**1. Introduction**

Integration and miniaturization are the two main factors in the device engineering research, and these two factors are also the main features of the lab‐on‐a‐chip technology. The main contributor for this major trend is the fast advances of integrated circuit (IC) technology. As a result of miniaturization, portable and cheaper equipment replaced the traditional bulky and expensive equipment.

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In the field of cell biology, the use of micro‐ and nano‐fabricated lab‐on‐a‐chip using active substrates and integrated sensors [1] is still at experimental stage. In order to replace traditional optical inspection techniques, these techniques have been tested for the identification of objects at microscopic level to envision several applications [2]**.**

Many research groups believe that lab‐on‐a‐chip technology will be the major contributor to novel diagnostic tools. The target of these research groups is to have lab‐on‐a‐chip systems that will allow healthcare suppliers and workers in poorly equipped clinics to do different tests which need more sophisticated equipment in a simpler way, for example, nucleic acid assays and immunoassays tests [3]. Thus, lab‐on‐a‐chip innovation is a vital part of the endeavors to upgrade and enhance worldwide healthcare system, especially through the advancement of clinics and laboratories' testing techniques [4].

One dynamic area of lab‐on‐a‐chip research includes approaches to analyze and oversee HIV infections. In view of UNAIDS 2014 report [5], more than 40 million individuals are tainted with HIV around the world; 13 million of these individuals get hostile to retroviral treatment. Around 55% of individuals with HIV have never been tested for the infection [5].

Measuring the quantity of CD4+ T lymphocytes in a man's blood is an exact and right approach to figure out whether a man has HIV and to track the advancement of a HIV contamination. Right now, flow cytometry is the brilliant standard for acquiring CD4 checks; yet, flow cytometry is a confounded method that is not accessible in most developing regions since it requires prepared professionals and costly instruments [6, 7].

The vicinity of the complementary metal oxide semiconductor (CMOS) technology allows the integration of sensors, amplifiers, filters and other circuitry on a single chip. Also, CMOS technology leads to a totally electronic integrated lab‐on‐a‐chip utilizing a homogenous technology [8, 9]. CMOS is an innovation used in building integrated circuits. It is used in microcontrollers, static RAM, registers, microchips and other digital circuits. CMOS technol‐ ogy is utilized also for a wide assortment of analog circuits, for example, image sensors, amplifiers, analog to digital converters, and transceivers for communication modules. Low static power consumption and high noise immunity are two advantages of CMOS devices [10].

Numerous specialists and research teams have begun to utilize the CMOS technology in lab‐ on‐a‐chip applications [11, 12]. These applications incorporate, yet not restricted to, identifi‐ cation of neurons' activities, microorganism discovery and portrayal, electric field imaging, flow cytometry and polymerase chain reaction (PCR) applications [13–17].

This chapter covers and presents the state of the art in CMOS circuits and systems for lab‐on‐ a‐chip applications. It summarizes and reviews different circuits and systems of CMOS‐based lab‐on‐a‐chip technology at the cell level. These circuits and systems include: polymerase chain reaction (PCR), microorganism detection and characterization, neuronal activity detection, flow cytometry and electric field imaging applications.
