What is CMOS?

CMOS - or Complementary metal-oxide-semiconductor - is a semiconductor element used in many modern computers and other electronic products. For example, the static RAM device can store, process, and forward digital and analog data simultaneously. In addition to its versatile applications, it is characterised by comparatively low power consumption and lower sensitivity to radiation. Here you can find out what CMOS is, how the semiconductor component works, and which areas it is primarily used in.

CMOS is a semiconductor device familiar to many PC and laptop users, for example, in the form of CMOS memory. This type of memory requires hardly any power to store information. It can therefore be powered by a small battery over long periods of time. How does it work?

CMOS works through an interplay between two transistors - an N-Channel MOSFET and the P-channel MOSFET. As soon as the N-channel MOSFET conducts, the P-channel MOSFET is simultaneously turned off and vice versa. Due to the low voltage in the input, the cross current remains at zero, and power is only required when switching. CMOS RAMs require their own power supply but have low consumption of their own in the process. Typical operating voltage is in the range of 0.75 to 15 volts, much lower than bipolar or MOS circuits. The CMOS transistor itself is only a few millimeters in size and is considered to be exceptionally durable. The actual lifetime depends on the type and frequency of use.

The CMOS basically consists of an n-channel and a p-channel. In the idle state, the power dissipation is only 10 nW. During switching, this depends on the type, but for the standard series, it is at about 1 mW/MHz. If a logic gate is integrated, it is about 10 µW/MHz. Since CMOS is a slow memory chip, there are operating systems that do not write to it directly. In this case, the contents and the configuration are stored as a copy in RAM.

In a computer, CMOS is used to store the BIOS parameter of the mainboard. So even if the device is disconnected from the power supply for an extended period of time or the power supply is unexpectedly interrupted, the CMOS memory ensures that the data required, in particular for the configuration of the computer and its hardware, is saved. All the system configuration data needed to start the system is stored here. For this, CMOS supplies power to the system clock.

If you interrupt the contact between the computer and CMOS and also interrupt the power supply to the CMOS RAM, all configuration data and passwords will be reset after a while. Thus, the computer is reset to its default settings. This principle also comes into effect with a BIOS update. So you can start BIOS and adjust the hardware or react to error messages by BIOS beep codes without having to worry. The CMOS works detached from the rest of the power supply.

The powerful and long-lasting battery supplies the CMOS with continuous power. A CR2032 lithium battery is used in the standard series. Its service life is primarily dependent on external influences. If the battery is exposed to large temperature fluctuations or if the power supply to the computer is interrupted over a longer period of time, this may affect the performance of the battery. As a result, the life expectancy fluctuates but is usually at least two years. Under ideal conditions, however, it can supply the CMOS for much longer.

The HCT-CMOS technology was developed to enable compatibility with TTL voltage. This was necessary because conventional HC inputs were previously not fully compatible with TTL output levels. With the new technology, the transistor structure of the CMOS has been adapted to the output voltage levels of the TTL voltage. This not only enables the unrestricted connection of TTL circuits and HCT-CMOS circuits within one voltage but also results in the speed of TTL gates being achieved in the process.

The so-called Bi-CMOS combines field-effect transistors with bipolar transistors. This combination generates high current drive capability while reducing dependence on capacitive load. This is used for radar systems, radio systems, and signal circuits, for example. However, they are disproportionately more expensive to manufacture.

CMOS technology is used in numerous electronic areas. In addition to its use in processors, microprocessors, and memory elements, other devices such as digital watches are also equipped with CMOS. Furthermore, the technology is used in the field of onboard electronics. CMOS sensors are installed in digital cameras and spectrometers. The positive characteristics of the technology, its high resistance, long life expectancy, and low energy requirements ensure that CMOS has become an indispensable part of many modern devices.

When the battery of the CMOS is removed, the content of the memory is erased. Depending on the type, this process can take a few seconds or even several days. Therefore, the battery should only be removed when the memory is to be erased. This may be necessary if the computer has problems booting or if a different BIOS chip has been inserted. On some mainboards, this procedure is also recommended after a BIOS update. On many PCs, you can also reset the BIOS password via the CMOS. However, with most notebooks, the built-in theft protection ensures that this method does not lead to the desired success.

Information on how to erase the CMOS memory can be found in the manual of the respective motherboard. Follow the instructions there precisely to avoid damaging the mainboard or individual parts, for example, due to a short circuit!