ISFET's using inorganic gate thin films

The characteristics of various types of ISFET's using inorganic gate films are described. The pH and pNa selectivities are investigated for SiO
2
, Si
3
N
4
, Al
2
O
3
, alumino-silicate, and sodium-aluminosilicate gate dielectrics. The transient response and device stability are also studied for different values of solution pH. The Al
2
O
3
gate shows a nearly ideal pH response, excellent stability, and selectivity to other cations. On the other hand, the Si
3
N
4
gate is also a good pH sensor, but it is proved by the studies of SiO
2
and SiO
x
N
y
films that the oxygen content in its surface degrades its properties as a pH sensor. Sodium-alumino-silicate, which is generally known as a material for pNa selective glass electrodes, is utilized as a gate film for the pNa ISFET. The pNa selectivity of this device is comparable to that of the conventional glass electrode. The alumino-silicate gate has also a pNa selectivity, but it is inferior to the sodium-alumino-silicate gate.