pH sensor on O-terminated diamond using boron-doped channel

A concept of an ion sensitive FET (ISFET) on diamond using a thin and highly boron-doped channel is described. The doped channel is in direct contact to the electrolyte solution. The diamond surface is oxygen terminated to provide pH sensitivity and chemical stability. The first set of pH-sensitive ISFET microstructures with the extrinsically doped channel is fabricated on 100-oriented single crystal diamond substrate using a solid doping source technique and a wet chemical oxidation for the O termination. The fabricated structures show pH sensitivity close to the Nernst's limit of 59 mV/pH. The ISFET characteristics have been reproducible and did not degrade after repeated cycling between 0.1 M H2SO4 and 0.1 M KOH solutions. It is shown that only a part of the applied gate potential to the diamond–electrolyte interface drops within the delta-doped channel. The possible origin of this effect is discussed by analysing the electrochemical impedance characteristics of the boron-doped diamond electrode.