In-situ thermo-reflectance imaging is used to show that the discontinuous, snap-back mode of current-controlled negative differential resistance (CC-NDR) in NbO x -based devices is a direct consequence of current localization and redistribution. Current localisation is shown to result from the creation of a conductive filament either during electroforming or from current bifurcation due to the super-linear temperature dependence of the film conductivity. The snap-back response then arises from current redistribution between regions of low and high current-density due to the rapid increase in conductivity created within the high current density region. This redistribution is further shown to depend on the relative resistance of the low current-density region with the characteristics of NbO x cross-point devices transitioning between continuous and discontinuous snap-back modes at critical values of film conductivity, area, thickness and temperature, as predicted. These results clearly demonstrate that snap-back is a generic response that arises from current localization and redistribution within the oxide film rather than a material-specific phase transition, thus resolving a longstanding controversy.Current-controlled negative differential resistance (NDR) is observed in a wide range of amorphous transition metal oxides (e.g. TiO x 1 , TaO x 2 , VO x 3 and NbO x 4,5 ) and is being used to fabricate devices for brain-inspired computing, including: trigger comparators 6 , self-sustained and chaotic oscillators 7-10 , threshold logic devices 11,12 and the emulation of biological neuronal dynamics 13,14 . In their simplest form such devices consist of simple metal-oxidemetal structures and exhibit a smooth transition from positive to negative differential resistance under current-controlled operation (hereafter referred to as S-type NDR) due to a rapid increase in device conductance, as shown in Figure 1a. In general this can arise from electronic, thermal or a combination of electronic and thermal processes but for amorphous

Alternating current electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically relevant liquids in microchannels using simple electrode designs. However, Joule heating may cause significant temperature rises, which can degrade biological species, and hence, ACET may become impractical for biomicrofluidic sensors and other possible applications. In this study, the temperature rise at the electrode/electrolyte interface during ACET flow is measured using a high-resolution, noninvasive, thermoreflectance imaging method, which is generally utilized in microelectronics thermal imaging applications. The experimental findings reveal that Joule heating could result in an excessive temperature rise, exceeding 50 °C at higher voltage levels (20 Vpp). The measured data are compared with the results of the enhanced ACET theoretical model, which predicts the temperature rise accurately, even at high levels of applied voltages. Overall, our study provides a temperature measurement technique that is used for the first time for electrode/electrolyte systems. The reported results are critical in designing biomicrofluidic systems with significant energy dissipation in conductive fluids.

The negative-differential-resistance (NDR) response of Nb/NbO x /Pt cross-point devices is shown to have a polarity dependence due to the effect of the metal-oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snapback NDR, and combined S-type and snapback NDR, depending on the stoichiometry of the oxide film and device area. In situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localization, thereby justifying the use of a previously developed two-zone, core-shell thermal model of the device. The observed polarity-dependent NDR responses, and their dependence on stoichiometry and area, are then explained by extending this model to include the effect of the polaritydependent contact resistance. This study provides an improved understanding of the NDR response of metal-oxide-metal structures and informs the engineering of devices for neuromorphic computing and nonvolatile memory applications.

No abstract

Metal−oxide−metal (MOM) devices based on niobium oxide exhibit threshold switching (or current-controlled negative differential resistance) due to thermally induced conductivity changes produced by Joule heating. A detailed understanding of the device characteristics therefore relies on an understanding of the thermal properties of the niobium oxide film and the MOM device structure. In this study, we use timedomain thermoreflectance to determine the thermal conductivity of amorphous NbO x films as a function of film composition and temperature. The thermal conductivity is shown to vary between 0.86 and 1.25 W•m −1 • K −1 over the composition (x = 1.9 to 2.5) and temperature (293 to 453 K) ranges examined, and to increase with temperature for all compositions. The impact of these thermal conductivity variations on the quasistatic current−voltage (I−V) characteristics and oscillator dynamics of MOM devices is then investigated using a lumped-element circuit model. Understanding such effects is essential for engineering functional devices for nonvolatile memory and brain-inspired computing applications.

No abstract

Gallium nitride (GaN) high electron mobility transistors (HEMTs) operate at high power levels and are thus especially thermally-critical devices. Not only do they require innovative thermal management strategies, but can also benefit from advanced experimental thermal characterization, both numerical and experimental, in their design and system integration stages. The thermal numerical analysis of microelectronic devices faces the challenges of complex physics and uncertain thermophysical properties which leads to numerically expensive models that are prone to error. By the use of an innovative reverse modeling approach to mitigate the above challenges, this work presents the full thermal characterization of GaN power devices with different substrates aimed at managing performance-limiting self-heating. The approach develops and optimizes a thermal simulation model to match the numerical results to experimentally-obtained thermal maps of the devices under test. The experimentally-optimized simulation model can then be used to extract full 3D temperature distributions, infer in-situ thermal properties, and provide a numerical platform that can be used to conduct further parametric studies and design iterations. The presented analysis provides a full thermal characterization of different GaN HEMT devices and compares the thermal performance of different substrates on the basis of thermal properties. The extracted properties for HEMTs on Si, SiC, and Diamond substrates are compared and a set of conclusions are presented to guide further developments in GaN HEMT thermal management strategies.

No abstract