Low Power Design | Physical Design | Static & Dynamic Power | Leakage Current | Part 2

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Low Power Design | Physical Design | Static & Dynamic Power | Leakage Current | Part 2


The goal of low power design is to reduce the individual components of power as much as possible, thereby reducing the overall power consumption. The power equation contains components for dynamic and static power. Dynamic power is comprised of switching and short-circuit power; whereas static power is comprised of leakage, or current that flows through the transistor when there is no activity. 




The higher the voltage, the higher the power consumed by each component, resulting in higher overall power. Conversely, the lower the voltage, the lower the overall power. To achieve the best performance with the lowest power consumption, tradeoffs for each of these different factors are tried and tested via various low power techniques and methodologies.


Static Power Consumption


When a circuit is in a quiescent state, meaning there is no activity or clock signal, it still consumes power, known as static power. This occurs even when the inputs to the circuit remain unchanged, and the only presence is the supply voltage. Static power consumption is a critical aspect of modern integrated circuit design.


Leakage Currents:


The primary contributor to static power consumption is leakage current, which flows when transistors are in an off-state.


1. Reverse Bias Leakage Current

Reverse bias leakage current occurs when the junction diodes within transistors are reverse biased, meaning the voltage across the diode is negative. This causes a small amount of current to flow through the diode, even when it's supposed to be turned off. Reverse bias leakage current is a significant contributor to static power consumption in modern ICs.

2. Sub-Threshold Leakage Current

Sub-threshold leakage current flows from drain to source through the channel when the gate-source voltage (VGS) is less than the threshold voltage (Vth). This means that even when the transistor is supposed to be turned off, a small amount of current can still flow through the channel. Sub-threshold leakage current is a major concern in low-power design, as it can significantly impact battery life.

3. Gate Leakage Current

Gate leakage current occurs due to the flow of current through the gate oxide of a transistor. This can happen when the gate voltage is high, causing electrons to tunnel through the oxide layer and create a leakage path. Gate leakage current is a significant concern in modern ICs, as it can lead to increased power consumption and reduced device reliability.

4. Junction Leakage Current

Junction leakage current flows through the p-n junctions of a transistor, even when the transistor is supposed to be turned off. This can happen due to the presence of minority carriers in the junction, which can flow across the junction and create a leakage path. Junction leakage current is a significant contributor to static power consumption in modern ICs.

5. Gate-Induced Drain Leakage (GIDL) Current

Gate-induced drain leakage (GIDL) current occurs when the gate voltage is negative with respect to the source voltage. This can cause the drain voltage to become positive, leading to a leakage path between the drain and source. GIDL current is a significant concern in low-power design, as it can significantly impact battery life.

6. Drain-Induced Barrier Lowering (DIBL) Current

Drain-induced barrier lowering (DIBL) current occurs when the drain voltage is high, causing the threshold voltage to decrease. This can lead to a leakage path between the drain and source, even when the transistor is supposed to be turned off. DIBL current is a significant contributor to static power consumption in modern ICs.

7. Punch-through Current

Punch-through current occurs when the depletion regions of the source and drain merge, creating a leakage path. This can happen when the transistor is operating at high voltages or high temperatures. Punch-through current is a significant concern in low-power design, as it can significantly impact battery life.

8. Hot Carrier Injection (HCI) Current

Hot carrier injection (HCI) current occurs when high-energy electrons or holes inject into the gate oxide, causing leakage current. This can happen when the transistor is operating at high voltages or high frequencies. HCI current is a significant concern in modern ICs, as it can lead to increased power consumption and reduced device reliability.

9. Tunneling Current

Tunneling current occurs due to the flow of current through the thin gate oxide of a transistor. This can happen when the gate voltage is high, causing electrons to tunnel through the oxide layer and create a leakage path. Tunneling current is a significant contributor to static power consumption in modern ICs.



Relationship Between Leakage Current & Threshold Voltage:

The leakage power dissipation in a transistor is inversely proportional to its threshold voltage (Vth). This means that as the threshold voltage increases, the leakage power dissipation decreases, and vice versa. In other words, transistors with higher threshold voltages tend to have lower leakage power dissipation, while those with lower threshold voltages tend to have higher leakage power dissipation.

Leakage current is a parasitic current that flows when a transistor is in the off state, and it can have a significant impact on circuit performance and reliability. A lower threshold voltage (Vth) increases leakage current, which can hinder circuit performance and reliability. On the other hand, as Vth increases, the delay of the transistor increases, but the leakage current decreases. This tradeoff between delay and leakage current is a critical consideration in transistor design, as it affects the overall performance, power consumption, and reliability of the circuit. By increasing Vth, designers can reduce leakage current and improve circuit reliability, but this may come at the cost of increased delay and reduced performance. Conversely, decreasing Vth can improve performance and reduce delay, but this may come at the cost of increased leakage current and reduced reliability.

#scripting #coding #Tcl #pd #physicaldesign #physicalverification #pv #vlsi #design #india #semiconductor #freshers #professional #hardware #floorplanning #motivation #leadership #magnet #bound #placement #synopsys #cadence #interview #vlsi #question #power #irdrop #powerswitches #rushcurrent #lowpower #staticpower #dynamicpower #wifi #bluetooth #leakage #current

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