Yes, you can operate circuits below the 0.9 volts where some transistors can run. The result is mind-blowingly low power consumption, as in a full-featured Real Time Clock (RTC) with a supply current below 14 nA (that’s nanoamps).
First, some background on how transistors work
Transistors operate much like an electronic switch. Apply voltage above a certain threshold, and the transistor will conduct current. Turning the transistor “off” means that there is still some leakage current through the transistor, although it’s minimal. Transistors are in the majority of electronics today, as our world has become an on/off, digital world where transistors dominate electronic circuits. Circuits designed to operate with very low power consumption is desirable in smartphones, wearables, remote monitoring, and the Internet of Things, to name a few, if battery energy is to be conserved.
Some ways to reduce power consumption are to turn things off or put them in various states of sleep or shutdown, depending upon how fast the circuit needs to wake when it’s needed. This reduces the need to use dynamic power. Other ways to reduce dynamic energy consumption in a circuit are to minimize the total activity, switching load capacitances of, operating frequency of, and operating (supply) voltage of the transistors in the circuit.
Here, we will discuss lowering the supply voltage, which is critical, since dynamic energy changes with the square of the operating voltage: PD = ½ C • V2 • A• f, where C is capacitance, V is operating voltage, A is activity factor, and f is frequency (or clock rate). PTotal = PD + PS, and static power (PS) is the power consumed when there is no change of state going on (i.e., not dynamic). Static power is related to leakage current (P = V•I).
However, lowering the operating voltage comes with some trade-offs. Operating voltages have gotten lower over the years, decreasing from 5.5 volts in the ‘90s to 3.3 volts a decade later, to 1.8 volts a few years after that. Some integrated chips require only 0.9 volts to operate. The trade-off for low power consumption with a low supply voltage, however, is that at very low supply voltages, headroom for doing things at different voltage levels decreases significantly. For instance, at 0.9 volts, your binary “1” is high at 0.9 volts but a binary “0” can never be exactly 0 volts every time. Thus, the voltage level threshold for a binary “0” is closer to the voltage threshold for “1” if the highest level you have to work with is 0.9 volts and not 5.5 volts. Noise also becomes harder to ignore when supply voltages are low.
Sometimes performance is traded in for low power consumption, however.
However, operating at the sub-threshold level means that no voltage in the integrated chip (IC) rise above the threshold voltage. Transistors would never turn on, thus a logic “high,” to register as a binary “1,” means that the transistors are still “off.” How is that possible?
A different way to use the transistor model
The transistor model is what everything is based upon in IC design. Operating at higher supply voltages is where transistor models are focused, however. Transistors still respond at very low voltages, the response is just not shaped the same, and is not as linear. Accurate transistor models emulate the behavior of the transistor in it’s “on” state quite well. No one cares what the transistor does in it’s “off” state, as long as it responds with low current flow under a certain threshold voltage level. Why take pains to model the transistor’s response accurately when it’s “off”? However, the transistor does have a change in current flow below an arbitrary threshold voltage, although it is a poor model fit. (Figure 2)
If most modern circuit design is based on modeling and simulations, though, why couldn’t you design a circuit based on accurate models of predictable transistor behavior below the threshold voltage? The answer is, you can, but an entirely new design approach is required where models are accurate below VTH and where you can tolerate designing with exceedingly small currents that change exponentially in response to changing voltages.
Other areas of concern with this kind of “out of the box” design thinking are temperature sensitivity and logistical challenges. However, the progenitors of a company called Ambiq Micro started tackling sub-threshold design in 2004 at University of Michigan. Ambiq Micro has since then resolved many challenges with working below VTH.
Ambiq Micro’s Sub-threshold Power Optimized Technology (SPOT) addresses all of the challenges with working in the traditionally “off” state area of the transistor model. Ambiq Micro has spent the intervening years in an effort involving multi‐faceted solutions, beginning with recharacterizing existing low-power transistors in the sub-threshold region. Both analog and digital circuits for use on ICs were either created from scratch or modified to operate at such low voltages.
According to Ambiq Micro, “Ambiq Micro’s circuits are successful because they pick and choose from amongst different techniques, applying some or all of them in different parts of the integrated circuit. There is no magic formula that dictates what to use where; it takes solid engineering and good design to pull together the right combination that provides the required performance with the lowest energy, while at the same time paying attention to chip area and cost.”[i] The design strategy for any circuit is to use above threshold circuits where it makes sense, for instance, with non-volatile memory and data-transfer circuits if they are only used at power-up and shut completely off.
Ambiq Micro has created different circuits and catalog devices, building up a library of designs to date, with the same assurances for correct operation over the life of the chip as one would have with any other transistor-based design. Utilizing the sub-threshold region of transistors means that Ambiq Micro’s chips can provide the same functionality on just a fraction of the energy that others require. And it’s good to know that sub-threshold techniques can be applied to nearly any type of IC device.
[i] Sub-Threshold Design – A Revolutionary Approach to Eliminating Power. (2014). [Whitepaper] Ambiq Micro. Available at: https://ambiqmicro.com/ [Accessed 17 Jul. 2018].