Exploiting subthreshold MOSFET behavior in analog applications
Subthreshold voltage is still difficult to control, but, as the era of nanopower dawns and starts to expand, engineers can begin to use leakage current that they previously deemed unusable, he says. Energy harvesting, for example, is a rapidly expanding field of R&D. Efforts are now just beginning to unlock the possible applications of a field that benefits from nanopower operations. Engineers have yet to tap the potential for this technology, however, because of the limitations of harnessing, storing, and distributing micropower energy. The ability to store energy with minimal leakage is a key factor in the continuing advancement of applications in this area.
Engineers commonly consider a device to be off when current drops below the gate-threshold voltage (see sidebar “Threshold-voltage background”). For example, they would consider unusable any current lower than a gate-threshold voltage of 1V. Historically, it has been difficult to control this leakage current below the threshold voltage, or subthreshold leakage current, within a certain range. To understand the limitations holding back the development of nanopower, consider a MOSFET device or any similar device in which the gate voltage falls below the gate-threshold voltage. Any remaining voltage below the gate-threshold voltage is in the device’s subthreshold region; however, current drops off exponentially when there is even a relatively minimal drop from 1 to 0.9V. Designers usually refer to this drop as 100 mV/decade of current, so for every 0.1V drop in voltage, current drops by 10 times, or an order of magnitude.
Therefore, a MOSFET device with a VGS of exactly 0.5V has a drain-to-source current, IDS, of 0.03 nA when its gate is grounded. When gate voltage dips to 0V, current has decreased by approximately 30,000 times below threshold. It’s easy to imagine why engineers would consider it to be off.
This reduced current, however, operates on a well-behaved curve. When you turn off a switch, power is still behind the gate of the circuit below threshold, so the device is often not completely off. This same concept enables cell phones to operate in sleep mode, for example, conserving battery life when not in use and then waking up when receiving a call or a text message. In a similar fashion, many security systems, such as battery-powered alarm backups, sometimes must operate in deep-sleep mode for as long as five or 10 years and then wake up to complete their functions when they sense a specified event or behavior.
When a design does not require a subthreshold current, that current becomes parasitic leakage current, which dissipates power for the circuit without serving any useful purpose. This parasitic leakage joins and combines with other types of leakage currents, such as junction leakage, gate-oxide leakage, package-level surface leakage, and PCB leakage currents. Most of these leakage currents, however, are the products of contamination and imperfection in manufacturing and fabrication. These leakage currents are generally targeted to be minimized or eliminated with only a maximum value, but usually with no minimum value. In other words, a leakage current of exactly 0A would be ideal.
For analog-circuit design, subthreshold leakage currents differ from these other types of leakage currents. A 0A value for this subthreshold leakage current is unacceptable because the resistor value becomes infinitely large when the current denominator is zero. Engineers, therefore, minimize it and treat it as a “junky” current. When you control VGS and VGSTH, you can then control and reproduce the leakage current. When you control this current within a certain maximum and minimum range, then it becomes a resistive element that you can use.
An infinitely large resistor value is simply an “open circuit,” which would not lend itself to being useful as an analog- circuit element. A resistor element, therefore, must have a maximum and a minimum value to have any meaningful circuit function. MOSFETs have low threshold voltages and precise tolerance ranges, and the devices can be electrically trimmed at the factory for these precise specifications. These devices play an important role in many analog circuits, such as current sources, current mirrors, discrete differential amplifiers, and analog multiplexers. With such precise design techniques, manufacturers can produce MOSFETs with a VGS of 0.2V±0.02V at 1 μA, 0.4V±0.02V at 1 μA, or even 0.8V±0.02V at 1 μA. You can use the threshold voltage and the relative subthreshold voltage of a MOSFET as a voltage comparator and a voltage reference. Historically, you would have needed both a voltage reference and a voltage comparator for this task.
Analog engineers seeking an advantage for operating their designs below gate threshold will soon discover that nanopower is not a panacea because this approach provides less current to work with and requires acute sensitivity. For example, a threshold voltage may vary between 0.4 or 0.1V. If an engineer sees a 0.3V drop in input voltage, current will have decreased by 1000 times. This example underscores the importance of precision control in subthreshold voltages and gate voltages. When a current can fluctuate so greatly, it is difficult to build a tangible circuit.
Ultraprecise enhancement-mode MOSFET arrays provide designers with accuracy in controlling VGS and VGSTH. With extremely low VGSTH characteristics, these devices are essential in creating nanopower circuits. Examples include quad and dual N-channel, matched-pair enhancement-mode MOSFET arrays, which provide a VGSTH of 0.4V and precision tolerances of ±20 mV.
The ability to control the subthreshold characteristics of certain devices yields the ability to repurpose some leakage currents as useful sources for a variety of low-power circuits. This ability is unlocking the potential of new areas in nanopower circuitry and allowing electronic systems to use less power from a power source. As designers become more aware of the subthreshold characteristics of MOSFET devices and how more precise tolerances can help control voltages and currents that they previously deemed unusable, they will be able to tap little-used resources and open new developments in nanopower design.