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 SiO2, Si3N4, A12O3, alumino-silicate, and sodiumalumino-silicate gate dielectrics. The transient response and device stability are also studied for different values of solution pH. The Al2O3 gate shows a nearly ideal pH response, excellent stability, and selectivity to other cations. On the other hand, the Si3N4 gate is also a good pH sensor, but it is proved by the studies of SiO2 and SiOxNy 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 iti s inferior to the sodium-alumino-silicate gate.