Managing Static (Leakage) Power - Computer Architecture Techniques for Power-Efficiency - page 136

V gs , voltage differential between the gate and source : Regarding subthreshold leakage
for devices in their normal “off” state, this factor can be set to zero, so it is not
a concern. Butts and Sohi use this assumption to arrive at their simplified leakage
model [ 41 ]. However, V gs plays a significant role in the gate-oxide leakage discussed in
Section 5.1.2.

V T , threshold voltage : The threshold voltage—the voltage level that switches on the
transistor—significantly affects the magnitude of the leakage current in the off state.
The exponential dependence of subthreshold leakage on ( V T ) − 1 is evident in the last
factor of the BSIM3 formula: the smaller the V T , the higher is the leakage. Raising the
threshold voltage reduces the subthreshold leakage but compromises switching speed.
Many circuit-level techniques, e.g., MTCMOS, reverse body bias (RBB) and larger-
than- V dd forward body bias [ 13 , 174 , 14 , 222 ], have been developed to provide a choice
of threshold voltages. These techniques provide multiple threshold voltages at the
process level (for example, MTCMOS offers high- V T and low- V T devices) or vary the
threshold voltage dynamically by applying bias voltages on the semiconductor body
(e.g., RBB and larger-than- V dd FBB). Architectural techniques based on manipulating
the threshold voltage are presented in Section 5.4.

T , temperature : Last but not the least, subthreshold leakage exponentially depends on
temperature, T , via the thermal voltage term

t . This is actually a dangerous dependence
since it can set off a phenomenon called thermal runaway . If leakage power—or for that
matter any other source of power consumption—causes an increase in temperature,
the thermal voltage

v

t also increases linearly to temperature. This leads, in turn, to
an exponential increase in leakage, which further increases temperature. This vicious
circle of temperature and leakage increase can be so severe as to seriously damage the
semiconductor. The solution is to keep the temperature below some critical threshold
so that thermal runaway cannot happen. Cooling techniques, combined with accurate
thermal monitoring, are used for this purpose. 3
Architecturally, the dependence of leakage to temperature is quite interesting. This
is because at low temperatures it might not be so important to engage architectural
techniques that could hurt performance with little payoff. As temperature rises and
leakage power becomes the dominant component of power consumption (and hence
heat generation ) architectural techniques that can curb leakage become much more
appealing. One such example is presented in Section 5.3.4.

v

3 Unfortunately, the subject of thermal management, despite its importance, is too extensive to receive other
than superficial coverage in the space of this topic. Here, it is only mentioned briefly with respect to leakage
(Section 5.3.4).