors

Abstract

The big difference between an ISFET (Ion Sensitive Field Effect Transistor) and a conventional MOSFET (Metal Oxide Semiconductor FET) is contained in the absence of the upper electrode on the gate oxide of the device, as shown in Figures 1 (a) and 1(b), respectively. In the case of an ISFET (Figure 1(a)), the gate reference electrode with the solution (of chemical or biological materials), which is in contact with the gate oxide (dielectric), work as the upper electrode, inducing the channel in the semiconductor and enabling the conduction of electric current between source and drain terminals. On these two terminals, there are the polymer layers as barrier against chemical or biological solutions. Therefore, the choice of the gate oxide is of great importance, since it must present chemical stability when in contact with the solution that will be measured. The solution pH is the main parameter which is measured. The gate dielectric chosen for application as a pH sensor must carry out measurements in acidic and basic media and be capable of forming hydrogen bonds. This ability is associated with atoms that have greater electronegativity such as fluorine, oxygen and nitrogen. Furthermore, the water contamination metals, DNA, RNA, enzymatic, antigen-antibody and cell related detections can be obtained by ISFETs.Thus, thin films of SiNx, TiOx, TaOx, AlOx and AlN were chosen as gate dielectrics because are compatible to chemical or biological substances. Various results from literature will be present. For example, pH ISFET devices with silicon nitride (SiNx) as gate dielectric were fabricated [2]. Silicon nitride (SiNx) films have been obtained by Low Pressure Chemical Vapor Deposition (LPCVD) at temperature of 720oC for 30 min, using different ratios of [SiH2Cl2]/[NH3] reagent gases. These films been used as ISFET gate dielectric. Figure 2 presents a characteristic of the current between source and drain (IDS) versus voltage between source and gate (VGS) of ISFETs (for voltage VDS of 2V) in related to the four values of pH solution. In this case, sensibility of S=51mV/pH [2]. was estimated, which is close to the expected 59mV/pH determined by the Nernst limit. Other application: Controlling the water quality has become an important issue nowadays, especially due to its contamination along the years which may cause significant damage to human health and in this context, phosphate in water reuse or lead deserves a great attention. The Electrolyte-Insulator-Semiconductor (EIS) structure, which is similar to the gate structure of ISFETs, with TiO2 thin films as gate dielectric, were fabricated and Capacitance x Voltage (CxV) curves to estimate the sensitivity values through the flat-band voltage variation for each curve. In order to enhance the Pb+ detection an additional cerium phosphate layer (TiO2 surface functionalization) was deposited over the TiO2 thin film as selective membrane for Pb+ measurements and the device presented 40 mV/100 ppm sensitivity. For the case of phosphate detection in waste water, EIS devices with TiO2 films as gate dielectric, without functionalization layer, were used and the good sensitivity to phosphate ions of 66 mV/ppm was obtained. Most of results with ISFET devices are based on Si semiconductor conduction channel between the source and drain regions. Others materials have been used as conduction channels of transistors, such as 2D layer, mainly graphene. One example is the biosensor based on Graphene Field Effect Transistor (GraFET), using the TiO2 dielectric gate, capable of being applied effectively for early diagnosis of COVID-19. The graphene channel has ten parallel ribbons, and the virus of SARS-Cov-2 interacts with the unprotected TiO2 gate dielectric. Unlike rapid tests, which depend on the body's immune response (production of IgM and/or IgG antibodies), this project seeks to detect components of the infectious agent itself. It makes graphene-based biosensors very advantageous compared to current tests available since there will be no dependence on the immune response to infection by COVID-19 but only on the presence of the virus in the tested samples. Thus, these devices can reduce the time involved in the analyses.In conclusion, as observed in this review, the ISFET devices are mandatory sensors to develop the future instrumentation, because allow the detection of various chemical and biological species, using different materials as gate dielectrics or electrodes, and conduction channels. Figure 1