Role of deposition and annealing of the top gate dielectric in a-IGZO TFT-based dual-gate ion-sensitive field-effect transistors

Abstract

The deposition of the top gate dielectric in thin film transistor (TFT)-based dual-gate ionsensitive field-effect transistors (DG ISFETs) is critical, and expected not to affect the bottom gate TFT characteristics, while providing a higher pH sensitive surface and efficient capacitive coupling between the gates. Amorphous Ta 2 O 5 , in addition to having good sensing properties, possesses a high dielectric constant of ∼25 making it well suited as the top gate dielectric in a DG ISFET by providing higher capacitive coupling (ratio of C top /C bottom ) leading to higher amplification. To avoid damage of the a-IGZO channel reported to be caused by plasma exposure, deposition of Ta 2 O 5 by e-beam evaporation followed by annealing was investigated in this work to obtain sensitivity over the Nernst limit. The deteriorated bottom gate TFT characteristics, indicated by anincrease in the channel conductance, confirmed thatplasma exposure is not the sole contributor to the changes. Oxygen vacancies at the Ta 2 O 5 /a-IGZO interface, which emerged during processing, increased the channel conductivity, becamefilled by optimum annealing in oxygen at 400 °C for 1 h, which was confirmed by anx-ray photoelectron spectroscopydepth profiling analysis. The obtained pH sensitivity of the TFTbased DG ISFET was 402 mVpH −1 , which is about 6.8 times the Nernst limit (59 mVpH −1 ). The concept of capacitive coupling was also demonstrated by simulating an a-IGZO-based DG TFT structure. Here, the exposure of the top gate dielectric tothe electrolyte without applying any top gate bias led to changes in the measured threshold voltage of the bottom gate TFT, and this obviated the requirement of a reference electrode needed in conventional ISFETs and other reported DG ISFETs. These devices, with high sensitivities and requiring low volumes (∼2 μl) of analyte solution, could be potential candidates for utilization as chemical sensors and biosensors.

Sensitivity Enhancement of Dual Gate Field-Effect Transistors Using Stacked Bottom Gate Dielectrics

“…Similarly, D3 and D4 were fabricated by deposition of 30 nm Ta 2 O 5 as the TG dielectric over T3 and T4 by RF sputtering at 100 W power and 10% oxygen in Ar. Finally, a reservoir of size 10 × 0.1 mm 2 was made on top of all DGISFETs by photolithography with epoxy (SU 8 2002), , as shown in Figure f.…”

“…The values of sensitivity obtained by simulations were then compared with the experimental and theoretical values. The equations used in this simulation are the same, as used in our previous work, , except that a stacked BG dielectric is used here instead of a single-layer BG dielectric.…”

“…The TFT with a stacked gate dielectric has been used to take advantage of the properties of both the gate dielectrics. For DGISFETs, Al 2 O 3 /SiO 233 as the BG dielectric stack has been studied while Y 2 O 3 /SiO 2 as BG dielectric has not been studied yet and is the subject of this work. The Y 2 O 3 / IGZO interface was studied in our previous work,7 and low leakage current, high stability, and high sensitivity of DGISFETs were obtained with Y 2 O 3 as the TG dielectric− IGZO interface.…”

Amorphous InGaZnO and metal oxide semiconductor devices: an overview and current status

“…By reducing the level of native defects through optimising the deposition process for the gate dielectric, much additional processing is eliminated. However, it has been shown that where non-optimised deposition is necessary, annealing can significantly improve the device performance 242,243 .…”

Highly Sensitive Magnesium-Doped ZnO Nanorod pH Sensors Based on Electrolyte–Insulator–Semiconductor (EIS) Sensors

“…Lately, the development of pH sensors with a high stability and sensitivity for various biochemical and biological applications has attracted much attention from researchers. Since Bergveld introduced ion-sensitive field-effect transistors (ISFET) in 1970, different types of sensors have been used to estimate pH as well as other biochemical solutions, such as extended-gate field-effect transistors (EGFETs) [ 4 ], ion-sensitive field-effect transistors (ISFETs) [ 5 ], light-addressable potentiometric sensors (LAPSs) [ 6 ], and electrolyte insulator semiconductors (EISs) [ 7 ]. Among various types of sensors, the EIS system is considered one of the most exciting systems for chemical and biological sensing.…”