Back Gate Bias Influence on BESOI ISFET Sensitivity
Authors: Duarte Pedro Henrique , Rangel Ricardo Cardoso , Ramos Daniel Augusto , Yojo Leonardo , Sasaki Katia , Agopian Paula , Martino Joao 
Published: September 2023

The application of Field Effect Transistors (FETs) to sensors and biosensors allows a mass production, low cost, small size, fast response and, also, the possibility of integrating the device and conditioning signal circuit in the same integrated circuit (IC). The Ion Sensitive Field Effect Transistor (ISFET), invented in 1970 by Piet Bergveld (1), is a device similar to the conventional Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), except that the metal gate is replaced by an inert electrode/pseudo electrode and a sample solution, in order to expose the gate oxide layer to ions present in the solution. The solution charges change the electrical potential in the gate oxide surface and, consequently, the device threshold voltage changes as a function of the ion concentration in the sample solution, being used as a pH sensor. Thus, the ISFET is a very promising device to Point-of-Care (POC) monitoring, such as COVID-19 control (2) and for continuous in vivo monitoring ion activity in biological processes (3)-(6).
The BESOI (Back Enhanced Silicon-On-Insulator) MOSFET is a device patented in 2015 by João Antonio Martino and Ricardo Cardoso Rangel (7). This device is easy to fabricate and low cost. In the last years, the research using this device as a sensor/biosensor has been done (8)-(14). This paper presents, for the first time, the BESOI MOSFET working as an Ion Sensitive Field Effect Transistor (ISFET): the BESOI ISFET. Therefore, the focus of this work is to study the device electrical behavior influenced for pH standard solutions.
The BESOI ISFET was fabricated at Integrated System Laboratory (LSI) from University of Sao Paulo (USP), Brazil. It is a planar device made on a SOI (Silicon-On-Insulator) wafer with three conventional photolithography steps, in an analogous way to previous work (15) and one more photolithography step to create microchannel and microreservoirs with SU-8 (16) layer to contain the sample solution. The wafer has no doping process (there is only the natural wafer doping of 1015 cm-3). The silicon channel layer and the gate oxide (SiO2) thicknesses are 10 nm and 25 nm, respectively. The buried oxide layer thickness is 200 nm. The drain and source contacts are fabricated with nickel and the contact pads using aluminum. The Figure 1 presents the final device layout and the Figure 2 shows the device schematic drawn.
Differently from a conventional ISFET, where the drain current flows at the front interface, in the BESOI ISFET the drain current flows at the back interface. As the BESOI ISFET is a non-intentionally doped device, the free conduction charge layer is formed by biasing the Back Gate electrode (VGB). When a positive enough VGB is applied, electrons are attracted and accumulated at the back interface, and therefore, current flows from source to drain when VDS (Drain bias) is applied. A Platinum (Pt) pseudo-electrode is used to apply front-gate voltage (VGF) to the sample solution.
The experimental drain current (IDS) as a function of the front-gate voltage (VGF), for 25 V applied at the back-gate (VGB), is presented in Figure 3.A, which is possible to observe that the neutral pH (pH7) curve stayed between the acid pH (pH4) curve, in the left, and alkaline pH (pH10) curve, in the right. This result shows that the threshold voltage (VTH) is lower for acid pH and higher for alkaline pH, what is compatible with the conventional ISFET results present in the literature (17). To corroborate the experimental measurement result, the device simulation was made with TCAD-Sentaurus (18), based in the paper (19). Figure 3.B shows the same trend for the simulated device with pH solution for any VGB value.
In Figure 4, the threshold voltage was analyzed as a function of the pH for different VGB values. The VTH increases (becomes more positive) for alkaline pH values, regardless of the VGB applied. However, the VTH variation in the pH range, between pH4 and pH10, is different for different VGB values. With higher VGB applied, higher is the VTH variation. In Figure 5 Is possible to see clearly that the sensitivity (ΔVTH /ΔpH) increases with the VGB increase probably due to the small influence of the series resistance. In another hand, the acquired sensitivity values for VGB higher than 15 V in this work (about 25-33 mV/pH, approximately), is compatible with results present in the literature for conventional ISFETs with silicon dioxide (SiO2) for gate insulator (20).

Figure 1