Gate-controlled diodes for ionic concentration measurement

We have designed and built a new type of chemically sensitive semiconductor device based on the operation of a gate-controlled diode. This structure provides both electrical and chemical shielding of the contact regions from the solution. Ions in the solution provide the driving force for the chemically sensitive layer. The ion-controlled diode (ICD) is biased into inversion from a reference electrode. The substrate source admittance of the ICD is then measured at sufficiently high frequencies so that it is transit-time limited. Under these conditions, the capacitance-gate voltage response becomes frequency dependent and, at constant capacity, the pH response becomes simply dependent on the frequency. We describe the temperature gradient zone melting process used to fabricate the ICD and present operational characteristics of the device.