| Home > Novel Digital and Radio Frequency Electronic Functions Enabled by Steep-Slope Phase-Change Devices |
In the past half-century, the semiconductor industry has relied on the scaling of complementary metal-oxide-semiconductor (CMOS) transistor dimensions and on material and geometrical innovations to increase the computational density. Approaching the end of the CMOS scaling era, the quest for novel materials and device principles is critical to address the challenges of energy efficiency, sustainability and computational power that the society poses. Beyond CMOS, many "More than Moore" devices have benefitted of the innovation in material and miniaturization techniques. One of today¿s novel research directions focuses on the exploitation of the unique physical properties of some functional oxides. Among these, there is a set of correlated transition metal oxides (TMOs) that exhibit a reversible insulator to metal transition (IMT) when heated across a certain temperature TIMT. The aim of this thesis is to demonstrate that IMT can be used as booster in "More Moore" devices and as enabler for "More than Moore" functions in a CMOS compatible platform. To experimentally validate the proposed devices and architectures, vanadium dioxide (VO2) has been selected, being of particular interest thanks to the above room temperature TIMT (68~°C) and the high resistivity contrast between metal and insulating phase that reaches five orders of magnitude in bulk crystal. Moreover, IMT in VO2 can be triggered by applying an electrical stimulus, resulting in ultra-steep transition characteristic. Magnetron sputtering deposition and pulsed laser depositions (PLD) of VO2 were exploited to provide thin films, whose quality was analysed and confirmed with different characterization techniques, including electrical resistivity measurements, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Two terminal planar switches on VO2 are presented as fundamental building blocks for more complex devices. The extensive electrical characterization confirms the abrupt electrical switching due to the IMT, and correlates parameters such as actuation voltage and current, switching power and hysteresis with the geometry of the switch. IMT was implemented as a booster for "More Moore" devices by combining it with band to band tunnelling (BTBT) principle in the demonstration of a Phase-Change Tunnel Field Effect Transistor (PC-TFET). The device principle is presented and experimentally validated for two possible configurations, resulting in the first solid-state VO2-based 3-terminal switch with simultaneous very low IOFF current, high ION/IOFF ratio and ultra-steep subthreshold swing. The great potential of VO2 as an enabler for ¿More than Moore¿ functions has been expressed in the design, fabrication and characterization of three types of reconfigurable radio-frequency devices: phase shifters, inductors and filters. If compared to existing MEMS solutions, the proposed distributed loaded line phase shifter working in C-band offers shorter switching time and lower fabrication complexity. The reconfigurable inductor for C and X-band offer state-of-the art performances for 2D CMOS-compatible reconfigurable inductors with the added merits of a simple design and actuation mechanism. Reconfigurable band-stop filters for Ka-band are made with an original and compact design by short-cutting with a VO2 switch spirals shaped defected ground structure (DGS) to modify the resonant frequency.
