Tiwari, Shree Prakash

BiFeO3 based resistive random access memory (RRAM) devices are fabricated using a low-cost solution process to study the effect of an Al top electrode on switching behavior and reliability. Fabricated devices demonstrated bipolar switching characteristics with a moderate Ion/Ioff ratio, set and reset voltages of ∼-1.3 V and ∼0.8 V, DC and AC endurance of more than 250 cycles and 7100 cycles, respectively, and a retention time of over 104 s, confirming the non-volatile resistive switching behavior. The ohmic and trap filled space charge limited conduction dominates the conduction mechanism in the devices at lower and higher voltages, respectively. Moreover, impedance spectroscopy measurements substantiate the presence of an AlOx layer at the Al/BiFeO3 interface resulting from the Al-O interaction at the junction, which is the possible rationale of reliable complementary switching in these RRAM devices. The switching mechanism is elucidated using the formation and rupture of the oxygen vacancy mediated filament, assisted by the participation of a thin AlOx layer at the Al/BFO interface. The role of the thin AlOx layer is explained by modeling of impedances.

High-performance flexible resistive random access memory (RRAM) devices were demonstrated by engineering the switching layer with PVK:MoS2 composite and TiO2 bilayer. These flexible RRAM devices exhibited excellent switching with low SET (1.2 V) and RESET (-1.5 V) voltages, a high repeatability of 1000 cycles, and an excellent retention time of 5000 s with an ON/OFF current ratio more than 102 at a read voltage of 0.2 V, significantly better than neat PVK/TiO2 devices. Moreover, devices with PVK:MoS2/TiO2 bilayer exhibited high stability upon bending with radii up to 7 mm for 100 cycles. Our results indicate that PVK:MoS2/TiO2 composite bilayer can be a promising switching layer candidate for high-performance flexible RRAM devices.

This paper for the first time presents a comprehensive computational modeling approach for AlGaN/GaN high electron mobility transistors. Impact of the polarization charge at different material interfaces on the energy band profile as well as parasitic charge across the epitaxial stack is modeled and studied. Furthermore, impact of surface and bulk traps on two-dimensional electron gas, device characteristics, and gate leakage is accounted in this paper. For the first time, surface states modeled as donor type traps were correlated with gate leakage. Moreover, a new approach to accurately model the forward gate leakage in Schottky gate devices is proposed. Finally, impact of lattice and carrier heating is studied, while highlighting the relevance of carrier heating, lattice heating, and bulk traps over the device characteristics. In addition to this, modeling strategy for other critical aspects like parasitic charges, quantum effects, S/D Schottky contacts, and high field effects is presented.

In an attempt to improve ON current and current ON/OFF ratio in tunnel field-effect Transistors (TFETs), a new wedge shaped structure for TFET named Wedge-TFET (WTFET) is proposed. This proposed device is a double gate structure, showing ∼3 times higher ON current compared to planar double gate TFET device working at supply voltage of 0.6 to 1.0 V. This increase is mainly due to increase in the tunneling area. In addition to increased current, W-TFET also shows high current ON/OFF ratio of the order ∼1012. The subthreshold swing values are slightly higher for W-TFET compared to our simulated DG-TFET, however, both of them fall in the range of 40 to 50 mV/dec. The W-TFET device with higher ON current and current ON/OFF ratio and with almost same area requirement can be a potential candidate for future low voltage applications.

A comprehensive investigation of long-term environmental stability and performance of flexible hybrid resistive random access memory (RRAM) devices with PVK:MoS2/TiO2 bilayer is presented. These devices were systematically characterized for their switching behavior for a very long duration of 20 months, and it was found that devices were able to maintain their switching behavior with maximum average VSET of 1.5 V and lowest average VRESET of -1.8 V, indicating high shelf life and low-voltage operation. Excellent retention of 104s was exhibited by devices even after being in ambient environment for 20 months. Although the devices exhibited almost unchanged VSET and VRESET values upon variation of humidity exposure (RH 42% to 99%), an increase in the magnitude of both was observed upon temperature variation from room temperature (RT) to 100 °C. A typical decay in IONIOFF by more than an order of magnitude (from 103) was observed upon aging and application of temperature; however, an increase in the same was observed upon exposure to high humidity on the order of 102-103. Our investigation indicates that (PVK:MoS2/TiO2) hybrid switching layer can be suitable for environmentally stable nonvolatile memory devices for flexible electronics.

Organic transistors fabricated with high-k P(VDF-TrFE) as gate dielectric and TIPS-pentacene:PS blend as active layer were investigated for light sensing and circuitry applications. These devices operated at low voltage of -5V with high Ion/Ioff of 103. These devices were explored for light sensing, and it was found that these devices exhibited higher sensitivity to blue light illumination compared to the other counterparts, red and green. In addition, circuit applicability of these devices was demonstrated through resistive load inverters.

Top-gate bottom-contact field-effect transistors based on solution-processed films of molecules in which two naphthalene-1,8:4,5- bis(dicarboxdiimide)s are bridged by thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene and dithieno[3,2-b:2′, 3′-d]pyrrole exhibit electron mobility values of up to 1.5 cm2 V-1 s-1. © 2011 American Chemical Society.

BiFeO3 thin films are deposited on FTO coated glass substrates using a simple sol-gel process, limiting thickness about 70 nm and Ag/BiFeO3/FTO RRAM devices are prepared. The devices showed low-voltage bipolar switching with the maximum Ion/Ioff ratio ∼450, and low set and reset voltages ∼1.1 V and −1.5 V, respectively. The devices are stable against on-off cycles with ∼104 s retention time without any significant degradation. The variations in the set and reset voltages are 0.4 V and 0.6 V, respectively. We found that ohmic and trap-controlled space charge limited conductions are responsible for low and high resistance states, respectively. The resistive switching mechanism is attributed to the formation and rupturing of the metal filament during the oxidation and reduction of Ag ions for the set and reset states. The devices showed strong robustness against environmental conditions even after ten months from their synthesis and first measurements, exhibiting good reproducibility, retention and endurance.

The current heat treatment processes make use of electricity or fuel for generating heat. Solar energy is available in abundance in the states of Rajasthan and Gujarat in India. Annual global solar radiation of about ≥2400 kWh/m2 is received in these regions. Use of the abundant solar thermal energy in heat treatment of metals would saveenergy and fuels. In view of this a concept of solar convective furnace system is described in this paper. As a starting point, heat treatment of aluminium is considered. For this system, requirements of industrial furnace are taken as basis.A scale-down retrofitted furnace is designed and analysed.The importance of different process stages like, solar thermal energy absorption, storage and utilization in design of such a system is presented.A thermodynamic analysis is performed to derive requirements for the achievement of a uniform heating at a pre-determined rate. Control strategy to meet the requirements is worked out. Air flow profile in the furnace is analyzed using CFD as a tool. Laser Doppler Velocimetery technique is used for measurement of velocity in a Plexiglas model of the scale-down furnace. Flow analysis using theadopted CFD tool shows non uniformities in air flow profile. To counter this, further modifications and improvements in the furnace structure are suggested. Evaluation of Open Volumetric Air Receiver (OVAR) using installed solar air tower simulation facility is also presented in the paper.

In this paper, a bi-layer combination of high-k/low-k gate dielectric consisting of poly (vinyl alcohol) (PVA) and poly (4-vinylphenol-co-methyl methacrylate) (PVP-Co-PMMA) polymers was explored to fabricate high performance flexible organic transistors. Solution processed TIPS-Pentacene: polystyrene (PS) was used to demonstrate p-channel transistors with excellent electrical characteristics. These devices exhibited average and maximum field-effect mobility (μ) values of 0.2 (±0.1) cm^2/Vs and 0.5 cm^2/V s in the saturation regime with high current (Ion/Ioff) ratio of 10^4 at an operating voltage of -8 V, with negligible hysteresis in transfer curves. Moreover, excellent repeatability in electrical characteristics with high cyclic and bias stress stability were also achieved. These findings suggest that this unique bi-layer gate dielectric combination can act as an excellent alternative for solution processed organic transistors.