Leakage flows even when nothing switches. Subthreshold conduction dominates in modern processes, with gate and junction leakage adding smaller shares.
Subthreshold current grows exponentially as threshold voltage falls. It also rises steeply with temperature, which couples power to thermals.
Static power is voltage times leak current. Assume V = 0.8 V and I_leak = 0.5 A for one block.
P_static = V x I_leak
= 0.8 V x 0.5 A
= 0.4 W = 400 mWAssume a sleep transistor cuts that block to 8 mW in sleep. The saving is 392 mW, about 98 percent of the leakage.
P_saved = P_static - P_sleep
= 400 mW - 8 mW
= 392 mW (392 / 400 = 98 percent)Technique | What it does | Cost |
|---|---|---|
Multi-Vt cells | Slow low-leakage cells off critical paths | Less speedup where used |
Power gating | Sleep transistor cuts supply to idle blocks | Wakeup latency and rush current |
Retention registers | Keep state at low voltage during sleep | Extra area and control |
Body biasing | Shift threshold to trade speed for leakage | Process and design complexity |
Full gating loses state, so blocks needing fast resume use retention. The choice is wakeup latency against sleep leakage.
Static leakage remains at full value. Only the switching term fell.
Leakage rises with temperature, and the extra power raises temperature further. The loop stops only when cooling balances it.