Complete Introduction to CMOS (Complementary Metal-Oxide-Semiconductor)

CMOS (Complementary Metal Oxide Semiconductor) is a technology utilized for the fabrication of large-scale integrated circuit chips, specifically tailored for the production of low-power digital and analog circuits. Its widespread adoption in smartphones, computers, digital cameras, and various other devices can be attributed to its numerous advantages, including exceptional reliability, minimal power consumption, efficient heat dissipation, and low power supply requirements. As the electronics manufacturing industry continues to evolve, CMOS technology has emerged as a pivotal catalyst in the progression towards intelligent and high-performance electronics.

Table of Contents

CMOS Technical Characteristics

  • Low power consumption: CMOS circuits exhibit exceptionally low power consumption due to their characteristic bias voltage implementation, which effectively minimizes circuit current in an ideal state. The complementary pair of MOSFETs ensures that one transistor is always on while the other is off during stable signal input, resulting in minimal current loss. Additionally, the logic gates enter a high-impedance state when inactive, further reducing power consumption. The complementary design of CMOS logic gates and memory cells contributes to the overall power efficiency of the circuit.
  • Massive integration: Running extensive transistor integration on smaller chips, facilitating highly integrated circuits. Firstly, the adoption of standard semiconductor manufacturing processes simplifies the realization of integrated manufacturing. Secondly, the circuit’s streamlined structure allows for continuous reduction in transistor size through scaling techniques, enabling large-scale integration. Finally, the low power consumption characteristic ensures that CMOS chips do not exceed power limits or encounter excessive heat dissipation issues.
  • Cost-effectiveness: CMOS manufacturing offers cost advantages compared to traditional chip fabrication methods. Firstly, the simplicity of the process allows manufacturers to produce highly efficient circuits with minimal control voltage and materials. Secondly, CMOS utilizes readily available and common materials. Moreover, the high level of integration minimizes the need for unnecessary components, contributing to cost savings.
  • High reliability: This technology enhances circuit reliability while supporting high-speed operations, meeting the demanding efficiency requirements of modern electronic devices like CPUs. Firstly, it possesses noise suppression capabilities, counteracting environmental and power supply noise interference. Secondly, CMOS exhibits resistance to radiation and electromagnetic interference. Finally, CMOS circuits can operate within a wider voltage range, accommodating various power supply voltage requirements while maintaining reliability.

CMOS Composition and Principle


CMOS, as an advanced semiconductor device, operates on the fundamental principle of the metal oxide semiconductor field-effect transistor (MOSFET). A MOSFET consists of three key components: the gate, drain, and source.

  • Gate: The gate comprises metal electrodes, separated by a layer of oxide, which forms the gate oxide layer of the MOSFET.
  • Source and Drain: The source and drain regions are connected by a conductive channel. The conductive characteristics of this channel can be controlled by applying a voltage to the gate.

In essence, envision a MOSFET as a faucet, where the gate corresponds to the handle, and the source and drain represent the two ends of a water pipe. By turning the handle, water flows from the source to the drain.

Logic Gates

Within digital circuits, logic gates serve as crucial components for processing input and output signals, enabling logical operations and the realization of various digital circuit functionalities. CMOS logic gates, in particular, are comprised of both nMOS and pMOS transistors, allowing for the implementation of fundamental logic operations like AND gates, OR gates, and NOT gates. The fundamental building block is the NOT gate, where the input signal is connected to the MOSFET’s gate. When the input is at a high level, the valve opens, resulting in a low-level output. Conversely, when the input is at a low level, the valve remains closed, generating a high-level output.

Memory Unit

The fundamental building block of memory is the static random access memory (SRAM), which employs six MOSFETs to store a single bit of information. Two MOSFETs are responsible for controlling the read and write operations, while the remaining four MOSFETs store the actual bit value. SRAM operates on the principle of utilizing a feedback loop to retain a high or low signal within the four MOSFETs. When a read operation is required, applying the appropriate read control signal to the MOSFET enables the retrieval of the stored bit value.

Two Main Application Areas of CMOS

Computer Information Storage

CMOS in computer

Within the realm of computer applications, CMOS technology finds extensive use in memory construction, encompassing static random access memory (SRAM), dynamic random access memory (DRAM), and non-volatile memory (flash memory).

  • SRAM: Constructed using CMOS technology, SRAM stores one bit of data through multiple memory cells, enabling rapid read and write operations, low latency, and high stability.
  • DRAM: Each memory cell in DRAM consists of a capacitor and an access transistor. However, the capacitor gradually loses charge during operation, necessitating periodic refreshing. CMOS manufacturing of the main memory (RAM) allows for the storage of larger data.
  • Flash memory: Flash memory adopts a specialized charge accumulation and erasing mechanism to represent stored data by altering the charge state within the transistor. CMOS technology enables higher storage density, making it widely employed in mobile devices, solid-state drives (SSDs), and other fields.

More specifically, CMOS chips are commonly utilized in computer motherboards to preserve BIOS (Basic Input/Output System) settings. These chips offer read and write capabilities, allowing for repeated data manipulation and long-term data retention. This information encompasses critical parameters such as date, time, and startup settings, which are essential for configuring computer hardware parameters.

It is important to distinguish between CMOS and BIOS, as they are distinct concepts. BIOS refers to the fundamental input/output system of a computer, responsible for hardware initialization during system startup and serving as an interface between the operating system and hardware components. In contrast, CMOS denotes a separate chip that is both readable and writable, exclusively used for data storage. Typically, CMOS chips are physically mounted on the motherboard, with a battery providing power to sustain their operations, while BIOS resides in firmware and contains essential system programs.

Digital Imaging Field

CMOS in camera

When discussing CMOS, the term most commonly associated with it is “image sensor,” indicating its wide-ranging development as a cost-effective photosensitive element in the realm of digital imaging. In contemporary photographic digital products that demand higher frame rates, increased dynamic range, and reduced read noise, CMOS image sensors are often preferred over traditional Charge-Coupled Device (CCD) image sensors.

As an image sensor, although its application differs significantly from CMOS circuits employed in firmware or computing tools, it fundamentally utilizes the CMOS manufacturing process. The primary transformation involves converting the function of pure logical operations into electrical energy upon receiving external light. The obtained image signal is then converted into a digital output through the on-chip analog-to-digital converter (ADC). There are four key components in an image sensor: microlense, color filter (CF), photodiode (PD), and pixel design.

Why is CMOS crucial in the field of digital imaging?

There is a popular adage in this domain: “Larger the bottom, the better.” Here, “bottom” refers to the CMOS image sensor, and this statement holds true. A larger sensor equates to a greater light-sensing area, resulting in improved image quality under low-light conditions.

It’s akin to viewing the world with narrow, squinted eyes versus wide-open eyes. The CMOS image sensor serves as the device’s visual system, and it’s undoubtedly advantageous for it to be larger!

The sensor is populated with numerous minuscule photosensitive elements, with each element representing a pixel. As the CMOS size increases, so does the pixel count. However, there is an upper limit to this expansion. Once a certain size is reached, further increasing the number of pixels becomes meaningless.

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