Overview of PMOS
When p-type dopants are utilized in the gate area (the channel), the transistor is known as a p-channel metal-oxide semiconductor (pMOS) or PMOSFET. The device activates when the gate receives a negative voltage. PMOS or pMOS also referred to as P-type metal-oxide-semiconductor logic, is a category of the digital circuit built employing metal-oxide-semiconductor field effect transistors (MOSFET) with a p-type semiconductor source and drain printed on a bulk n-type “well.”
By reducing the high voltage on the logic gates, the resulting circuit can be “turned on” by allowing electron holes to flow between the source and drain. PMOS circuits are simpler to manufacture than NMOS circuits because they are less vulnerable to electronic noise.
In the 1970s, when microprocessor technology was still in its infancy, they were frequently used. In comparison to the NMOS and CMOS alternatives, PMOS has many drawbacks, such as the requirement for a variety of supply voltages (both positive and negative), high power dissipation in the conducting state, and very big features. Additionally, the changeover pace is slower overall.
Overview of NMOS
NMOS devices are N-channel metal-oxide semiconductors (NMOS) often called the NMOS logic family. They are a type of microelectronic circuit used in the design of complementary metal-oxide semiconductors (CMOS) and logic and memory circuits. A greater number of NMOS transistors can be accommodated on a single chip than their P-channel metal-oxide semiconductor (PMOS) cousin.
The p-type transistor body in these nMOS transistors is converted into an inversion layer to carry out their function. Between the n-type “source” and “drain” terminals, this inversion layer, known as the n-channel, can conduct electrons. The third terminal, known as the gate, receives voltage to produce the n-channel MOSFET. With the development of superior fabrication methods, including the continued removal of impurities from the silicon stock that reduced noise, PMOS was superseded by NMOS.
NMOS Transistor Working
The working of the NMOS transistor is; when the NMOS transistor receives a non-negligible voltage then it forms a closed circuit which means the connection from the source terminal to the drain works as a wire. So the current flows from the gate terminal to the source. Similarly, when this transistor receives a voltage at approximately 0V then it forms an open circuit which means the connection from the source terminal to the drain will be broken, so current flows from the gate terminal to the drain.
PMOS Transistor Working
The p-type transistor working is quite opposite to the n-type transistor. This transistor will form an open circuit whenever it gets non-negligible voltage which means, there is no flow of electricity from the gate (G) terminal to the source (S). Similarly, this transistor forms a closed circuit when it gets a voltage at around 0 volts which means the current flows from the gate (G) terminal to the drain (D).
This bubble is also known as an inversion bubble. So the main function of this circle is to invert the input voltage value. If the gate terminal provides a 1 voltage, then this inverter will change it into zero and functions the circuit accordingly. So the function of the PMOS transistor and NMOS transistor is quite opposite. Once we merge them into a single MOS circuit, then it will become a CMOS (complementary metal-oxide semiconductor) circuit.
Cross Section of NMOS Transistor
Generally, an NMOS transistor is simply built with a p-type body by two n-type semiconductor regions which are adjacent to the gate known as the source & the drain. This transistor has a controlling gate that controls the electron flow between the source & drain terminals.
In this transistor, since the body of the transistor is grounded, the PN junctions of the source & drain toward the body are reverse-biased. If the voltage at the gate terminal is increased, an electric field will start to increase and attracts free electrons to the base of the Si-SiO2 interface.
Once the voltage is high enough, then electrons wind up filling all the holes & a thin region below the gate known as the channel will get inverted to perform as an n-type semiconductor. This will create a conducting lane from the source terminal to the drain by allowing the flow of current, so the transistor will be turned ON. If the gate terminal is grounded then no current flows in the reverse-biased junction so the transistor will be turned OFF.
Cross Section of PMOS Transistor
The cross-section of the PMOS transistor is shown below. A pMOS transistor is built with an n-type body including two p-type semiconductor regions which are adjacent to the gate. This transistor has a controlling gate as shown in the diagram which controls the electrons flow between the two terminals like source & drain. In the pMOS transistor, the body is held at +ve voltage. Once the gate terminal is positive, then the source & drain terminals are reverse-biased. Once this happens, there is no flow of current, so the transistor will be turned OFF.
Whenever these transistors deal with digital logic there are usually have two different values only like 1 & 0 (ON and OFF). The transistor’s positive voltage is known as VDD which represents the logic high (1) value within digital circuits. The VDD voltage levels in TTL logic were generally around 5V. At present transistors cannot actually withstand such high voltages because they typically range from 1.5V – 3.3V. The low voltage is frequently known as GND or VSS. So, VSS signifies the logic ‘0’ and it is also set normally to 0V.
NMOS vs PMOS Transistor
|PMOS Transistor||NMOS Transistor|
|This transistor’s threshold voltage (Vth) is a negative value.||This transistor’s threshold voltage (Vth) is a positive value.|
|These are more noise-resistant.||These are not noise-proof in comparison to PMOS.|
|Once a low voltage is applied to the gate of a PMOS transistor, it will begin to conduct.||Once a high voltage is applied to the gate, an NMOS transistor will begin to conduct.|
|Compared to NMOS devices, PMOS devices cannot be switched more quickly.||NMOS devices can be switched faster than PMOS devices.|
|PMOS devices are not smaller than NMOS ones.||NMOS devices are considerably more compact than PMOS devices.|
|In PMOS, holes make up the majority of the charge carriers.||Electrons make up the majority of charge carriers in NMOS.|
|This transistor’s substrate is comprised of an n-type semiconductor.||This transistor’s substrate is comprised of p-type semiconductor material.|
|Simple n-type semiconductors are used to provide the source and drain in PMOS transistors.||P-type semiconductors are used to make the source and drain in NMOS transistors.|
|P-channel metal-oxide-semiconductor transistor is referred to as a PMOS transistor.||N-channel metal-oxide-semiconductor transistor is referred to as an NMOS transistor.|