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CMOS

CMOS or Complementary Metal Oxide Semiconductor is a combination of NMOS and PMOS transistors that operates under the applied electrical field. The structure of CMOS was initially developed for high density and low power logic gates.

The NMOS and PMOS are the types of Metal Oxide Semiconductor Field Effect Transistors (MOSFET). The CMOS transistors are used in various applications, such as amplifiers, switching circuits, logic circuits, Integrated circuit chips, microprocessors, etc.

The importance of CMOS in semiconductor technology is its low power dissipation and low operating currents. Its manufacturing requires fewer steps as compared to the Bipolar Junction transistors.

As discussed, CMOS is a combination of NMOS and PMOS transistors. Let's discuss a short description of the NMOS and PMOS transistor before beginning with CMOS.

NMOS

The symbol of the NMOS transistor is shown below:

CMOS

The n-channel MOSFET is called NMOS. It has a substrate of p-type, which consists of majority carriers holes. The n-channel consists of majority carriers electrons. The flow of electrons is fast as compared to holes. Hence, NMOS transistors are more rapid than PMOS transistors.

PMOS

The symbol of the PMOS transistor is shown below:

CMOS

The p-channel MOSFET is called PMOS. It has a substrate of n-type, which consists of majority carriers electrons. When a negative voltage is applied to the gate end of the PMOS, it repels the electrons. The attraction of holes results in the formation of the channel called the p-channel. The channel is formed between the source and drain.

The slow flow of holes makes the current controlled process of PMOS easy as compared to NMOS transistors.

Connection setup of CMOS

The practical construction of the CMOS transistor is shown in the below image:

It comprises the NMOS transistor that has N++ regions at the source and drain terminal and p-type substrate. Similarly, the PMOS transistor has two P++ regions and an n-type substrate.

Working of CMOS

The structure as shown consists of the NMOS transistor inverted on the top of the PMOS transistor. The substrate is of the P-type, and three N++ regions. The two N++ regions are small and the third N++ region is large. The two smaller regions are a part of the NMOS transistor, while the third N++ region is a part of the PMOS transistor. The two P++ regions are diffused into the larger N++ region to form the PMOS transistor. The top surface is protected and covered using the Silicon dioxide layer (SiO2) with aluminum's metallization.

CMOS has the least amount of power dissipation in the switching applications. It is because when one transistor is OFF, the other becomes ON. For example, if PMOS is ON, the NMOS transistor will be OFF.

The value of VDD voltage is generally selected between 5V and 15V.

The symbol of CMOS is shown below:

CMOS

Here, G, S, and D specifies the Gate, Source, and Drain terminal of the NMOS and PMOS.

The practical construction of CMOS is shown below:

CMOS

Let's consider the two states of the CMOS, when the input voltage (A) is 0 and 1.

a) A in the above diagram is the input voltage that is fed to the NMOS and PMOS transistors. When the input voltage (A) = 0V, the PMOS conducts, and NMOS will remain in the OFF A' will become 1 when A is 0. Let's represent such condition in the form of NO and NC switches, as shown below:

CMOS

It means that the output voltage is considered as logic 1 because the PMOS switch is closed and the NMOS switch at the bottom is open.

b) Similarly, when the input voltage A = 1, A' will be 0. In such a state, the PMOS will be OFF, and NMOS will conduct. Such state is represented below:

CMOS

It means that the output is 0 when the input is 1. The output voltage is considered as logic 0 because the PMOS switch at the top is open and the NMOS switch is closed.

CMOS Logic Gates

CMOS logic gates are manufactured using the combination of NMOS and PMOS field-effect transistors. In the case of NMOS logic gates, we generally use NMOS depletion type transistors as the load resistance. In CMOS logic gates, we use a complementary structure in which one transistor acts as a load to the other transistor.

The NMOS transistors are designed to work as positive logic elements, while PMOS works as negative logic elements. It means that both the transistors in a CMOS perform complementary logic functions.

Features of CMOS Logic Gates

The features of CMOS Logic Gates are listed below:

  • Reduced cost as it requires only a single power supply.
  • Large logic swing.
  • Large fan-out capability.
  • Very high noise margin.
  • Lower propagation delay
  • High speed as compared to NMOS transistors.
  • Lower power dissipation.
  • Excellent temperature stability.
  • Less packaging density.

Types of CMOS logic gates

The Complementary Metal Oxide Semiconductors are categorized as:

  • CMOS Inverter
  • CMOS NAND
  • CMOS NOR
  • CMOS Operational Amplifiers

CMOS Inverter

The CMOS inverter is formed by connecting the PMOS and NMOS transistors in cascade, as shown below:

CMOS

The top of the CMOS inverter is the PMOS transistor, while the bottom transistor is NMOS. The positive voltage of +VDD at the gate input of the NMOS transistors will turn it ON, while the same positive voltage at the gate input of the PMOS transistor will keep it OFF. Similarly, the voltage of 0 volts at the gate input of the NMOS transistors will keep it OFF, while the same 0 at the gate input of the PMOS transistor will turn it ON.

It means that NMOS transmits logic 1 or VDD, and PMOS transmits logic 0.

CMOS NAND

CMOS NAND is a combination of NMOS NAND and PMOS NOR. It consists of an NMOS NAND gate with the PMOS NOR as its load. CMOS NAND gate can also include a PMOS NOR with the NMOS NAND as its load. It means that NMOS and PMOS transistors' combination in the desired manner forms a CMOS logic gate.

The circuit diagram of CMOS NAND is shown below:

CMOS

The input terminal of the transistors is A and B, as shown above. If A = 0 and B = 0, the NMOS transistors will remain off, and the two PMOS transistors will conduct.

Consider the NOR table shown below:

A B A NOR B
0 0 1
0 1 0
1 0 0
1 1 0

It clearly shows that the output is 1 when the two inputs are 0. Hence, PMOS NOR will conduct. The output Y will be:

Y = logic 1 = VDD

Similarly, when both the input is 1, the NMOS NAND will conduct, and PMOS NOR will remain OFF. But, the output will be 0 because the power will pass to the ground.

Consider the CMOS NAND table shown below:

A B CMOS NAND
0 0 1
0 1 1
1 0 1
1 1 0

CMOS NOR

CMOS NOR logic gate is a combination of NMOS NOR and PMOS NAND. The circuit diagram is shown below:

CMOS

When both the input A and B are 0, NMOS NOR will remain OFF, and PMOS NAND will conduct. It means that transistors T1 and T2 will be OFF, while T3 and T4 will conduct. Thus, the output will be logic 1 when the voltage VDD reaches through the conduction of transistors T3 and T4.

CMOS NOR will conduct only when both the inputs are 0 as discussed above. At all the other input conditions, the output will be 0, as listed below:

A B CMOS NOR
0 0 1
0 1 0
1 0 0
1d 1 0

CMOS Operational Amplifiers

The CMOS structures can also be used as an amplifier when the operating point is fixed in the active region.

Let's consider an example of a CMOS differential amplifier using constant current sources.

CMOS

Advantages of CMOS

Let's discuss the advantages of the Complementary Metal Oxide Semiconductor, which are listed below:

  • Very low power dissipation
    There is no continuous current path from the positive terminal of the transistor to its negative terminal throughout the circuit except the switching instants. Hence, it has the least amount of power dissipation.
  • Reduced circuit complexity
    CMOS requires fewer components, due to which the circuit complexity reduces.
  • Produces very less heat
    CMOS produces significantly less heat as compared to other transistors, such as NMOS and TTL (Transistor-Transistor Logic). Other transistors have some standing current even in the unchanged state, while CMOS does not have.
  • Low static power Consumption
    In an ideal state, CMOS dissipates almost zero or no power as compared to other circuits. It means that it only dissipates power while switching. The lesser dissipation results in lower power consumption. Hence, CMOS has very low static power consumption.
  • Temperature stability
    CMOS family is stable in a wider temperature range compared to other logic circuits, such as TTL. The operating range of CMOS is around -55 degrees Celsius to 125 degrees, while TTL is 25 to 70 degrees Celsius.
  • Improved Noise immunity
    Noise immunity refers to the ability of a system to function in the presence of noise interference. CMOS has the highest noise immunity as compared to the circuit of logic families. Hence, it is highly preferred in high noise automotive applications.
  • High fan-out
    Fan out specifies the input gates driven by the output of the other gate. It means the highest number of input gates of a particular type to which the output can be connected. The fan-out feature measures the load driving ability of a logic gate. Thus, CMOS has a high fan out.

Applications of CMOS

The applications of CMOS are listed below:

  • Integrated Circuits
    CMOS consumes less current than other logic devices, such as TTL. Hence, the use of CMOS in the Integrated Circuit applications forms the production of ICs that has lower consumption and low dissipation.
  • Chip designing
    The use of CMOS in chip designing allows the high-density logic functions to be integrated on a chip.
  • Microprocessor designing
    CMOS requires current only during the switching state. It means that CMOS uses the power efficiently. Hence, CMOS is used in most modern processors, such as microprocessors.
  • ASIC designing
    It stands for Application Specific Integrated Circuits. CMOS is considered the standard transistor for the fabrication of chips. Hence, it is used in ASIC designing.
  • CPU Memories
    The two major advantages of CMOS are high noise immunity and low static power consumption. Due to this, it is used in the CPU (Central Processing Units) Memories.

CMOS vs. NMOS

Let's discuss some differences between CMOS and NMOS for a better understanding. It will help us to analyze the applications of both these transistors in electronics.

Category CMOS NMOS
Full Form Complementary Metal Oxide Semiconductor. N-type Metal Oxide Semiconductor.
Formation It is a combination of NMOS and PMOS. It only consists of NMOS.
Logic transmission CMOS transmits both logic 1 and logic 0. NMOS transmits only logic 1.
Advantages High fan-out, lower power dissipation, etc. Lowest required diffusion steps, etc.
Applications Batteries, sensors, cameras, etc. Logic gates and digital circuits.

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