Hydration Effect Analysis of Ion-sensitive Field Effect Transistor
- PDF / 657,659 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 46 Downloads / 232 Views
1253-K05-34
Hydration Effect Analysis of Ion-Sensitive Field Effect Transistor
Lihong (Heidi) Jiao and Mykhaylo Rybachek School of Engineering, Grand Valley State University, Grand Rapids, Mi, 49504, U.S.A.
ABSTRACT In this work, the drift of ISFET characteristics due to the hydration effect was studied. The ISFETs were fabricated using the standard NMOS process and the AgCl reference electrode was fabricated with the electrolysis method. The ISFET modeling was carried out in MATLAB to study the time variation of surface potential and the drain-source current. In addition, the surface morphology and dielectric constant of silicon dioxide were measured to study the hydration effect. It was found that the dielectric constant of the gate oxide increases exponentially with hydration time. The surface morphology studied with the atomic force microscope (AFM) showed no significant change after immersion in water. It is believed that the main cause of the drift is the hydration effect, which is due to the change in dielectric constant of the sensing material and a small number of surface binding sites that react slowly to a change in pH.
INTRODUCTION The Ion-Sensitive Field Effect Transistor (ISFET) is a device where the gate oxide is in direct contact with an analytic solution. This device is a good option for continuous monitoring applications to determine concentration of various ion species due to its advantages over the conventional electrodes such as high sensitivity, fast response time, micro-size and on-chip circuit integration. The operation of an ISFET is shown in figure 1 and can be described by comparing it with its electronic analogue, the MOSFET.
Figure 1 The cross sectional structure of an ISFET
The metal gate of the MOSFET is replaced by a metal of the reference electrode. The gate insulator senses the specific ion concentration (H+ ions in pH detectors) and generates an interface potential on the gate. Hence the gate potential depends on the number of H+ ions in
contact with it. Consequently, the corresponding drain-source current is related to the number of H+ ions. The technology harbors limitations including threshold voltage time drift, temperature dependence and technological difficulty in packaging of a small reference electrode [1]. Therefore, the commercial viability of the ISFET applications in medical and chemical analyses is limited. The drift is the main constrain. It has been reported that an ISFET sensor with a pH gate had a drift of 0.02-0.06 pH/hour, and the initial drift could be even higher [2]. Most of the proposed methods for time and temperature drift compensation involve in redundant data calculation and additional circuitry. It is believed that the main cause of the drift is the hydration effect, which is due to the change in dielectric constant of the sensing material and a small number of surface binding sites that react slowly to a change in pH. The hydration mechanism involves the dissociation of an absorbed water molecule, where an H+ ion bonds to an oxygen ion and OH– i
Data Loading...
