POTASSIUM ION-SELECTIVE FIELD-EFFECT TRANSISTOR (ISFET) SENSORS - STUDIES ON GATE DIMENSIONS AND ION-IMPLANTATION LEVELS

Silicon nitride membrane ISFET sensor chips have been produced with va rying gate dimensions. A series of width/length (W/L) aspect ratios ha ve been examined, combined with three levels of boron ion-implant. The level of ion-implantation affects the threshold voltage; this is impo rtant as a low threshold voltage allows the use of low noise operating conditions. Gate dimensions are also important factors for they deter mine the level of drain current for a given gate and drain voltages. A novel design feature, aimed at achieving wide gates, is the use of fo lded gates as well as having a straight structure. The evaluation of d evices with gates covered with poly(vinyl chloride) (PVC)-valinomycin- dioctyl adipate was based on their response to potassium chloride stan dards when it was shown that there may be a maximum width of gate abov e which there is no improvement of response. Also, the effect of foldi ng the gate structure is discussed and shown to be tenable, thus permi tting greater miniaturisation.