ISFET with built-in gold electrode and readout circuit with frequency-adjustable pulse output

Abstract

In this paper, an ISFET with a built-in gold reference electrode and an integrated readout circuit with frequency-adjustable pulse output is presented. The CMOS chip is based on the TSMC 0.35mm process with post-process gold metallization. The sensing membrane is surrounded with two gold rings, which can be used as a reference electrode or an enzyme-immobilizing material for pH-value or biomolecule-concentration detection. The ISFET is one of the input differential transistor pair of an operational amplifier in its readout circuit, which mainly consists of a voltage-to-current converter and a current-controlled oscillator. The output pulse frequency is linearly proportional to the effective gate voltage of the ISFET, which is immersed in the analyzed solution with a stably biasing reference electrode. A current-offset structure is added into the oscillator to shift the transfer characteristic line of the output frequency versus the effective input voltage, which usually varies due to process variation. The transfer characteristics of the pulse frequency versus pH value are measured by an external Ag/AgCl reference electrode and an on-chip gold reference electrode, respectively. The built-in gold reference electrode brings about a larger sensitivity. The measured output transfer characteristics show the sensitivity of more than 2kHz/pH with linearity of 99% at least.