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!