Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 270

i s-d

V gate

Source

Si Chip

Drain

Encapsulation
insulator

FIGURE 10.3 A schematic diagram of an ISFET pH measurement system. A potential responding to
the hydrogen ions concentration in the solution is established on the pH sensitive gate-solution interface of
the ISFET. It electrostatically infl uences the current ( i s-d ) fl ow between the source and drain, therefore, the
current i s-d change is directly related to the pH change in the solution.

drain is directly related to the pH change in solution. A reference electrode with a stable
voltage, independent of pH changes, is required to complete the voltage circuit between
the gate and source [79] and to control the gate voltage. This voltage ( V gate ) circuit has
extremely high impedance, while the current circuit via source and drain has a low
impedance of a few ohms. For such low impedance, a pre-amplifi er, which is required
for glass electrodes, may not be necessary, and a long cable can be used for in-line
pH measurements.
The heart of an ISFET is the double-layer gate [80]. The ideal gate material should
provide a Nernstian pH response, a low alkali metal ion sensitivity, a small hysteresis,
a low drift, and a high corrosion resistance [81]. When a native SiO 2 gate surface from
the MOSFET process is used, the pH sensitivity is only about 30-40 mV/pH depend-
ing on the electrolyte concentration, and the gate is sensitive to both sodium and
hydrogen ion concentrations in the solution. Early ISFETs use a silicon nitride (Si 3 N 4 )
coated gate, which has good insulating properties and provides a pH sensitive interface
with a Nerstian response of 40-60 mV/pH [56]. However, it suffers from problems of
light sensitivity, and both short- and long-term drift [52]. To overcome such problems,
research efforts have been made to develop other pH sensitive gate materials, such as
Al 2 O 3 , and Ta 2 O 5 [82].
Another example of an Si-based pH sensing device is an electrolyte-insulator-
semiconductor (EIS) structure. In a typical EIS sensor, a voltage applied to the semi-
conductor back gate ( V gate) attracts ions in the electrolyte to the oxide surface. The