Research
Self-powered Time-stamped Sensing
Self-powered sensing of the time-of-occurrence of an event is challenging because it requires access to a reliable time reference or a synchronized clock. We propose for the first time a self-powered integrated circuit that is capable of time-stamping asynchronous mechanical events of interest. The work presents a 4x4 fully programmable timer array system-on-chip (SoC) together with a linear injector array SoC . The synchronization error of the timer array with respect to an external software clock was measured to be less than 1% over a duration of 100 hours and the average accuracy in sensing the time-of-occurrence of the event was measured to be 6.9%. The minimum activation energy of the self-powered system was measured to be 840 nJ. This technique could be used for applications such as time-sensitive recording of destructive mechanical events in civil structures or detection of tampering events in supply chain. (JSSC2018PAPER, MWSCAS2018PAPER1, MWSCAS2018PAPER2)
Dynamic authentication of passive IoT devices using FN Timers
Passive Internet of Things (IoT) like radio frequency identification (RFID) tags can be used to offer a wide range of services such as object tracking or classification, marking ownership, noting boundaries, and indicating identities. Many authentication protocols have been proposed in literature, however, they either are vulnerable to certain types of attacks or require prohibitively a large amount of computational resources to be implemented on a passive tag. In this work we present variants of a novel authentication protocol that can overcome the security flaws of previous protocols while being well suited to the computational capability of the tags. At the core of the proposed approach is our recently demonstrated self-powered timing devices that can be used for robust time-keeping and synchronization without the need for any external powering. The proposed protocol also incorporates margins of tolerance that make the authentication process robust to any deviations in the timer responses due to fabrication artifacts. (ISCAS2017PAPER, IoT2018PAPER, ICC2018PAPER)
Self-powered time-temperature indicator for supply chain monitoring
Temperature management of the food supply-chain is important for ensuring compliance and the quality of perishable products like vaccines and fish. While conventional strategies have relied on using monitors attached to packaging containers, self-powered time-temperature monitoring is attractive because the technology can be embedded with passive RFID tags and can be integrated with every food or medical package. In this project we propose a self-powered sensor that can monitor the time-temperature information without the need for any external powering. The sensor exploits the physics of Fowler-Nordheim (FN) tunneling where electrons are thermally excited and are continuously integrated on a floating-gate. Measured results from sensors prototyped in a 0.5 um CMOS process show a temperature sensitivity of 1.5mV/℃ over a monitoring duration of 100 hours. (MWSCAS2017PAPER)
Self-powered time-tracking using thermodynamically driven quantum fluctuations in CMOS process
Self-powered timers provide a mechanism to achieve temporal synchronization between two passive devices (for e.g., RF tags, credit/access cards, and thumb drives) without the need for any external powering or clocks. As a result, the timers could be used to implement dynamic SecureID type authentication involving random keys and tokens that need to be periodically generated and synchronized. We report a novel solid-state self-powered timer, which exploits a self-compensating mechanism in the physics of Fowler–Nordheim quantum transport of electrons tunneling onto a floating gate. The proposed devices have been fabricated using standard CMOS processing and are demonstrated to be operational for durations greater than three years using extrapolation studies. The fabricated devices were also found to be extremely robust to device mismatch and as a result of which, the proposed selfpowered timers can be synchronized with respect to each other with an accuracy greater than 0.5%. (ISCAS2013PAPER, SRCTECHCON2016PAPER, arXiv2017PAPER, TED2017PAPER)
Low-power current bias array with temperature compensation using varactors
Floating-gate (FG) transistors are commonly used in synthetic neural systems for implementing analog multipliers. However, conventional floating-gate multipliers are sensitive to variations in temperature which limit their application to only controlled environments. In this project, we report a continuous-time circuit implementation of the temperature compensation algorithm and show that it enables on-chip implementation of temperature compensated current amplifiers and analog multipliers. Using measured results from fabricated prototypes in a 0.5μm CMOS process, the temperature coefficient is reduced from 4000 ppm/℃ to 500 ppm/℃. (ISCAS2015PAPER)
Low-power floating-gate array with high programming accuracy
While floating-gate transistors are attractive as a compact non-volatile storage of analog parameters, precise and fast adaptation of the stored parameters through digital command and control has been a challenge. In this paper we present the design of a high-density array of analog floating-gate memory that can be independently and precisely updated using digital timing interrupts. The proposed array uses a digital isolation method where FN tunneling can be independently applied to each of the memory cell without affecting the stored values in the other cells. As a result, bi-directional updates better than 500 uV accuracy and with linearity greater than 60dB can be achieved using digital control. (ISCAS2014PAPER)
Low-power in-vivo mechanical sensing use PFG sensors
Piezoelectricity-driven hot-electron injectors (p-HEI) are used for self-powered monitoring of mechanical activity in biomechanical implants and structures. Previously reported p-HEI devices operate by harvesting energy from a piezoelectric transducer to generate current and voltage references which are then used for initiating and controlling the process of hot-electron injection. As a result, the minimum energy required to activate the device is limited by the power requirements of the reference circuits. In this paper we present a p-HEI device that operates by directly exploiting the self-limiting capability of an energy transducer when driving the process of hot-electron injection in a pMOS floating-gate transistor. As a result, the p-HEI device can activate itself at input power levels less than 5 nW. Using a prototype fabricated in a 0.5-um bulk CMOS process we validate the functionality of the proposed injector and show that for a fixed input power, its dynamics is quasi-linear with respect to time. The paper also presents measurement results using a cadaver phantom where the fabricated p-HEI device has been integrated with a piezoelectric transducer and is used for self-powered monitoring of mechanical activity. (ISCAS2016PAPER, TBCAS2016PAPER, TBCAS2017PAPER)