What is CMOS (Complementary Metal Oxide Semiconductor) Technology?
CMOS or Complementary Metal Oxide Semiconductor refers to the specific type of technology that is used to make ICs or Integrated Circuits that are used in different digital and analog devices.
Technically, it signifies the transistors called MOSFETs or Metal Oxide Semiconductor Field-Effect Transistors that have a ‘source’ and a ‘drain’ as the two terminals, and an insulated gate.
- CMOS is more efficient in power management with better control on the flow of electrons between the source and drain terminals and no consumption of current in the idle state.
- CMOS technology is extensively used in the making of different Integrated Circuits, ASIC chips, microprocessors, CPU memory and others having a wide range of applications.
- A combination of CMOS VLSI design is very useful in creating several new Integrated Circuits, System on a Chip and other different modules.
- CMOS can be categorized into different groups based on its design such as HCT-CMOS, Bi-CMOS, CMOS NAND, CMOS NOR, CMOS Inverter, and CMOS Operational Amplifier.
- The working process of CMOS involves the interaction between the N-Channel MOSFET and the P-channel MOSFET where one is turned off when the other conducts.
Understanding CMOS Technology
CMOS is actually the battery powered semiconductor chip on the motherboard and is located inside computers to store information.
CMOS may be referred in different names such as:
- Complementary Metal Oxide Semiconductor
- RTC or Real Time Clock
- NVRAM or Non Volatile RAM and
- CMOS RAM.
The term ‘metal’ can be quite misleading because there is nothing as such in it.
Modern MOSFETs usually use a poly-silicon conductive material rather than aluminum.
And the term ‘complimentary’ in CMOS signifies the two kinds of N-type and P-type semiconductors present in each transistor.
There are two terminals in CMOS and the gate is insulated from the transistor body.
When adequate voltage is applied between the body and the gate electrons start to flow between the two terminals, source and drain.
The N-type semiconductors in it contain more electrons than holes in it and therefore there is a shortage of space where these electrons can reside. On the other hand, in the case of the P-type semiconductors, the situation is just the opposite.
When these two semiconductors work in concert logic gates are formed depending on the design of the circuit.
At the same time they also restrict the voltage output and therefore results in a low-power design that generates very little heat.
The CMOS transistors use electrical power most efficiently because no current is used in the idle state like other circuits.
It is required only when there is a change in state from ON to OFF and vice versa.
It is for these specific benefits in its working process, the CMOS has replaced the CCDs or Charge Coupled Device in the camera sensors and are also used in the modern computer processors.
Need for CMOS
The need of CMOS today can be seen in manufacturing different Integrated Circuits called chips or microchips. It is due to the efficiency and benefits offered by the MOSFET technology.
The different reasons that the CMOS today is required in ASIC and chip designing, microprocessor designing, and CPU memories are summarized as follows.
- CMOS is needed to make the Integrated Circuits because it consumes less power than TTL and other logic devices.
- It is needed in chip designing because it allows integrating high-density logic functions on the chips.
- It is needed in microprocessor designing because it uses power most efficiently and only when it is required.
- It is used in ASIC or Application Specific Integrated Circuits designing because it is taken as the standard transistor for fabricating chips.
- It is also used in memories of the Central Processing Units or CPUs due to its low static power utilization and high noise resistance.
CMOS in VLSI
CMOS is used in VLSI or Very Large Scale Integration design because it enables substantial scaling in a wide range of semiconductor devices.
When the CMOS technology is combined with VLSI it eventually helps in reducing the cost of manufacturing and at the same time it pushes the package to a smaller level.
Since MOSFET technology is not as dense as the FinFET technology, CMOS plays a major role in designing a new IC, SoC, or any other module using the combination of CMOS VLSI design.
Typically there are two specific steps in CMOS VLSI design such as:
- Physical Design and
- Circuit blocks design.
Designing the circuit blocks involve connecting different transistors into the logic blocks and are then integrated into a bigger IC.
This eventually helps in extracting the specific electrical characteristics of the circuit blocks with the use of different simulation tools for VLSI.
Typically, using CMOS VLSI design in the making the ICs is a more modular approach where the complete circuit is built in two specific steps such as:
- Connecting the smaller circuit blocks to the larger circuit blocks and
- Connecting the larger circuit blocks at the system level.
The complete circuit can be a digital, analog or mixed signal circuit.
However, the process is not as simple as it sounds. In fact, there are two specific challenges in CMOS VLSI design that are to needed to overcome such as:
- Designing the smaller and larger circuit blocks and simulating them and
- Linking these circuit blocks together into the larger system.
The foundation of the CMOS VLSI design is the digital CMOS circuit blocks that are based on the CMOS inverter.
However, the analog CMOS circuit blocks can also be created from the CMOS circuitry, which may be old but there are several modern devices that are created on such age-old circuit designs.
Once done, there are a lot of things to analyze in the CMOS VLSI design and its physical layout, both before and after creation.
During this analysis process, simulations are needed to be conducted at three basic levels to deduce the electrical behavior of every level.
The different sets of simulations to perform are:
- Small-scale circuit simulations, which involve operating point analysis for nonlinear components, frequency sweeps for linear components, and transient analysis to deduce the quantitative behavior of the circuits within the frequency domain and time
- Large-scale circuit simulations, which involve running logic simulations for digital circuits and a set of analog simulations to see how outputs are produced by a particular larger circuit block for a specified set of inputs and
- System-level simulations, which involve using the results from block-level simulations in the high-level system simulations to see how a set of circuit blocks of the CMOS VLSI design functions by using special modeling tools which will lessen circuit blocks in the functional blocks.
To use in graphical simulations in a circuit simulation, specific models such as SPICE models are needed to perform the required simulations particularly for the MOSFETs that are typically present in the CMOS inverters.
Types of CMOS
There are typically a few different types of CMOS that can be categorized based on their design and characteristics such as HCT-CMOS, Bi-CMOS, CMOS Inverter, CMOS NAND, CMOS NOR, and CMOS Operational Amplifiers.
First, the HCT-CMOS technology was originally designed to make it compatible with TTL voltage so that the HC inputs can be worked on.
This technology changed the CMOS structure so that it can adapt to the different levels of output voltage of the TTL.
This allowed unrestricted connection of the HCT-CMOS and TTL circuits in a single voltage which enhanced the speed of the TTL gates in the process.
Second, the commonly known Bi-CMOS technology combines both bipolar and field-effect transistors that reduce capacitive load dependence and generate high current drive. Though it is expensive to manufacture it is used in radio systems, radar systems, signal circuits, and more.
CMOS can also be categorized as follows:
- CMOS Inverter – This is formed when the NMOS and PMOS transistors are connected in cascade where the top one is the PMOS transistor and the bottom one is the NMOS transistor
- CMOS NAND – This is the combination of PMOS NOR and NMOS NAND where the former is the load and the latter is the gate which may also contain PMOS NOR with NMOS NAND as the load
- CMOS NOR – This is the combination of PMOS NAND and NMOS NOR and it conducts only when both the inputs are 0 and
- CMOS Operational Amplifiers – This refers to a CMOS structure when the point of operation is set in the active area.
Depending on the type of the CMOS, it is used for different purposes as already mentioned above.
How Does CMOS Work?
Typically, the working process of CMOS involves the interaction between the two transistors that make these chips namely, the N-Channel MOSFET and the P-channel MOSFET.
The entire process typically involves two specific steps as follows:
- The P-channel MOSFET is turned off as soon as the N-channel MOSFET starts to conduct and
- The N-channel MOSFET is turned off simultaneously as soon as the P-channel MOSFET starts to conduct.
During this entire process power is needed only when switching is made and at the other times the cross current stays at zero. This is because the input has a very low voltage.
During the working process, there are a few specific things that happen such as:
- Both the N-type and P-type transistors design the logic functions
- The same signal that turns ON a transistor of a specific type is utilized to switch OFF a transistor of another type
- The design of logic devices use simple switches not needing a pull-up resistor
- A set of N-type MOSFETs is arranged at the CMOS logic gates during a pull-down network between the low voltage power supply rail and the output named VSS and
- A set of P-type MOSFETs is arranged at the CMOS logic gates during a pull-up network between the high voltage power supply rail and the output often named VDD.
This specific and simultaneous pull-up and pull-down network ensures that when the gates of both the P-type and N-type transistors are linked to the same input, the N-type MOSFET will be OFF and the P-type MOSFET is ON and vice-versa.
This actually is the basic working principle of CMOS, as mentioned earlier.
Ideally, if the voltage of the source is fixed, CMOS can work on a varied range of input and source voltages.
Characteristics of CMOS
CMOS is characterized by its versatile applications, huge noise immunity, low sensitivity to radiation and low power consumption. It comes with an n-channel and a p-channel and the power is supplied to it by a durable CR2032 lithium battery.
Out of all, low static power consumption is the most significant characteristic of CMOS. This is actually the result of the switching OFF of the single transistor from the MOSFET transistor pair.
This also means that CMOS will not generate as much heat as other logic circuits such as NMOS and Transistor-Transistor Logic that use some amount of current even in their idle state.
Some other notable characteristics of CMOS are:
- Very simple gates
- High input impedance
- Negligible power dissipation
- High fan out
- TTL compatibility
- Stable temperature
- Low packing density and more space consumed than NMOS
- Well and mechanically robust design and
- Large logic swing.
All these CMOS characteristics allow integrating logic functions on an Integrated Circuit with high density which is why CMOS technology is most frequently and extensively used in the Very Large Scale Integration or VLSI chips.
Where is CMOS Suitable?
Usually, on most modern motherboards the CMOS is integrated with the Real Time Clock or RTC located on the Southbridge. Some motherboards may also have CMOS as a separate chip.
In addition to that, CMOS also has some other significant uses in different components of a computer such as:
- In the microcontrollers
- In the static RAM
- In the microchips
- In the registers and several other digital circuits.
Not only that, CMOS technology is also used in a wide array of analog circuits such as image sensors, transceivers used as communication modules, amplifiers, and Analog to Digital Converters or ADCs.
What is Stored in CMOS Chip?
Different types of information can be stored in the CMOS chip. The most important information is the current date and time of the system clock.
It may also include information regarding the types of drives installed in the system as well as the booth sequence of the computer system
So, coming to the end of the article, you now know what CMOS is and what the importance of it is in modern computing.
It may be an ingenious and petite part of the computer that you need not worry about but it should not be tinkered unless troubleshooting or reverting BIOS or UEFI to their default settings is required.