Bottom-gate amorphous InGaZnO₄thin-film transistor pH sensors utilizing top-gate effects

Abstract

We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO 4 thin-film transistor (a-InGaZnO TFT) pH sensors from the viewpoint of top-gate effects. Comparing the top-gate effects in a-InGaZnO TFTs having TaO x and SiO x ion-sensitive insulators, we draw an analogy between the operations of dual-gate TFTs and TFT pH sensors. Our new concept for enhancing pH sensitivity is characterized by a high capacitance ratio of the ion-sensitive insulator to the bottom-gate insulator and pH sensing utilizing threshold-voltage shifts in bottom-gate transfer characteristics. The close similarity between top-gate effects and pH sensitivity strongly suggests that a common mechanism underlies the phenomena. We discuss the mechanism on the basis of the material properties of a-InGaZnO and the silicon-on-insulator (SOI) model that relates bottom-and top-gate electric fields in fully depleted operations. We believe that the pH-sensitivity enhancement utilizing top-gate effects is one of the potential applications that would make the most of the intrinsic features of a-InGaZnO TFTs.

Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

“…120,171 The increased sensitivity (over 100 mV per pH) is benecial for the detection of small pH changes. [172][173][174][175] In terms of free chlorine sensors, a chemoresistor was recently fabricated in contrast to conventional amperometric sensors. 176 The resistor consisted of single-wall CNTs modied by phenyl capped aniline tetramer (PCAT), which could be oxidized by the free-chlorine and alter the resistivity of CNTs.…”

Low temperature annealed amorphous indium gallium zinc oxide (a-IGZO) as a pH sensitive layer for applications in field effect based sensors

“…For a-IGZO, higher temperatures (i.e. above 400 o C) are known to deteriorate the TFT performance with the films being amorphous till about 400 o C and reported to be polycrystalline above 500 o C. 14 The a-IGZO films annealed at 400 o C have reported [1][2][3] to provide the optimum TFT performance. Hence, the annealing temperature of 400 o C was used for processing a-IGZO to study its sensing characteristics in N 2 and O 2 environments.…”

“…The use of a-IGZO as the active layer and different dielectrics such asTiO 2 , ITO, SiOx, and TaOx as the pH sensitive layers in TFT based ion sensitive field effect transistors (ISFETs) have been reported. [1][2][3][4] High-k dielectrics (Ta 2 O 5 , Al 2 O 3 , Pr 2 O 3 ,TiO 2 , Er 2 O 3 , and HoTiO 3 etc.) and their stacks to reduce lattice mismatch are used to enhance the pH sensitivity with an objective to achieve the maximum sensitivity of 59.5mV/pH (the Nernstian limit) of field effect sensors.…”

“…[5][6][7][8] But to achieve these higher sensitivity values with low drift and hysteresis, these materials are annealed at high temperatures (600 -900 o C). TFT based sensors fabricated using sputtered a-IGZO film as the active layer showed optimized performance at an annealing temperature of 400 o C; 1,3 therefore subjecting such devices to higher temperatures needed for annealing the high-k dielectrics could affect the transistor characteristics. Thus the use of a-IGZO itself as a sensitive layer could be advantageous in reducing the number of fabrication steps, and overcoming the need of a separate high-k dielectric where the different deposition methods or high temperature annealing will be required.…”

High Sensitivity of Dual Gate ISFETs Using HfO2 and HfO2/Y2O3 Gate Dielectrics

“…On the other hand, sputtering is a method for deposition of HfO 2 at lower temperatures and lower processing costs than ALD. There are reports in the literature for DGISFETs with HfO 2 as the gate dielectric deposited by ALD, 5,18,19 but there are no reports on DGISFETs with sputter-deposited HfO In the present work, deposition and annealing of HfO 2 were studied for TFTs and the sensing layer. Deposition of HfO 2 and Y 2 O 3 was optimized by RF sputtering at different powers and deposition pressures.…”