Unveiling the Power and Applications of CMOS Technology

 What is a complementary metal-oxide semiconductor (CMOS)?

A complementary metal-oxide semiconductor (CMOS) is the semiconductor technology used in most of today's integrated circuits (ICs), also known as chips or microchips. CMOS transistors are based on metal-oxide semiconductor field-effect transistor (MOSFET) technology. MOSFETs serve as switches or amplifiers that control the amount of electricity flowing between source and drain terminals, based on the amount of applied voltage.

MOSFETs use semiconductor materials to conduct electricity under certain conditions but not others. A semiconductor falls somewhere between a conductor and insulator in terms of conductivity. It typically consists of silicon and a mix of impurities that together strike the right balance of conductivity. Silicon in its pure form is not conductive.

The process of adding impurities to a material such as silicon is sometimes referred to as doping. Semiconductor doping can be applied in different degrees to control conductivity. However, other factors can also impact conductivity, such as the type of impurities used.

In MOSFETs, the impurities used for the semiconductor material depend on the semiconductor type. MOSFET semiconductors fall into one of two categories: p-type or n-type. Boron, gallium and indium are commonly used for p-type semiconductors. Phosphorus, arsenic and bismuth are commonly used for n-type semiconductors.

The p-type semiconductor, which is positively charged, carries current as electron deficiencies called holes. A hole has a positive charge that is equal to and opposite an electron charge. The electrons flow in a direction opposite the holes. The n-type semiconductor is negatively charged. In this case, the semiconductor carries current in the form of negatively charged electrons.

However, it is sometimes referred to as Real-Time Clock (RTC), CMOS RAM, Non-Volatile RAM (NVRAM), Non-Volatile BIOS memory, or complementary-symmetry metal-oxide-semiconductor (COS-MOS).

Introduction to MOS Technology

In the IC design, the basic and most essential component is the transistor. So MOSFET is one kind of transistor used in many applications. The formation of this transistor can be done like a sandwich by including a semiconductor layer, generally a wafer, a slice from a single crystal of silicon; a layer of silicon dioxide & a metal layer. These layers allow the transistors to be formed within the semiconductor material. A good insulator like Sio2 has a thin layer with a hundred molecules thickness.

The transistors which we use polycrystalline silicon (poly) instead of metal for their gate sections. The Polysilicon gate of FET can be replaced almost using metal gates in large scale ICs. Sometimes, both polysilicon & metal FET’s are referred to as IGFET’s which means insulated gate FETs, because the Sio2 below the gate is an insulator.

NMOS

NMOS is built on a p-type substrate with n-type source and drain diffused on it. In NMOS, the majority of carriers are electrons. When a high voltage is applied to the gate, the NMOS will conduct. Similarly, when a low voltage is applied to the gate, NMOS will not conduct. NMOS is considered to be faster than PMOS, since the carriers in NMOS, which are electrons, travel twice as fast as the holes.

PMOS

P- channel MOSFET consists of P-type Source and Drain diffused on an N-type substrate. The majority of carriers are holes. When a high voltage is applied to the gate, the PMOS will not conduct. When a low voltage is applied to the gate, the PMOS will conduct. The PMOS devices are more immune to noise than NMOS devices.

CMOS Working Principle

In CMOS technology, both N-type and P-type transistors are used to design logic functions. The same signal which turns ON a transistor of one type is used to turn OFF a transistor of the other type. This characteristic allows the design of logic devices using only simple switches, without the need for a pull-up resistor.

In CMOS logic gates a collection of n-type MOSFETs is arranged in a pull-down network between the output and the low voltage power supply rail (Vss or quite often ground). Instead of the load resistor of NMOS logic gates, CMOS logic gates have a collection of p-type MOSFETs in a pull-up network between the output and the higher-voltage rail (often named Vdd).

Thus, if both a p-type and n-type transistor have their gates connected to the same input, the p-type MOSFET will be ON when the n-type MOSFET is OFF, and vice-versa. The networks are arranged such that one is ON and the other OFF for any input pattern as shown in the figure below.

CMOS offers relatively high speed, low power dissipation, high noise margins in both states, and will operate over a wide range of source and input voltages (provided the source voltage is fixed). Furthermore, for a better understanding of the Complementary Metal Oxide Semiconductor working principle, we need to discuss in brief CMOS logic gates as explained below.

Which Devices use CMOS?

Technology like CMOS is used in different chips like microcontrollers, microprocessors, SRAM (static RAM) & other digital logic circuits. This technology is used in a wide range of analog circuits which includes data converters, image sensors & highly incorporated transceivers for several kinds of communication.

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.

CMOS logic gates

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

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.

CMOS Inverter

Disadvantages of CMOS

The disadvantages of CMOS include the following.

• The cost will be increased once the processing steps increases, however, it can be resolved.
• The packing density of CMOS is low as compared with NMOS.
• MOS chips should be secured from getting static charges by placing the leads shorted otherwise; the static charges obtained within leads will damage the chip. This problem can be solved by including protective circuits otherwise devices.
• Another drawback of the CMOS inverter is that it utilizes two transistors as opposed to one NMOS to build an inverter, which means that the CMOS uses more space over the chip as compared with the NMOS. These drawbacks are small due to the progress within the CMOS technology.

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 logic ICs

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.

CMOS Fabrication

The fabrication of CMOS transistors can be done on the wafer of silicon. The diameter of the wafer ranges from 20mm to 300mm. In this, the Lithography process is the same as the printing press. On every step, different materials can be deposited, etched otherwise patterned. This process is very simple to understand by viewing the wafer’s top as well as cross-section within a simplified assembling method. The fabrication of CMOS can be accomplished through using three technologies namely N-well pt P-well, Twin well, an SOI (Silicon on Insulator). Please refer to this link to know more about CMOS Fabrication.

A Lifetime of CMOS Battery

The typical life span of a CMOS battery is approximately 10 Years. But, this can change based on the utilization and surroundings wherever the PC resides.

Failure Symptoms of CMOS Battery

When the CMOS battery fails, then the computer cannot maintain the exact time & date on the computer once it is switched off. For instance, once the computer is ON, you may see the time and date like 12:00 PM & January 1, 1990. This fault specifies that the battery of CMOS is failed.

• The boot-up of the laptop is difficult
• The beep sound can be generated continuously from the motherboard of the computer
• The time & date have reset
• Peripherals of the computers don’t respond correctly
• The drivers of hardware have vanished
• The Internet cannot be connected.