Subthreshold leakage

Definition

Subthreshold leakage is the current that flows through a transistor when it is in the off state, which occurs below the threshold voltage. This form of leakage is a significant contributor to power consumption in modern processors, particularly as transistor sizes shrink and more transistors are packed into integrated circuits. As the operating voltages decrease to improve energy efficiency, the impact of subthreshold leakage becomes more pronounced, making it an important consideration in processor design.

5 Must Know Facts For Your Next Test

1. Subthreshold leakage increases with temperature, leading to higher power consumption in hot operating conditions.
2. As transistors shrink in size (following Moore's Law), the ratio of subthreshold leakage to total power consumption tends to rise significantly.
3. Techniques like voltage scaling and transistor sizing are used to manage and reduce subthreshold leakage.
4. Subthreshold leakage is particularly critical in low-power applications like mobile devices, where battery life is a major concern.
5. In advanced manufacturing processes, specialized materials and device architectures, such as FinFETs, are used to mitigate subthreshold leakage.

Review Questions

• How does subthreshold leakage impact power consumption in modern processors?
  • Subthreshold leakage significantly impacts power consumption in modern processors as it represents the current that flows even when transistors are off. As transistors become smaller and more densely packed, the percentage of total power attributed to subthreshold leakage increases. This creates challenges for designers who must balance performance with energy efficiency, especially in applications where low power consumption is critical.
• Discuss strategies that can be employed to mitigate subthreshold leakage in processor design.
  • To mitigate subthreshold leakage, designers can implement several strategies, such as using higher threshold voltage transistors or utilizing multi-threshold voltage (MTV) designs where different transistors within a circuit have varying threshold voltages. Additionally, techniques like dynamic voltage scaling allow systems to reduce supply voltage during idle states, further minimizing leakage currents. Specialized transistor architectures like FinFETs also help reduce leakage by improving control over the channel.
• Evaluate the trade-offs associated with reducing subthreshold leakage while maintaining performance in high-speed processors.
  • Reducing subthreshold leakage often involves increasing the threshold voltage or implementing complex circuit designs that may lead to slower switching speeds. While this can enhance energy efficiency and prolong battery life, it may compromise performance due to longer signal propagation times and reduced transistor drive current. Designers must carefully balance these trade-offs, as excessive focus on minimizing leakage could result in underperforming systems in applications demanding high speed and responsiveness.

Related terms

Threshold Voltage: The minimum gate voltage required to create a conductive path between the source and drain terminals of a transistor.

Dynamic Power Consumption: The power consumed by a circuit when it is switching states, primarily due to charging and discharging capacitances.

Static Power Consumption: The power consumed by a circuit when it is not switching, mainly arising from leakage currents, including subthreshold leakage.