Abstract
AbstractThe ion‐sensitive field effect transistors (ISFETs), proposed little over 50 years ago, today make the most promising devices for lab‐on‐a‐chip, implantable, and point‐of‐care (POC) diagnostics. Their compatibility with CMOS (Complementary Metal Oxide Semiconductor) technology and the low cost through mass production have been the driving factors so far. Nowadays, they are also being developed in flexible form factors for new applications such as wearables and to improve the effective usage in existing applications such as implantable systems. In this regard, the CMOS ultra‐thin chip (UTC) technology and the bonding by printing are the noteworthy advances. This paper comprehensively reviews such new developments in the CMOS‐compatible ISFETs, along with their theory, readout circuitries, circuit‐based techniques for compensation of the ISFET's instabilities, such as the offset, flicker noise, and drift. The sensing mechanisms and the properties of interface between the electrolyte under test and the metal‐oxide based ion‐sensitive electrodes have been discussed along with a brief overview of the metal‐oxide based pH sensors. An overview of the reported mechanically flexible pH sensors, including ISFETs, is provided and the history of ISFET applications are also covered. Finally, established models that can be used to design flexible circuits are presented, and possible opportunities to use circuit techniques to compensate for mechanical deformation are discussed.
Highlights
Field-effect transistor (FET) based solid state sensors have attracted considerable attention since late 1960s, that is, immediately after the introduction of standard silicon planar technology for integrated circuits (IC)
We present the evolution of ion-sensitive field effect transistors (ISFETs) in terms of the device structure, materials, and advances such as physical flexibility
We focus on the current trends attributed to the development of flexible and conformable ISFETs in terms of technology, material, response as well as their capability to be monolithically integrated with on-chip electronics
Summary
Field-effect transistor (FET) based solid state sensors have attracted considerable attention since late 1960s, that is, immediately after the introduction of standard silicon planar technology for integrated circuits (IC). With the advancement of CMOS technology, bulky passive pH sensors such as glass probes are being replaced by their miniaturized active ISFET counterparts that can be developed on semiconducting substrates They enable the development of System-onChip architectures with integrated signal conditioning circuits, read-out topologies, digitization, and wireless modules.[26] This paves the way for the development of Point-of-Care (PoC) devices and diagnostic tools deployable in resource-deprived areas as well as for in-vivo modules capable of being affixed to tissues, blood vessels, and even cells.[24,27] They provide the opportunity for the fabrication of large-scale chemical sensing arrays.
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