Anna Krammer

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Tunable RF Phase Shifters Based on Vanadium Dioxide Metal Insulator Transition

This paper presents the design, fabrication, and electrical characterization of a reconfigurable RF capacitive shunt switch that exploits the electro-thermally triggered vanadium dioxide (VO2) insulator to metal phase transition. The RF switch is further exploited to build wide-band RF true-time delay tunable phase shifters. By triggering the VO2 switch insulator to metal transition (IMT), the total capacitance can be reconfigured from the series of two metal–insulator–metal (MIM) capacitors to a single MIM capacitor. The effect of bias voltage on losses and phase shift is investigated, explained, and compared to the state of the art in the field. We report thermal actuation of the devices by heating the devices above VO2 IMT temperature. By cascading multiple stages a maximum of 40° per dB loss close to 7 GHz were obtained.

MoS 2 /VO 2 vdW heterojunction devices: Tunable rectifiers, photodiodes and field effect transistors

In this work we report a new class of ultra-thin film devices based on n-n van der Waals (vdW) heteroj unctions of MoS<inf>2</inf> and VO<inf>2</inf>, which show remarkable tunable characteristics. The favorable band alignment combined with the sharp and clean vdW interface determines a tunable diode-like characteristic with a rectification ratio larger than 10³. Moreover, the heterojunction can be turned into a Schottky rectifier with higher on-current by triggering the VO2 insulator to metal transition (IMT), by either applying a sufficiently large voltage or increasing the temperature above 68 °C. The proposed devices are photosensitive with linear photoresponse and temperature tunable photoresponsivity values larger than 1 in the 500/650 nm wavelength range. We finally report the first ever field-effect transistor based on gated MoS<inf>2</inf>/VO<inf>2</inf> heterojunctions, which is a true low power FET exploiting a phase change material where the electrostatic doping effect of the gate on the junction results in a subthreshold slope (SS) of 130 mV/dec at room temperature, I<inf>ON</inf>/I<inf>OFF</inf> > 10³ and I<inf>oFF</inf> < 5 pA/μm at Vd = 1.5V.

Shunt capacitive switches based on VO2 metal insulator transition for RF phase shifter applications

This paper presents a wide-band RF shunt capacitive switch reconfigurable by means of electrically triggered Vanadium Oxide (VO2) phase transition to build a true-time delay (TTD) phase shifter. The concept of VO2-based reconfigurable shunt switch has been explained and experimentally demonstrated by designing, fabricating and characterizing an 819 μm long unit cell. The effect of bias voltage on losses and phase shift has been studied and explained. By triggering the VO2 switch insulator to metal transition (IMT) the total capacitance can be reconfigured from the series of two metal-insulator-metal (MIM) capacitors to a single MIM capacitor. Higher bias voltages are more effective in this reconfiguration and give a higher phase shift. The optimal achievable performance has been shown heating the devices above VO2 IMT temperature. A maximum of 16° per dB loss has been obtained near the design frequency (10 GHz).

Van der Waals MoS2/VO2 heterostructure junction with tunable rectifier behavior and efficient photoresponse

Junctions between n-type semiconductors of different electron affinity show rectification if the junction is abrupt enough. With the advent of 2D materials, we are able to realize thin van der Waals (vdW) heterostructures based on a large diversity of materials. In parallel, strongly correlated functional oxides have emerged, having the ability to show reversible insulator-to-metal (IMT) phase transition by collapsing their electronic bandgap under a certain external stimulus. Here, we report for the first time the electronic and optoelectronic characterization of ultra-thin n-n heterojunctions fabricated using deterministic assembly of multilayer molybdenum disulphide (MoS2) on a phase transition material, vanadium dioxide (VO2). The vdW MoS2/VO2 heterojunction combines the excellent blocking capability of an n-n junction with a high conductivity in on-state, and it can be turned into a Schottky rectifier at high applied voltage or at temperatures higher than 68 °C, exploiting the metal state of VO2. We report tunable diode-like current rectification with a good diode ideality factor of 1.75 and excellent conductance swing of 120 mV/dec. Finally, we demonstrate unique tunable photosensitivity and excellent junction photoresponse in the 500/650 nm wavelength range.

Elevated transition temperature in Ge doped VO2 thin films

Thermochromic GexV1−xO2+y thin films have been deposited on Si (100) substrates by means of reactive magnetron sputtering. The films were then characterized by Rutherford backscattering spectrometry (RBS), four-point probe electrical resistivity measurements, X-ray diffraction, and atomic force microscopy. From the temperature dependent resistivity measurements, the effect of Ge doping on the semiconductor-to-metal phase transition in vanadium oxide thin films was investigated. The transition temperature was shown to increase significantly upon Ge doping (∼95 °C), while the hysteresis width and resistivity contrast gradually decreased. The precise Ge concentration and the film thickness have been determined by RBS. The crystallinity of phase-pure VO2 monoclinic films was confirmed by XRD. These findings make the use of vanadium dioxide thin films in solar and electronic device applications—where higher critical temperatures than 68 °C of pristine VO2 are needed—a viable and promising solution.