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64 l New-Tech Magazine Europe
Swiss watchmakers noticed the
potential of operating select transistors
in the sub-threshold region. The
idea was picked up for pacemakers
and RFID tags, but never saw much
acceptance beyond that.
What’s changed since the 70s, when
the first commercial sub-threshold
devices were created, is the scale of
implementation. Designs of the past
used a few critical sub-threshold
transistors - on the order of 10. At that
level, each transistor can be optimized
by hand.
Ambiq creates entire chips that
primarily use sub-threshold transistors.
This equates to millions of transistors
and cannot be done by hand but relies
on the standard design tools and flows
used to create super-threshold chips.
It is these that Ambiq has achieved in
order to commercialize sub-threshold
circuits.
Ambiq Micro’s approach moves
beyond the incremental improvements
that other semiconductor companies
have taken and makes revolutionary
advances through a unique approach
to the problem: sub-threshold circuit
design. These considerations drove the
development and commercialization
of Ambiq’s patented Sub-threshold
Power Optimized Technology (SPOT™)
the transistors “off.” This means that
completely new design approaches
are required.
The challenges of modern
sub-threshold
Adapting the standard super-
threshold flows and infrastructure
for sub-threshold design presents
numerous challenges, starting with
the transistors themselves:
1. Poor transistor models
The transistor model forms the basis
of everything in an integrated circuit
design. All of the simulations, all of the
abstractions and automation, the very
process of design closure: they all rely
on an accurate transistor model. Most
transistor modeling has focused on
the “on” characteristics of the device,
with little attention given to “off.” The
entire region between 0 V and Vth
typically does not get modeled as
accurately, and so existing models are
inadequate for sub-threshold design,
as shown in Figure 2.
2. Logic swings and noise
The output response of a transistor
in the sub-threshold regime is subtle;
detecting it requires great sensitivity.
Currents change exponentially in
Figure 1 - Dynamic current dominates with
higher operating voltage
Figure 2 - Transistors haven't been well
modeled below threshold
platform, which Ambiq uses to build
reliable, robust circuits that consume
dramatically less energy on a cost-
effective, mainstream manufacturing
process.
Electronics energy
consumption
Because dynamic energy varies as the
square of the operating voltage, that
voltage becomes the biggest lever for
reducing dynamic energy consumption
(while also having a tangible, but less
dramatic, impact on leakage). For
example, when compared to a typical
circuit operating at 1.8V, a “near-
threshold” circuit operating at 0.5V
can achieve up to a 13X improvement
in dynamic energy. An even more
aggressive “sub-threshold” circuit
operating at 0.3V can achieve up to a
36X improvement!
Traditional digital designs use the
transistor state - “on” or “off” - as a
critical concept for implementing logic.
Analog designs likewise assume that a
transistor is “on” to some controlled
degree, using it for amplification. But
sub-threshold operation means that
none of the voltages in the chip rise
above the threshold voltage (Vth),
so the transistors never turn on.
Even a logic “high” voltage keeps