CMOS (Complementary Metal-Oxide-Semiconductor)

CMOS (Complementary Metal-Oxide-Semiconductor)

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Complementary Metal-Oxide-Semiconductor (CMOS) is a type of integrated circuit process used to create PMOS (P-channel MOSFET) and NMOS (N-channel MOSFET) transistors on a silicon wafer. Due to the complementary nature of PMOS and NMOS, it is called CMOS. This process is used to create microprocessors, microcontrollers, static random-access memory (SRAM), and other digital logic circuits.

CMOS has the advantage of consuming power only when transistors need to switch on or off, making it very energy-efficient and generating low heat. Early read-only memory (ROM) was mainly made with this circuitry. Because BIOS programs and parameter information in early computer systems were stored in ROM, in many cases, when people referred to "CMOS," they were actually referring to the BIOS unit, and "setting CMOS" meant configuring the BIOS.

The term "Metal-Oxide-Semiconductor" actually reflects the early construction of field-effect transistors (FETs), where the gate electrode was a layer of metal over an insulating material (such as silicon dioxide). Today, most metal-oxide-semiconductor field-effect transistors (MOSFETs) use polysilicon as the gate material, but the term "MOS" is still used in the names of modern components and processes.

Nowadays, CMOS technology is also commonly used as an image sensor in digital imaging devices, also known as an active pixel sensor (APS). This includes high-resolution digital cameras, digital camcorders, and especially larger-format digital single-lens reflex (DSLR) cameras. Consumer digital cameras have also started using back-illuminated CMOS to improve image quality. Compared to traditional charge-coupled devices (CCDs), CMOS sensors have an amplifier at each pixel, resulting in faster data transmission.

When Micro-Electro-Mechanical Systems (MEMS) sensing elements and CMOS signal processing circuits are integrated on a single chip, it is often called CMOSens.

In 1963, Frank Wanlass of Fairchild Semiconductor invented the CMOS circuit. By 1968, a research team led by Albert Medwin at RCA successfully developed the first CMOS integrated circuit. Although early CMOS components had lower power consumption than common transistor-transistor logic (TTL) circuits, their slower operating speed meant that most CMOS applications focused on reducing power consumption and extending battery life, such as electronic watches. However, after years of research and improvement, modern CMOS components have advantages over another mainstream semiconductor process, Bipolar Junction Transistor (BJT), in terms of area, speed, power dissipation, and manufacturing cost.

Early standalone CMOS logic components included the "4000 series" (RCA 'COS/MOS' process). Later, with the "7400 series," many logic chips could be implemented using CMOS, NMOS, or even BiCMOS (Bipolar Complementary Metal-Oxide-Semiconductor) technology.

Early CMOS components were more susceptible to Electrostatic Discharge (ESD) damage compared to their main competitor, BJT. Newer generations of CMOS chips usually include ESD protection circuits at input/output pins and power/ground terminals to prevent large currents induced by ESD from damaging internal circuits. However, most chip manufacturers still warn users to take electrostatic precautions to avoid exceeding the energy that ESD protection circuits can handle, such as wearing anti-static wrist straps when installing memory modules into a personal computer.

Early CMOS logic devices (like the 4000 series) had an operating range of 3 to 18 volts DC, so the gates were made of aluminum. Over the years, most CMOS logic chips have operated under the TTL standard voltage of 5 volts, until the 1990s, when there were increasing demands for low-power requirements and new signaling standards, which replaced TTL. As MOSFETs became smaller, the thickness of gate oxides decreased, and the gate voltages also dropped. Some of the latest CMOS processes now have operating voltages below 1 volt, which further reduces power consumption and improves performance.

Modern CMOS gates are mostly made using polysilicon. Compared to metal gates, polysilicon has the advantage of better temperature tolerance, making the annealing process more successful after ion implantation. It also allows for self-aligning gate definitions, reducing gate size and minimizing stray capacitance. Since 2004, some new studies have begun using metal gates again, but most processes still use polysilicon gates. Many studies are also focused on using different gate oxide materials to replace silicon dioxide, such as high dielectric constant materials (high-K dielectrics), to reduce gate leakage current.

CMOS can refer to both the complementary metal-oxide-semiconductor device and the process. For the same functional requirements, integrated circuits (ICs) made with the CMOS process consume less power, which is why most IC products today are made using CMOS.

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