If the immediate future truly sees the networking of all previously unconnected electronic devices, then the real prize is not maximizing the technology's performance (performance is already fast enough to sustain IoT), but reducing the energy consumption of electronic devices sufficiently so they can function meaningfully within this new Internet.

AMB002 Fig 1Energy is consumed in two fundamental ways: as loss, when the state of a circuit is not changing, and dynamically as internal nodes charge and discharge. For real-world operating circuits, dynamic energy dominates—especially for the higher supply voltages used in most current designs (see Figure 1).

 

Dynamic energy is determined by the operating voltage

Subthreshold techniques are a way to create circuits that consume drastically less power than those built using standard design practices. Subthreshold design is challenging, but with the right expertise and diligence, it can be done with significant benefits.
The results are circuits that provide the same functions as more traditional ones but use a fraction of the power. There is no compromise on performance, robustness, or reliability. These chips can operate alongside their traditional counterparts with no visible difference from the outside—except for the amount of power required to run them. They can provide significant energy savings for designers building energy-efficient systems.
Due to the fundamental nature of these innovations, subthreshold design techniques can be applied to virtually any type of integrated circuit device.

Subthreshold design was tested decades ago.
Subthreshold design is not a new concept. As early as the 1970s, Swiss watchmakers realized the potential of operating transistors in the subthreshold region. The idea was adopted in pacemakers and RFID tags, but never gained widespread acceptance beyond that.

What has changed since the 1970s, when the first commercial subthreshold devices were created, is the scale of application. Past designs used a few critical subthreshold transistors—on the order of 10. At that level, each transistor can be optimized by hand.
Ambiq creates entire chips that primarily use subthreshold transistors. This amounts to millions of transistors and cannot be done by hand, but it relies on standard design tools and is used to create superthreshold chips. These are the chips Ambiq has achieved in order to commercialize subthreshold circuits.
Ambiq Micro's approach goes beyond the incremental improvements other semiconductor companies have taken and produces revolutionary advances through a unique approach to the problem: subthreshold circuit design. These considerations led to the development and commercialization of Ambiq's patented subthreshold Power Optimized Technology (SPOT™), which Ambiq uses to build reliable, robust circuits that consume drastically less power in a cost-effective manufacturing process.

Power Consumption in Electronics:
Because dynamic power varies exponentially with operating voltage, voltage becomes the biggest lever for reducing dynamic power consumption (while also having a tangible, but less dramatic, impact on losses). 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 power. An even more aggressive "subthreshold" circuit operating at 0.3V can achieve up to a 36X improvement!

Traditional digital designs use the transistor's "on" or "off" state as a fundamental concept for implementing logic. Analog designs, in parallel, assume that a transistor is "on" to some controlled degree, used for amplification. But subthreshold operation means that none of the voltages on the chips are above the threshold voltage (Vth), so the transistors never actually turn on. Even a "high" logic voltage keeps the transistors "off." This means that entirely new design approaches are required.

AMB002 Fig 2The challenges of modern subthreshold design:
Adapting standard superthreshold fluxes and infrastructure for subthreshold design presents numerous challenges, starting with the transistors themselves:

1. Poor Transistor Models
The transistor model forms the basis of everything in integrated circuit design. All simulations, all abstractions and automation, the design closure process itself: all rely on an accurate transistor model. Most transistor models have focused on the "on" characteristics of the device, paying little attention to the "off" characteristics. The entire region between 0 V and Vth is typically not modeled with the same accuracy, and existing models are unsuitable for subthreshold design, as shown in Figure 2.


AMB002 Fig 3 w2. Logic Changes and Noise
The output response of a transistor in the subthreshold regime is subtle; detecting it requires high sensitivity. Currents change exponentially in response to voltage changes, but they are extremely small currents. Furthermore, the ratio of "on" to "off" current is on the order of 1000, orders of magnitude less than in superthreshold design experience (see Figure 3). As expected, external noise can interfere much more easily with clean operation.
 

 

3. Sensitivity to Operating Conditions.
Subthreshold designs are also much more susceptible to process changes and environmental variations than superthreshold designs. For example, the current in a slow process curve can be 10–100 times lower than that of a nominal process. Considering that the on/off current ratio (above) is only on the order of a thousand, this cannot be ignored.
AMB002 Fig 4Temperature variations are a good example of how environmental conditions create a challenge for the designer. Vth depends on temperature, and Ion depends exponentially on V (as shown in Figure 4 below). As a result, the "off" current at elevated temperatures is similar in value to the "on" current at reduced temperatures for an uncompensated circuit. Therefore, designing subthreshold circuits requires additional effort to ensure that the circuits will perform as expected under all specified operating conditions.
   
4. Logistical Challenges.
Much of the manufacturing flow relies on reasonable assumptions for superthreshold designs, even though these assumptions break down for subthreshold designs. One obvious challenge lies in the testers used to validate silicon during production. Parametric measurement units (PMUs), which test voltages and currents, are designed to measure microamps, not nanoamps or picoamps.
Even something as simple as device characterization needs to be rethought simply because of the sensitivities that subthreshold circuits have, and that superthreshold circuits do not. Typical characterization flows may not be thorough enough to demonstrate that circuits function correctly under all conceivable conditions.
The fundamental nature of these challenges, combined with the fact that few engineers are experts in dealing with subthreshold problems, explains the challenge of commercializing subthreshold circuits.

Ambiq's solutions:
Ambiq's SPOT technology aims to overcome all these challenges. Furthermore, subthreshold design techniques can be applied to virtually any type of integrated circuit (IC). For example, Ambiq demonstrated the viability of this innovative approach with the introduction of the world's lowest-power real-time clock (RTC) in 2013. The upcoming release of the world's lowest-power 32-bit ARM-based microcontroller (MCU) further demonstrates the feasibility of extending these techniques to a completely different platform. Ambiq Micro is committed to expanding the SPOT Platform—and giving batteries a longer lifespan.