Unlock Layouts: Fingers and Multipliers in Transistors

Introduction

• Discussion on the concepts of fingers and multipliers in MOSFETs and CMOS.

Fingering Concept

Why Use Multiple Fingers in One MOSFET?

• Reduces the impact of gate resistance (sheet resistance).
• Figure 1: Demonstrates a large transistor with width (") and length (L).
• Sheet resistance (
• Metals, including gate metals, inherently have sheet resistance (
• In large transistors, voltage drops across the gate due to this sheet resistance.

Voltage Drop Across Nodes

• Input voltage (e.g., 5V) experiences a drop across successive resistances.
  • Node 1: Slight drop (e.g., 4.9V).
  • Node 2: Further drop (e.g., 4.8V).
  • Last node: Significant drop (e.g., 3V).
• Larger MOSFETs exhibit more voltage drop due to higher series resistance in the gate.
• Optimal source-to-drain conduction occurs only where the gate voltage is sufficient.

Solutions to Voltage Drop

1. Increase Input Voltage: Ineffective as it can permanently damage transistors due to high voltage at the first node.
2. Divide the Gate Resistance: Achieved by the fingering concept.

Implementing Fingering

• Split the Gate Resistance: Use parallel transistors to divide resistance.
• Equal voltage is maintained across divided paths, reducing individual node resistances.
• Ensures sufficient voltage (

Layout Structure

• Multiple gates are connected, sharing source and drain connections.
• Schematic Representation: Shows two-fingered transistor.
• Combats voltage drop due to gate resistance.

Multiplier Concept

Handling High Current

• Fingering addresses voltage issues, while multipliers address current issues.
• Increases overall current capacity without altering individual transistor dimensions.

How Multiplying Works

• Example: If each transistor handles 1mA, five transistors (M=5) can collectively handle 5mA.
• Each node maintains same voltage (
• Ensures high current capacity without damaging individual transistors.

Combining Fingers and Multipliers

• Used when both high voltage and high-current scenarios are present.
• Example: Finger = 2 and Multiplier = 2 results in effectively four transistors.
• Calculation: Ensures adequate voltage and current handling by combining both techniques.

Technical Notes

Transistor Dimensions

• Length: Fixed by technology node (e.g., 90nm or 45nm).
• Width: Adjustable based on design needs (to manage resistance and current capacity).

Cross-Sectional View

• Length: Source to drain distance or gate size.
• Width: Gate metal’s resistance impact area.

Conclusion

• Fingering: Important for managing voltage drops in large transistors.
• Multipliers: Essential for handling high-current scenarios.
• Combining both concepts: Effective for optimizing both voltage and current performance.

End Notes

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