PMOS Transistors | Structure, Advantages, Applications
2025-05-16 33163

Catalog

Figure 1. PMOS Transistor

What is PMOS Transistor?

PMOS transistor or P-channel Metal-Oxide Semiconductor, is a type of electronic switch used to control the flow of electricity in a circuit. It works by using holes (which are like missing electrons) to carry current. It has three main parts: the source, the drain, and the gate. The gate decides if electricity can flow from the source to the drain.

Figure 2. PMOS transistor Symbol

The PMOS turns on (lets current flow) when the gate voltage is lower than the source voltage. It turns off (blocks current) when the gate voltage is the same or higher than the source voltage. PMOS transistors are made using n-type material for the base, and p-type material for the source and drain.

PMOS work the opposite way of NMOS transistors. Both are used in CMOS circuits, which are found in devices like phones and computers. PMOS is a bit slower, but it uses less power when it's off, which helps save energy.

PMOS Transistor Structure

Figure 3. Cross-Sectional Structure of a PMOS Transistor

A PMOS transistor is built on n-type substrate, also called the bulk. Into this base, two p-type regions are placed—one acts as the source, and the other as the drain. On top of the area between the source and drain is a thin insulating layer called the gate oxide, and above that is the metal gate.

The gate controls whether current can flow between the source and drain. The metal contacts connect the source and drain to the rest of the circuit. When the gate voltage is low, holes (positive charges) form a path between the source and drain, allowing current to flow. When the gate voltage is high, this path disappears, and the transistor turns off.

Main parts:

• n-type substrate (bulk) – base layer

• p-type source and drain – allow current flow

• Gate oxide – insulation between gate and channel

• Metal gate – controls the switch

• Metal contacts – connect to the outside circuit

This structure allows the PMOS transistor to act like a controlled switch in digital and analog electronics.

Applications of PMOS Transistors

CMOS Circuits - PMOS is used together with NMOS in CMOS chips, which are found in phones, computers, and digital devices.

Works as a Switch - PMOS can turn parts of a circuit on or off, like an electronic switch.

Pull-Up in Digital Circuits - PMOS is used to keep a signal high (logic 1) when required.

Analog Circuits - It helps control current in things like amplifiers and mirrors.

PMOS Transistor Circuit Diagram

The diagram below shows how a NAND gate is built using PMOS and NMOS transistors. A NAND gate is a basic digital logic gate that outputs low (0) only when both inputs are high (1). If at least one input is low (0), the output stays high (1). This is the opposite of how an AND gate works.

Figure 4. CMOS NAND Gate Circuit

In the circuit:

• When A = 0 and B = 0, both PMOS transistors are on, and both NMOS transistors are off. The output is connected to the power supply (VDD) and not to ground, so the output is 1.

• When A = 0 and B = 1, one PMOS is on, and one NMOS is off. The output remains connected to VDD, so it stays 1.

• When A = 1 and B = 0, the other PMOS is on, and the other NMOS is off. The output is still connected to VDD, so it remains 1.

• When A = 1 and B = 1, both PMOS are off, and both NMOS are on. Now, the output is disconnected from VDD and connected to ground, so it becomes 0.

Truth Table

The threshold voltage (Vth) of a PMOS transistor is the gate-to-source voltage (Vgs) at which the transistor starts to turn ON by forming a channel.

In PMOS:

• The source is usually connected to the positive supply voltage (Vdd).

• To turn it ON, the gate voltage must be lower than the source voltage.

• Since the source is at a higher voltage (Vdd), and the gate is lowered, Vgs = Vg - Vs becomes negative, allowing the PMOS to turn ON.

When Vgs becomes more negative than the threshold voltage (which is itself a negative value), the PMOS channel inverts, and current flows from source to drain.

PMOS vs NMOS: What's the Difference?

PMOS and NMOS are two types of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) technologies used in digital and analog circuits. While both control the flow of current using an electric field, they behave differently based on voltage and internal structure. Here's a side-by-side comparison:

Conclusion

PMOS transistors are important electronic parts because they save power. They are perfect for devices like phones and tablets. Understanding PMOS helps you build better electronic circuits.