PMOS

Definition

PMOS (P-channel Metal-Oxide-Semiconductor) is a type of Field-Effect Transistor (FET) that uses P-type source and drain regions. In a PMOS transistor, the majority charge carriers are "holes," and the device is turned on when a negative voltage is applied to the gate terminal relative to the source.

Main Content

1. Structure of PMOS

• A PMOS transistor is built on an N-type silicon substrate.
• Two P-type regions (Source and Drain) are diffused into the N-type substrate, separated by a channel region.

2. The Gate Control

• A thin insulating layer (usually Silicon Dioxide, $SiO_2$) separates the metallic gate from the semiconductor body.
• Because of this insulation, PMOS is a voltage-controlled device, meaning almost no current flows into the gate terminal.

3. Voltage Logic

• PMOS is "Active Low," meaning it conducts current when the gate voltage is Low (Logic 0).
• When the gate voltage is High (Logic 1), the transistor acts as an open circuit (OFF).

      Gate (G)
         |
    _____|_____
   |   oxide   |
___|___________|___
| P+ |       | P+ |  <-- Source & Drain
|____|_______|____|
|   N-Substrate   |
|_________________|

Working / Process

1. Cut-off Region (OFF State)

• When a positive voltage (Logic High) is applied to the Gate, the holes in the channel region are repelled away from the gate.
• No conductive path exists between the Source and Drain, preventing current flow.

2. Linear Region (ON State)

• When a negative voltage (Logic Low) is applied to the Gate, it attracts holes from the substrate toward the gate oxide.
• These accumulated holes form a conductive "P-channel" that bridges the Source and Drain, allowing current to flow.

3. Saturation Region

• As the voltage difference between the Drain and Source increases beyond a certain threshold, the channel pinches off near the drain.
• The current becomes constant and independent of further increases in Drain-Source voltage.

Advantages / Applications

• PMOS transistors are highly resistant to noise when used in specific logic configurations.
• They are fundamental components in CMOS (Complementary MOS) technology, which combines both PMOS and NMOS to create efficient digital logic.
• PMOS is frequently used in A/D and D/A converters to act as high-side switches because they are easy to turn on relative to a positive supply rail.

Summary

PMOS is a transistor type that utilizes holes as charge carriers and requires a negative gate-to-source voltage to conduct current. It serves as an essential building block in digital circuits, particularly within A/D and D/A converter architectures where it functions as a reliable electronic switch.

Important terms to remember: P-channel, Holes, Gate Oxide, Substrate, and Logic High/Low.