HYSTERESIS IN AL2O3-GATE ISFETS

When used as pH-sensing surfaces, insulators such as SiO2, Si3N4 and Al2O3 are subject to non-idealities such as hysteresis and drift. These effects limit the accuracy obtainable from pH-sensitive ISFETs. This paper studies the hysteresis effects in Al2O3-gate ISFETs by exposing the devices to a cycle of pH values. When the pH is cycled between 3 and 11 in a 0.1 M NaCl solution, hysteresis of 3 to 6 mV is seen, which corresponds to 0.8 to 1.6% of the total response. The hysteresis is somewhat less when background phosphate buffers are used rather than a Tris buffer. Oxalate and aluminate ions have no significant effect. The presence of fluoride ions in acid solutions causes a strong irreversible drift. Alkaline solutions also induce such a drift, but to a much smaller extent. The observations can be explained by the presence of two different effects: first, part of the pH response is delayed, resulting in hysteresis. Secondly, OH- and F- ions cause an irreversible drift towards more negative threshold voltages, which will also affect measured hysteresis values. © 1990.