44 research outputs found
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
Wavelength-Controlled Photocurrent Polarity Switching in BP-MoS Heterostructure
Layered two-dimensional van der Waals (vdW) semiconductors and their
heterostructures have been shown to exhibit positive photoconductance (PPC) in
many studies. A few recent reports have demonstrated negative photoconductance
(NPC) as well that can enable broadband photodetection besides multi-level
optoelectronic logic and memory. Controllable and reversible switching between
PPC and NPC is a key requirement for these applications. This report
demonstrates visible-to-near infrared wavelength-driven NPC and PPC, along with
reversible switching between the two, in an air stable, high mobility,
broadband black phosphorus (BP) field effect transistor (FET) covered with a
few layer MoS flake. The crossover switching wavelength can be tuned by
varying the MoS bandgap through its flake thickness and the NPC and PPC
photoresponsivities can be modulated using electrostatic gating as well as
laser power. Recombination-driven NPC and PPC allows for reversible switching
at reasonable time scales of a few seconds. Further, gate voltage-dependent
negative persistent photoconductance enables synaptic behavior that is
well-suited for optosynaptic applications.Comment: Main Manuscript and Supporting Informatio
Electrically Controlled Reversible Strain Modulation in MoS Field-effect Transistors via an Electro-mechanically Coupled Piezoelectric Thin Film
Strain can efficiently modulate the bandgap and carrier mobilities in
two-dimensional (2D) materials. Conventional mechanical strain-application
methodologies that rely on flexible, patterned or nano-indented substrates are
severely limited by low thermal tolerance, lack of tunability and/or poor
scalability. Here, we leverage the converse piezoelectric effect to
electrically generate and control strain transfer from a piezoelectric thin
film to electro-mechanically coupled ultra-thin 2D MoS. Electrical bias
polarity change across the piezoelectric film tunes the nature of strain
transferred to MoS from compressive 0.23% to tensile 0.14% as
verified through peak shifts in Raman and photoluminescence spectroscopies and
substantiated by density functional theory calculations. The device
architecture, built on a silicon substrate, uniquely integrates an MoS
field-effect transistor on top of a metal-piezoelectric-metal stack enabling
strain modulation of transistor drain current 130, on/off current ratio
150, and mobility 1.19 with high precision, reversibility and
resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge
factors, easy electrical strain modulation, high thermal tolerance and
substrate compatibility make this technique promising for integration with
silicon-based CMOS and micro-electro-mechanical systems.Comment: Manuscript and Supplementary Informatio
Near-direct bandgap / 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) 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 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 on top of gate tunable 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 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 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 / heterostructure diode
technologically promising for next-generation optoelectronic applications.Comment: Manuscript- 27 pages, 8 figures. Supporting Information- 17 pages, 17
figure