The performance of semiconductor logic and memory devices has improved significantly with advancements in complementary metal-oxide-semiconductor (CMOS) manufacturing technology to miniaturize transistors, resulting in increased integrated circuit (“chip”) processing speed, energy efficiency, and cost per function. The proliferation of the Internet of Things (IoT) and the generation and processing of large data sets, commonly known as “big data,” have increased demand for non-volatile (NV) information storage that can be embedded with energy-efficient logic switches so that certain computational tasks can be performed in the memory itself. To achieve ultra-low-power electronics, alternative switching devices that can be operated with much smaller voltages than CMOS transistors are required. Micro-electro-mechanical (MEM) switches have been shown to be promising for ultra-low-power digital computing applications due to their negligible IOFF and abrupt switching behavior across a wide range of operating temperatures. Therefore, they have attracted growing interest for embedded energy-efficient logic and NVM applications. In addition to energy efficiency, it is also crucial to maintain the security and confidentiality of information collected and shared by IoT devices. To address this issue, memory devices that can store data and function as hardware security key generator are desirable. Emerging Resistive Random Access Memory (ReRAM) is a promising technology for hardware security applications due to its inherent variability.

This dissertation focuses on novel non-volatile memory devices and their applications. First, logic MEM switches are demonstrated to be operable as NV memory devices using controlled welding and unwelding of the contacting electrodes. Reprogrammability with consistently low programmed state resistance, and excellent (essentially infinite) retention time at elevated temperature, are experimentally demonstrated. These results indicate that MEM switches are promising for low-cost implementation of ultra-low-power integrated systems.

Next, this dissertation presents a prototype MEM switch design incorporating a floating gate (FG) for non-volatile charge storage. The FG-MEM NV switch potentially can achieve much longer data retention time than a conventional floating-gate MOS transistor, since there exists an air gap between conductive electrodes when the switch is in the OFF-state. Initial experimental results and FG-MEM NV switch design improvements are discussed.

Finally, this dissertation proposes a new method of generating Physically Unclonable Function (PUF) encryption keys that leverage the inherent random variability of ReRAM device programming time, for hardware security applications. The design and operation of a ReRAM device is presented, followed by a detailed discussion of the PUF key generation scheme. The randomness and reliability of the generated keys are then evaluated. The randomness of the ReRAM-based PUF is found to be of high quality compared to previous PUF implementations. Therefore, ReRAM technology enables the incorporation of both NVM and PUF functions within the same chip.

The proliferation of information and communication devices over the past few decades has been enabled by continual advancement of semiconductor manufacturing technology to steadily miniaturize semiconductor switching devices – most notably, metal-oxide-semiconductor field effect transistors (MOSFETs) – to increase the number of transistors in the most advanced integrated circuit (IC) products, at a pace set by Moore’s Law, for enhanced chip functionality and performance. In recent years, however, the incremental benefit of transistor scaling has diminished largely because the Boltzmann energy distribution of electrons in a semiconductor results in switching steepness (subthreshold swing) proportional to the thermal voltage (kT/q), which does not scale. As a result, conventional MOSFETs cannot switch ON/OFF more abruptly than 60 mV/decade at room temperature, which limits the extent to which the transistor threshold voltage (VT) can be reduced for a given OFF-state leakage current specification (IOFF). As the operating voltage (VDD) of a digital IC is reduced with increasing transistor density to meet power density constraints (set by chip cooling limitations), then, the gate overdrive voltage (VDD – VT) is disproportionately reduced, limiting the transistor ON-state current and hence IC performance. With the advent of the Internet of Things, the need for more energy-efficient electronics has emerged; alternative switching devices that can be operated at much lower voltage than the MOSFET will be required. Micro-electro-mechanical (MEM) relays are promising candidate switching devices for low-voltage digital ICs, since they can achieve immeasurably low IOFF and abrupt switching behavior across a wide range of operating temperatures. Since MEM relays exhibit hysteretic switching behavior (i.e., the value of the control/gate voltage at which a relay switches ON is different than that at which it switches OFF) the hysteresis voltage sets a lower limit for their operating voltage.

This dissertation discusses approaches and challenges for realizing milli-Volt MEM relay technology for energy-efficient computing. First the application of self-assembled molecular (SAM) anti-stiction coatings to reduce contact adhesive force and thereby the hysteresis voltage is investigated, and stable sub-50 mV operation is demonstrated. Next the issue of variability in relay performance parameters over many switching cycles and from device to device is systematically studied, and SAM coating is found to improve stability. Then the effects of contacting electrode design and body-biased operation on relay ON-state resistance (RON) are investigated. The direct source/drain contact design provides for lowest and least variable RON. Ultra-low-voltage relay operation facilitated by body-biasing results in lower contact velocity, which mitigates the need for a wear-resistant contacting electrode material while necessitating a contacting electrode material that is not susceptible to oxidation.