Semiconductor-Based Biosensing Chip for Point-of-Care Diagnostics - Point-of-Care Diagnostics on a Chip - page 121

electrical signals based on FET devices result in direct and quantitative analyses
of bio-samples. One positive or negative charge of ion or ionic molecule interacts
electrostatically with one electron charge in semiconductor device. Therefore,
ion behaviors based on biological phenomena can be directly detected using
semiconductor devices. Most of biological phenomena in vivo are closely related to
charged mediums, for example, such as DNA molecules with negative charges based
on phosphate groups, ions (potassium, sodium, and so on) through ion channel at
cell membrane keeping homeostasis.

6.2 Concept of Semiconductor-Based Biosensing Devices

6.2.1 Ion-Sensitive Field-Effect Transistor (IS-FET)

As a reason of a bad tooth, three elements of “ Streptococcus mutans ,” “quality of
tooth,” and “saccharinity” affect on a bad tooth in the course of time. S. mutans
induces acidification by dissolving saccharide in food and drink. As a result,
acidification of dental plaque is in progress on a tooth. That is to say, enamel of
tooth begins to dissolve less than pH 5.5, resulting a bad tooth. Therefore, it is
important to control meal considering pH variation in a mouth in order to prevent
a bad tooth. Thus, pH measurement is needed even for health care in daily life and
can be accomplished by engineering such as semiconductor technology.
The principle of IS-FET is based on potentiometric detection of charge density
changes induced at a gate insulator/solution interface accompanied by pH variation.
Hydrogen ions with positive charges at the gate insulator electrostatically interact
with electrons in silicon crystal across the thin gate insulator, resulting in the V T
change.
Typical drain voltage (V DS )-drain current (I D ) characteristic of the FET is shown
in Fig. 6.2 . It is found that the FET can be operated correctly. Since the fabricated
FET is depletion type as can be seen in Fig. 6.2 , the reference electrode is usually
connected to the ground for the measurement of the interface potential between
gate insulator and solution using the circuit shown in Fig. 6.3 . The pH-response
characteristics of the FET with a Si 3 N 4 gate are shown in Fig. 6.4 . The time course
of the interface potential was measured during calibration and is shown in Fig. 6.4 a.
The arrows indicate the timing to change the buffer solutions. The interface potential
changed rapidly after changing the buffer solution and became stable within 1 min.
The calibration curve for the Si 3 N 4 gate FET is shown in Fig. 6.4 b. The relationship
between pH and the output voltage is linear in the range from pH 1.68 to 9.18
with a correlation coefficient of 0.9999. The slope of the calibration curve was
57.52 mV/pH, which is close to the theoretical slope at 25 ı . On the basis of these
results, the operation of the Si 3 N 4 gate FET was considered to be stable, and no
leakage through the gate insulator and no defect of the encapsulation could be
observed.