CMOS vs NMOS: Which is Better?

What is CMOS Technology?

CMOS, namely complementary metal oxide semiconductors, employs complementary and symmetrical pairs of p-type and n-type MOSFETs to fabricate metal-oxide-semiconductor field-effect transistors (MOSFETs) for logic purposes. Integrated circuit (IC) chips, such as microprocessors, microcontrollers, memory chips (including CMOS BIOS), and other digital logic circuits, are built using CMOS technology. The analog circuits used in many other forms of communication also utilize CMOS technology, including image sensors (CMOS sensors), data converters, RF circuits (RF CMOS), and highly integrated transceivers. For your reference, click this blog to know more about CMOS technology: Understanding the CMOS Technology

Figure 1 CMOS  

Is CMOS Volatile or Nonvolatile?

Non-volatile memory, such as flash memory and EEPROM, is frequently built using CMOS technology. Instead of the memory cells themselves, CMOS is employed in the context of memory to address and control circuits. Thus, CMOS is neither volatile nor non-volatile in and of itself. This technology is employed in the manufacturing of a variety of electronic components, such as memory that are both volatile and non-volatile.  

What is NMOS Technology?

NMOS (N-type metal-oxide-semiconductor) technology is a fundamental semiconductor technology used in the construction of integrated circuits. It is widely employed in the design of digital logic circuits and has played a crucial role in the development of modern electronic devices. At the heart of NMOS technology lies the NMOS transistor, an enhancement-mode MOSFET (metal-oxide-semiconductor field-effect transistor). Unlike its counterpart, the PMOS transistor, the NMOS transistor utilizes electrons as its majority charge carriers, hence the designation "N-type." The transistor's structure consists of a silicon substrate hosting a source terminal, a drain terminal, and a gate terminal. The operation of an NMOS transistor begins with the application of a voltage to the gate terminal. This voltage creates an electric field that regulates the flow of electrons between the source and drain terminals. When the gate voltage surpasses a specific threshold, a conductive channel is formed, enabling current to flow through the transistor. In contrast, when the gate voltage is below the threshold, the channel remains non-conductive, and the transistor remains in the off state. NMOS technology finds extensive use in digital logic circuits, including microprocessors and memory chips. Its popularity stems from several advantages it offers. Firstly, NMOS transistors exhibit fast switching speeds, enabling efficient operation in high-speed applications. Additionally, they possess relatively low power consumption characteristics, making them suitable for various electronic devices. Moreover, NMOS technology can be integrated seamlessly with complementary metal-oxide-semiconductor (CMOS) technology, allowing for the construction of complex integrated circuits.

图2 NMOS  

Difference between CMOS and NMOS

The types of transistors employed and how they function are the primary distinctions between CMOS and NMOS.  

Feature	CMOS	NMOS
Transistor Types	Both NMOS and PMOS transistors	N-type (NMOS) transistors
Charge Carriers	Electrons (NMOS) and holes (PMOS)	Electrons (NMOS)
Power Consumption	Low static power consumption	Higher static power consumption
Noise Immunity	Better noise immunity and rejection	More susceptible to noise interference
Speed and Performance	Faster switching speeds and reduced delays	Slower switching speeds
Circuit Complexity	Supports complex circuit design and integration	Limited circuit complexity
Manufacturing Compatibility	Compatible with modern semiconductor processes	The similar manufacturing process to CMOS
Voltage Levels	Dual-voltage levels (high and low)	Single power supply voltage
Gate Voltage Requirement	Symmetrical (equal positive and negative)	Unipolar (positive voltage only)
Voltage Swing	Full rail-to-rail swing	Partial swing (limited by power supply)
Threshold Voltage	Symmetrical (positive and negative threshold)	Unipolar (positive threshold voltage)
Circuit Density	Lower circuit density due to dual transistors	Higher circuit density in comparison
Power Dissipation	Lower static and dynamic power dissipation	Higher static and dynamic power dissipation

 

How CMOS works?

Transistors are employed in CMOS technology to implement memory storage and logic functionality. Complementary pairs of p-type and n-type metal-oxide-semiconductor field-effect transistors (MOSFETs) make up these transistors. Since relatively little current passes through these transistors when they are not actively switching, their complementary nature enables low power consumption. Binary 1s and 0s are represented by the presence or absence of voltage in CMOS circuitry. It indicates a binary 1 in the presence of voltage and a binary 0 in the absence of voltage. This makes it possible for CMOS devices to store data and carry out logical processes. The CMOS battery, a tiny battery located on a computer's motherboard that powers the CMOS memory, which houses the BIOS settings, is likewise made using CMOS technology. As a result, even when the computer is turned off, its settings are preserved. In conclusion, low power consumption and strong noise immunity are two of the main benefits of CMOS technology, which implements logic functions and memory storage utilizing complementary pairs of transistors.  

How NMOS works?

The n-channel MOSFET is the fundamental building component of NMOS technology. The components of this transistor are a gate, a drain, and a source. An electric field is created by applying a voltage to the gate, allowing current to move between the source and the drain. The transistor is in the "off" state when the gate voltage is low and in the "on" state when the gate voltage is high. These transistors are used in NMOS logic circuits to implement digital logic functions. An NMOS transistor's gate receives a voltage that causes current to flow across it, indicating a logical "1." The transistor is off when there is no voltage applied to the gate, which corresponds to a logical "0." Due to the continuous current flow through the transistors when they are in the "on" state, one of the main features of NMOS technology is its higher power consumption compared to CMOS technology. Because of this, CMOS has essentially superseded NMOS in contemporary integrated circuits, as NMOS is less power-efficient than CMOS. In conclusion, NMOS technology implements digital logic operations utilizing n-channel MOSFETs, where binary 1s and 0s are represented by the presence or absence of voltage. However, CMOS has largely taken the place of NMOS in current integrated circuits because to its higher power consumption.  

Why is CMOS Better than NMOS?

Although both technologies are employed to build integrated circuits, CMOS has been the technology of choice due to NMOS's inferior performance and power efficiency. Metal-Oxide-Semiconductor Field-Effect Transistors, or N-type MOSFETs, are the transistors used in NMOS technology. The majority of the charge carriers in these transistors are electrons. An NMOS transistor's gate terminal receives a voltage that generates an electric field that regulates the movement of electrons between the source and drain terminals. A conductive channel forms when the gate voltage is high enough, allowing current to flow. A low gate voltage, on the other hand, disables the transistor by blocking the development of the conductive channel. Contrarily, NMOS, and PMOS (P-type MOSFET) transistors are combined in CMOS technology. In contrast to NMOS transistors, PMOS transistors behave differently since holes are used as the majority of the charge carriers. A pull-up network of PMOS transistors and a pull-down network of NMOS transistors make up the construction of a CMOS circuit. Together, these two networks carry out the logical operations needed in digital circuits. The low static power consumption of CMOS over NMOS is one of its main advantages. Even when the circuit is not in use, there is a steady current flowing from the power source to the ground in NMOS circuits. Higher power dissipation is the outcome. In contrast, because the PMOS network makes sure that the current only briefly flows during switching operations, CMOS circuits utilize very little static power. This feature makes CMOS technology extremely power-efficient, especially while it is idle or in standby mode. The noise immunity of CMOS technology is another important benefit. A single power supply voltage makes NMOS circuits more susceptible to noise interference. The noise can result in misleading signals and lower the circuit's overall reliability. The complementary behavior of the NMOS and PMOS transistors in CMOS technology aids in the reduction of noise-related problems. Stronger noise rejection is possible when both a high and low voltage supply is present. The performance of CMOS technology is also enhanced in terms of speed. Because electrons are used so frequently in NMOS circuits as charge carriers, these circuits have slower switching speeds. The complementary characteristic of CMOS enables quicker switching and shorter propagation delays. This benefit is especially significant for high-speed applications like microprocessors.  

CMOS Inverter VS NMOS Inverter

An inverter, often known as a NOT gate, is a logic gate used in digital logic to implement logical negation. When a bit is input, it outputs the exact opposite bit. Usually, the bits are implemented as two contrasting voltage levels.

CMOS Inverter

Construction: A CMOS inverter consists of both NMOS and PMOS transistors connected in series. The NMOS transistor is connected between the output and the ground, while the PMOS transistor is connected between the output and the power supply (VDD). Operation: When the input signal is low (logic 0), the NMOS transistor is turned off, and the PMOS transistor is turned on, connecting the output to VDD, resulting in a high output voltage (logic 1). Conversely, when the input signal is high (logic 1), the NMOS transistor is turned on, and the PMOS transistor is turned off, connecting the output to the ground, resulting in a low output voltage (logic 0). Advantages: CMOS inverters offer low power consumption, high noise immunity, and low static power dissipation. They also provide a full rail-to-rail voltage swing, enabling efficient signal transmission and compatibility with various logic families. Disadvantages: CMOS inverters tend to have larger circuit complexity due to the requirement of both NMOS and PMOS transistors. Additionally, they may have slightly slower switching speeds compared to NMOS inverters.

Figure 3 CMOS VS NMOS Inverter  

NMOS Inverter

Construction: An NMOS inverter consists of a single NMOS transistor connected between the output and the ground. Operation: When the input signal is low (logic 0), the NMOS transistor is turned off, and the output is pulled up to VDD through a pull-up resistor, resulting in a high output voltage (logic 1). When the input signal is high (logic 1), the NMOS transistor is turned on, providing a path to the ground, resulting in a low output voltage (logic 0). Advantages: NMOS inverters have simpler circuitry compared to CMOS inverters, which leads to lower circuit complexity. They can also achieve faster switching speeds due to the absence of PMOS transistors. Disadvantages: NMOS inverters suffer from higher power consumption due to the constant current flow from VDD to ground, even when the circuit is idle. They are also more susceptible to noise interference and have a limited voltage swing, which can affect signal integrity. Watch the below video to know more about their differences: Overall, CMOS inverters offer better power efficiency, noise immunity, and voltage swing, making them more commonly used in modern digital circuits. However, NMOS inverters can be simpler and faster, which can be advantageous in certain applications where power consumption and noise immunity are less critical.  

Conclusion

In conclusion, the particular needs of the application will determine whether CMOS and NMOS are superior. Due to its low power consumption, strong noise immunity, support for complicated circuit designs, and compatibility with cutting-edge fabrication techniques, CMOS is typically regarded as the better option. It has a well-balanced set of benefits that make it popular in contemporary digital circuits. Contrarily, NMOS has the potential to be faster and simpler, making it a good fit for specialized applications where speed is essential and power consumption is less important. The choice between CMOS and NMOS ultimately depends on the particular requirements and trade-offs of the application in question, with CMOS being the preferable choice in most cases due to its power efficiency and adaptability.