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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