Managing Static (Leakage) Power - Computer Architecture Techniques for Power-Efficiency - page 134

FIGURE 5.1: Example of an “ I - V ” curve for a semiconductor diode (introduced in Chapter 1).
Although we informally treat semiconductors as switches, their non-ideal analog behavior leads to
leakage currents and other effects.

leakage, gate-induced-drain leakage, gate-oxide leakage, gate-current leakage, and punch-
through leakage. The sub-threshold leakage and gate-oxide leakage dominate the total leakage
current in devices. Both increase exponentially with each new technology generation with the
gate-oxide leakage significantly outpacing the sub-threshold leakage.
In sub-micron technologies, subthreshold and gate leakage is the cost we have to pay for
the increased speed afforded by scaling. Supply voltage scaling attempts to curb an increase
in dynamic power. Unfortunately, this strategy also leads to an enormous increase in the
subthreshold and gate leakage problem. This explains why static power has been gaining on
dynamic power as a percentage of the total power consumption with every process generation.

5.1.1 Subthreshold Leakage
Subthreshold leakage increases with technology scaling due to V dd scaling. The supply voltage
( V dd ) is scaled along with other physical quantities to reduce dynamic power consumption.
Scaling solely the supply voltage, however, increases the delay (switching speed) of the transistor.
This is because the delay is proportional to the inverse of the current that flows in the on state—
the I on current (as in the I - V curve of Figure 5.1):

1
I on ∝

V dd

V T ) a .
This current, I on , is a function of the supply voltage and the difference between the supply
voltage and the threshold voltage ( V T ). The factor

Delay ∝

( V dd −

is a technology-dependent factor taking
values greater than 1 (between 1.2 and 1.6 for recent technologies) [ 195 ]. Since V dd is lowered
in order to maintain the speed increase from scaling, the only course of action is to also lower
the threshold voltage. Herein lies the problem: subthreshold leakage increases exponentially with
lower threshold voltage .

α