Ultra-Low Power Neuromorphic Obstacle Detection Using a Two-Dimensional Materials-Based Subthreshold Transistor

Accurate, timely and selective detection of moving obstacles is crucial for reliable collision avoidance in autonomous robots. The area- and energy-inefficiency of CMOS-based spiking neurons for obstacle detection can be addressed through the reconfigurable, tunable and low-power operation capabilities of emerging two-dimensional (2D) materials-based devices. We present an ultra-low power spiking neuron built using an electrostatically tuned dual-gate transistor with an ultra-thin and generic 2D material channel. The 2D subthreshold transistor (2D-ST) is carefully designed to operate under low-current subthreshold regime. Carrier transport has been modelled via over-the-barrier thermionic and Fowler-Nordheim contact barrier tunnelling currents over a wide range of gate and drain biases. Simulation of a neuron circuit designed using the 2D-ST with 45 nm CMOS technology components shows high energy efficiency of ~3.5 pJ/spike and biomimetic class-I as well as oscillatory spiking. It also demonstrates complex neuronal behaviors such as spike-frequency adaptation and post-inhibitory rebound that are crucial for dynamic visual systems. Lobula giant movement detector (LGMD) is a collision-detecting biological neuron found in locusts. Our neuron circuit can generate LGMD-like spiking behavior and detect obstacles at an energy cost of <100 pJ. Further, it can be reconfigured to distinguish between looming and receding objects with high selectivity.Comment: Main text along with supporting information. 4 figure

Near-direct bandgap WSe2WSe_2/ReS2ReS_2 type-II pn heterojunction for enhanced ultrafast photodetection and high-performance photovoltaics

PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct bandgap at the heterointerface that can significantly enhance optical generation, for high light absorbing few/multi-layer vdW materials, has not yet been shown. In this work, for the first time, few-layer group-6 transition metal dichalcogenide (TMD) $WSe_2$ is shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band alignment at its interface with the group-7 TMD $ReS_2$ through density functional theory calculations. Further, the type-II alignment and photogeneration across the interlayer bandgap have been experimentally confirmed through micro-photoluminescence and IR photodetection measurements, respectively. High optical absorption in few-layer flakes, large conduction and valence band offsets for efficient electron-hole separation and stacking of light facing, direct bandgap $ReS_2$ on top of gate tunable $WSe_2$ are shown to result in excellent and tunable photodetection as well as photovoltaic performance through flake thickness dependent optoelectronic measurements. Few-layer flakes demonstrate ultrafast response time (5 $\mu$s) at high responsivity (3 A/W) and large photocurrent generation and responsivity enhancement at the heterostructure overlap region (10-100X) for 532 nm laser illumination. Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 $\mu$A enables high output electrical power. Finally, long term air-stability and a facile single contact metal fabrication process makes the multi-functional few-layer $WSe_2$/$ReS_2$ heterostructure diode technologically promising for next-generation optoelectronic applications.Comment: Manuscript- 27 pages, 8 figures. Supporting Information- 17 pages, 17 figure

Reversible hysteresis inversion in MoS₂ field effect transistors

MoS2 devices: variable temperature measurements unveil reversible hysteresis mechanisms Defects and traps in MoS2 van der Pauw devices give rise to a hysteresis inversion mechanism which is reversible with temperature. A team led by Saurabh Lodha at the Indian Institute of Technology Bombay performed variable temperature hysteresis measurements on four- and two-terminal MoS2 devices, both suspended and supported on a SiO2 substrate. The onset of a clockwise hysteresis at room temperature was attributed to intrinsic MoS2 defects, whereas an additional mechanism resulting in an anticlockwise hysteresis was observed at higher temperature, and attributed to extrinsic charge trapping and de-trapping between the oxide and the silicon gate. By leveraging the temperature dependence of the hysteresis in MoS2, the authors developed a non-volatile memory and a temperature sensor

Enhanced responsivity and detectivity of fast WSe2 phototransistor using electrostatically tunable in-plane lateral p-n homojunction

In photodetectors based on 2D materials, a trade-off often exists between responsivity and speed. Here, the authors attenuate this issue via integration of a lateral, in-plane, electrostatically tunable p-n homojunction with a conventional WSe2 phototransistor