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Using sub-threshold techniques for IC design

Posted: 07 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Sub-threshold? energy consumption? transistors? super-threshold? non-volatile memory?

The founders of Ambiq were part of that academic revival, working at the University of Michigan to develop the technology more thoroughly. That effort was spun out so that it could be fully commercialized. Ambiq is the only company utilising sub-threshold design as a primary approach to reducing energy consumption.

It would be obvious to ask why, if this technology was developed in the 70s, it never caught on. One might even suspect that some flaw might have been uncovered that kept sub-threshold out of the mainstream. It begs the question, "If this is so easy, why isn't everyone doing it?"

The answer to that question is, "Because it's not so easy." There is no fatal flaw, but the transition from super-threshold techniques has not been trivial. Ambiq's founding team started their work at Michigan in 2004 and worked until 2010 to make the technology usable on a broad, commercial scale.

One might also ask what's changed since the 70s, when the first commercial sub-threshold devices were created. The difference is scale: Designs of the past used a few critical sub-threshold transistors C on the order of 10. At that level, each transistor can be optimised by hand. By contrast, Ambiq creates entire chips that primarily use sub-threshold transistors. That makes hand-crafting completely impractical. Designing millions of such transistors is possible only by using standard design tools and flows C preferably the same as those that have been used for super-threshold design. This is the work that Ambiq has done to commercialise sub-threshold circuits.

The challenges of modern sub-threshold
Adapting the standard super-threshold flows and infrastructure for sub-threshold design presents numerous detailed challenges. These start with the very transistors themselves.

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 modelling 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 modelled as accurately, and so existing models are inadequate for sub-threshold design, as shown in figure 2.

Figure 2: Transistors haven't been well modelled below threshold.

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 response to changing voltages, but they're exceedingly small currents. In addition, the ratio of "on" to "off" current is on the order of 1000, orders of magnitude less than what super-threshold designs experience (figure 3). As can be expected, external noise can much more easily interfere with clean operation.

Figure 3: The on/off current ratio is orders of magnitude smaller in the sub-threshold regime.

Sensitivity to operating conditions: Sub-threshold designs are also far more susceptible to process and environmental variation than are super-threshold designs. For example, the current in a slow process corner can be 10-100 times less than that for a nominal process. Given that the on/off current ratio (above) is only on the order of a thousand, this cannot be ignored.

Temperature sensitivity: Variations in temperature provide a good example of how environmental conditions create a challenge for the designer. Vth depends on temperature, and Ion depends exponentially on Vth (figure 4). As a result, the "off" current at elevated temperature is similar in value to the "on" current at reduced temperature for an uncompensated circuit. Sub-threshold circuit design therefore requires extra effort to ensure that the circuits will operate as expected under all specified operating conditions.

Figure 4: Sub-threshold circuits are exponentially sensitive to temperature.

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