The variation of human body fluid in tiny concentrations can be critical for clinical diagnosis. Therefore, the detection of chemical and biological species through microelectronic sensor devices has attracted great attention over the past decade. Ion-sensitive field-effect transistors [ISFETs] are one of the silicon-based potential metric sensors with the advantages of compatibility and integration with advanced complementary metal-oxide-semiconductor processes and cost reduction. Until now, plenty of high-k materials have been applied to the sensing membranes of ISFETs, including SiO2, Si3N4[1, 2], Ta2O5[3–5], Al2O3, TiO2[7, 8], HfO2[9, 10], SnO2, etc. Among numerous proposed high-k materials, hafnium oxide [HfO2], characterized by high pH sensitivity, low drift, low hysteresis, and low body effect, is a promising pH-sensing material [9, 12]. In recent years, there are more and more developments on ISFETs such as the chemical field-effect transistor, enzymatic field-effect transistor [EnFET], Bio-FET , DNAFET, etc. For the purpose of monitoring the small changes in body fluid during the early stages, an accurate and stable sensor is needed.
As mentioned above, EnFET is one of the sensors for many biomarkers. The earliest report of EnFET was proposed by Caras and Janata in 1980 . Subsequently, many biomarkers have been detected by EnFET, such as penicillin , urea , glucose , creatinine , etc. To fabricate the EnFET, a specific enzyme is immobilized on the surface of the sensing membrane of an ISFET. Moreover, to immobilize biomolecules (such as enzymes, antibodies, and probe-DNAs)  for monitoring the biomarkers (antigens and target-DNAs), many approaches have been developed, including physical adsorption [19, 20], covalent bonding , entrapment , etc. However, the silanization procedure for producing reactive groups (NH2) on the material is time-consuming and complicated.
In this paper, the pH sensing properties of HfO2 sensing layers with various thicknesses were fabricated by an atomic layer deposition [ALD] system and investigated by an electrolyte-insulator-semiconductor [EIS] structure. The EIS structure is a capacitive sensor in which the changes in surface potential between the electrolyte and the sensing insulator could be measured according to the shift of capacitance-voltage [C-V] curves. Compared to the complex processing of the ISFET, EIS is one of the simplest platforms as an ISFET replacement for the preliminary investigation of the properties of new sensing materials. For the purpose of saving the process time of the bioreactor immobilization, HfO2 sensing membranes with post-ammonia [NH3] plasma treatment were used to replace the chemical procedures.