CMOS-Compatible Ex-Situ Incorporated Junctionless Enhancement-Mode Thin Polysilicon Film FET pH Sensor

In this article, a junctionless enhancement-type field-effect-transistor based pH sensor is proposed and fabricated through low-cost bulk CMOS aligned process. The thin device layer consists of uniformly ex-situ doped p-type polySi, yielding a junctionless doping profile throughout the active region. A thin Al
₂
O
₃
layer is deposited through atomic layer deposition as a sensing membrane, providing high site binding density for pH solutions. The effect of channel width (W
_si
) ranging from 5 to 15 μm on threshold voltage and drain current sensitivities (ΔV
_th
/ΔpH}, ΔI/ΔI
_d
) respectively, have been investigated for different pH buffer solutions. For Ws of 5 μm, a maximum ΔV
_th
/ΔpH sensitivity of 66.7mV/pH and current sensitivity of 98.9% was observed. Further, the drain current of ≃65$ nA at V
_gs
= 0V and V
_ds
= -4V, yields low leakage operation of the proposed sensor. Furthermore, the repeatability of the proposed sensor is demonstrated by repeating the drain current measurements thrice with an interval of 10 minutes between corresponding measurements. A maximum shift of 30.9% among all measurements at pH = 6 which corresponds to 0.1 μA was observed, implying excellent repeatability of the proposed sensor.