Extended Gate Ion-Sensitive Field-Effect Transistors Using Al₂O₃/Hexagonal Boron Nitride Nanolayers for pH Sensing

A stack consisting of atomic layer deposited (ALD) aluminum oxide (Al2O3) and hexagonal boron nitride (h-BN) is proposed and demonstrated as the sensing layer of extended gate ion-sensitive field-effect transistors (EG ISFETs) in this work. The use of h-BN in the sensing stack is to suppress the voltage drift during pH measurement because of a low defect level inside the two-dimensional (2D) h-BN. The use of Al2O3 in the sensing stack is to improve the pH sensitivity due to the chemical inertness of h-BN and the resultant low sensitivity. A low-power oxygen plasma treatment on the surface of the h-BN flake is employed to obtain a uniform and high-quality Al2O3 layer on top of h-BN. The influences of Al2O3 thickness in the Al2O3/h-BN sensing stack on the sensitivity and drift characteristics of 2D EG ISFETs are investigated. It is found that the EG ISFET with molybdenum disulfide (MoS2) field-effect transistor (FET) and Al2O3/h-BN sensing stack with 5 nm thick Al2O3 provides a pH sensitivity of >50 mV/pH while at the same time exhibiting a low drift value of ∼4 mV/h. Moreover, a pH sensing resolution of 1.54 × 10–3 pH is extracted by using low-frequency noise measurement. All these suggest the Al2O3/h-BN stack as a highly promising sensing stack for high-sensitivity and low-drift ISFETs.