What is CMOS | A Complete Study about CMOS

What is CMOS ?

Complementary Metal Oxide Semiconductor (CMOS) technology is used for making memory chips,
microcontrollers and microprocessors. And also it is used for constructing analog circuits such as image sensors, transceivers, RF circuits and data controllers.

CMOS is a kind of Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) which uses p-type and n-type transistors. Due to the fact that CMOS uses both negative polarity (NMOS) and positive polarity (PMOS) circuits, CMOS consumes 7x lesser power than NMOS technology alone. This is one of the reasons why CMOS technology is used behind the battery powered devices. However the transistors can become hot when the direction of the current changes. Due to this the speed of the IC chips are limited.

Until 1980s, NMOS has remained as the dominant technology for fabricating Very Large Scale Integration (VLSI) chips. Since when CMOS overtook NMOS, today 99% of the IC chips are built using the CMOS technology.

CMOS technology in chips

The CMOS technology is used behind the semi conductor chip on the motherboard. Unlike a RAM which is volatile, the CMOS chip does not loose its data when there is no power since it is battery powered. This battery powered chip is responsible for storing BIOS settings including that of system time, date and hardware settings. 

 

To supply the relevant power needed for the CMOS chip for the functioning, there is a coin sized battery with the label CR2032 3V. This battery is known as the CMOS battery. Most of the motherboards has a separate spot for fixing this battery. 

You may find this CMOS battery used more frequently in laptop than in desktops because laptops are plugged without a power source most of the time. The moment you charge the laptop this battery will charge as well. 

 

And this battery is important to the extent that once it is dead, the whole CMOS chip has to be replaced. When this battery becomes dead it will show signs of incorrect system date and time. The average lifespan of a CMOS battery is around 10 years, after this period the battery has to be replaced.

How does CMOS and BIOS work together ?

The BIOS on your commuter performs the Power On Self Test (POST) that ensures that all of the hardware components connected to your computer are working properly. Without the BIOS the computer will not be able to understand how the hardwares work. 

 

The BIOS is just a firmware, therefore it cannot store data. The CMOS on the other hand, stores information related to the hardware settings, data and time. The information stored is only 256 bytes of data. 

When the computer boots, the BIOS pulls up information from the CMOS chips that includes hardware settings, date and time. Due to this the system will be able to display the accurate date and time even after a long period of computer power off.

Advantages of CMOS

CMOS transistors are well known for their power efficiency. Except when changing from one state to another, CMOS transistors use no power at all. Compared to a logic controller, CMOS only uses 50% of power to operate in both static and dynamic positions. 

 

Due to this the CMOS has limit the output voltage. Additionally, CMOS transistors perform fast operations without getting too hot. As a result no minimal heat is dissipated.