PMOS vs NMOS Transistors
PMOS and NMOS stands for P-channel MOSFET transistor and N-channel MOSFET transistor respectively. These complementary transistors are used in electronic circuits to control the flow of electrical current. They are fundamental building blocks in digital logic circuits and analog signal processing circuits. In CMOS circuits, they work together to create efficient digital logic gates.
Structure and Symbol diagrams of PMOS and NMOS
Figure-1 depicts symbols of N-channel MOSFET (NMOS) and P-channel MOSFET (PMOS). The symbol for an NOS transistor consists of an arrow pointing towards the gate, indicating the direction of electron flow when transistor is ON. The symbol for a PMOS transistor is a circle with an arrow pointing away from the gate, indicating the direction of hole flow when the transistor is ON.
Figure-2 depicts NMOS structure diagram. As shown it consists of three layers of semiconductor materials viz. source, drain and gate. A thin insulating layer of oxide separates the gate from semiconductor material. The gate terminal is typically made of conductive material like metal or polysilicon.
Figure-3 depicts PMOS structure diagram. PMOS transistor has similar structure to NMOS but with opposite polarity. As shown it consists of source, drain and gate terminals, but in PMOS, holes are majority carriers. The gate-source voltage controls flow of holes between source and drain.
Characteristics
Figure-4 depicts NMOS and PMOS enhancement MOSFET characteristics.
Figure-5 depicts NMOS and PMOS depletion MOSFET characteristics.
Difference between PMOS and NMOS
Following table compares PMOS vs NMOS MOSFET types with respect to various parameters.
| Parameters | PMOS (P-Channel MOSFET) | NMOS (N-Channel MOSFET) |
|---|---|---|
| Carrier type | Majority carriers are holes (Positive) | Majority carriers are electrons (negative) |
| Operation | Conducts when Vgs < Vth (Gate < Source) |
Conducts when Vgs > Vth (Gate > Source) |
| Symbol | Circular symbol with arrow pointing away from gate | Arrow symbol pointing towards the gate |
| Voltage levels | Positive gate source voltage (Vgs) | Negative gate source voltage (Vgs) |
| Logic level | Positive voltage represents logical '0' | Positive voltage represents logical '1' |
| Switching speed | Generally slower than NMOS | Generally faster than PMOS |
| Noise immunity | More noise immunity due to slower switching | Less noie immunity due to faster switching |
| Power consumption | Consumes more power when ON | Consumes less power when ON |
| Power supply polarity | Often used in circuits with negative power supplies | Often used in circuits with positive power supplies |
| Common use cases | Voltage level translators, analog circuitry | Digital logic gates, amplifiers in audio applications |
Conclusion :
We can infer following differences between PMOS and NMOS MOSFET transistor types.
• P-channel is much easier and cheaper to produce compare to N-channel MOSFET device.
• The N-channel MOSFET has high packing density. This makes it faster for switching applications due to smaller junction areas and low inherent capacitances.
• N-channel MOSFET is smaller for same complexity compare to P-channel MOSFET.
• Drain resistance of P-channel MOSFET is 3 times higher than identical N-channel MOSFET device.
• N-channel MOSFET has high false turn-on possibility compare to P-channel device. This is due to positively charged contaminants.
• For given drain current rating, P-channel MOSFET occupies larger area compare to N-channel MOSFET. This is due to the fact that electron mobility is 2.5 times than mobility of hole.
In summary, NMOS and PMOS transistors have distinct carrier types and operate with opposite polarities. They are often used together to create complementary logic gates in digital circuits, taking advantage of their differences to minimize power consumption and improve performance.