Silicon-on-Insulator Double-Gate Ion-Sensitive Field-Effect Transistors Using Flexible Paper Substrate-Based Extended Gate for Cost-Effective Sensor Applications

We developed ion-sensitive field effect transistors (ISFETs) with disposable paper extended gates (EGs). The sensing and the measuring part of conventional ISFETs are not separated and integrated on the same device. Therefore, if the sensing part is contaminated by reaction in a chemical solution, there is a problem that an expensive measuring part manufactured through a complicated process becomes also unusable. To overcome this problem and provide a cost-effective sensor platform, we constructed a high-sensitivity ISFET sensor with a measuring transistor part and a separate EG sensing part. In particular, in this experiment, a double-gate transistor capable of amplifying the sensitivity using the capacitive coupling effect between a top-gate and a bottom-gate, which differs from a general single gate transistor on an SOI substrate, was fabricated. As a result, the pH sensitivity of 1199.92 ± 32.4 mV/pH could be achieved using paper EG and dual-gate mode sensing operation, which greatly exceeds the theoretical Nernst limit (59.15 mV/pH at 25 °C) in single gate mode sensing operation. We also measured non-ideal effects such as the hysteresis and drift behavior of paper EGs, and demonstrated that they have excellent stability and reliability for long-term measurements. In addition, hysteresis and sensitivity were measured after 3, 7, 14 and 30 days to verify the aging effect of the continuous use of paper EGs. As a result, paper EGs showed stable operating characteristics for 30 days. Therefore, we expect that double-gate ISFETs with flexible paper EGs will have a significant impact on label-free, low-environmental impact, cost-effective, disposable, and flexible FET-based biosensor applications.