2D Steep Transistor Technology: Overcoming Fundamental Barriers in Low-Power Electronics and Ultra-Sensitive Biosensors
In order to sustain the unprecedented growth of the Information Technology, it is necessary to achieve dimensional scalability along with power reduction, which is a daunting challenge. In this dissertation, two-dimensional (2D) materials are explored as promising materials for future electronics since they can, not only enable dimensional scaling without degradation of device electrostatics but it is also shown here, that they are highly potential candidate for interconnects and passive devices. 2D semiconductors are investigated for transistor applications, and novel approach for doping using nanoparticle functionalization is developed. It is also demonstrated that these materials can lead to ideal transfer characteristics. Aimed towards on-chip interconnect and inductor applications, the first detailed methodology for the accurate evaluation of high-frequency impedance of graphene is presented. Using the developed method the intricate high-frequency effects in graphene such as Anomalous Skin Effect (ASE), high-frequency resistance and inductance saturation, intercoupled relation between edge specularity and ASE and the influence of linear dimensions on impedance are investigated in detail for the first time.
While 2D materials can address the issue of dimensional scalability, power reduction requires scaling of power supply voltage, which is difficult due to the fundamental thermionic limitation in the steepness of turn-ON characteristics or subthreshold swing (SS) of conventional Field-Effect-Transistors (FETs). To address this issue, a detailed theoretical and experimental analysis of fundamentally different carrier transport mechanism, based on quantum mechanical band-to-band tunneling (BTBT) is presented. This dissertation elucidates an underlying physical concept behind the BTBT process and provides clear insight into the interplay between electron and hole characteristics of carriers within the forbidden gap during tunneling. Moreover, a novel methodology for increasing the BTBT current through incorporation of metallic nanoparticles at the tunnel junction is proposed and theoretically analyzed, followed by experimental demonstration as proof of concept, which can open up new avenues for enhancing the performance of Tunneling-Field-Effect-Transistors (TFETs).
This dissertation, also establishes, for the first time, that the material and device technology which have evolved mainly with an aim of sustaining the glorious scaling trend of Information Technology, can also revolutionize a completely diverse field of bio/gas-sensor technology. The unique advantages of 2D semiconductor for electrical sensors is demonstrated and it is shown that they lead to ultra-high sensitivity, and also provide an attractive pathway for single molecular detectability- the holy grail for all biosensing research. Moreover, it is theoretically illustrated that steep turn-ON characteristics, obtained through novel technology such as BTBT, can result in unprecedented performance improvement compared to that of conventional electrical biosensors, with around 4 orders of magnitude higher sensitivity and 10-fold lower detection time.
With a view to building ultra-scaled low power electronics as well as highly efficient sensors, new generation of van-der Waal's BTBT junctions combining 2D with 3D materials is proposed and experimentally demonstrated, which not only retain the advantages of 2D films but also leverages the matured doping technology of 3D materials, thus harnessing the best of both worlds. These attributes are instrumental in the achievement of unprecedented BTBT current, which is more than 3 orders of magnitude higher than that of best reported 2D heterojunctions till date.
Finally, with the optimization of the novel heterojunctions, this dissertation also achieves a significant milestone, furnishing the first experimental demonstration of TFETs based on 2D channel material to beat the fundamental limitation in subthreshold swing (SS). This device is the first ever TFET, in a planar architecture to achieve sub-thermionic SS over 4 decades of drain current, a necessary characteristic prescribed by the International Technology Roadmap for Semiconductors and in fact, the only TFET to date, to achieve so, in any architecture and in any material platform, at a low power-supply voltage of 0.1 V. It also represents the world's thinnest channel sub-thermionic transistor, thus, cracking the long-standing issue of simultaneous dimensional and power supply scalability and hence, can lead to a paradigm shift in information technology as well as healthcare.