XXII International Workshop on Physics of Semiconductor Devices 2023 @ IIT Madras

Title: Reliable Sensing with Unreliable Sensors: Reading the Heartbeats of Field-Deployed Electron Devices in Uncontrolled/Extreme Environments
Abstract:The rapid rise in field-deployed electronics in automotive, health care, renewable energy, industrial automation, etc., is essential to address the emerging grand challenges of food, energy, water, environment, etc., for the full-earth scenario. The field-deployed electronics do not live in air-conditioned rooms/laboratories but must grapple with uncontrolled (and potentially extreme) fluctuations in environmental temperature, humidity, biofouling, and radiation. Learning to sustainably and predictably deploy these systems would require a foundational rethinking of how electronic devices are operated. In this talk, I will use examples from smart homes, solar farms, manufacturing machines, and wearable/implantable electronics to show that all electronic devices have a “heartbeat”, i.e., an intrinsic/characteristic power fingerprint that reflects their operation. By learning to interpret and manage the heartbeat and by collaborative/social sensing in a percolating sensor network, we can monitor the performance/reliability of these components and transform how these systems are sustainably deployed.

Title: Solution processible materials and printing techniques for 2D/3D microelectronic devices
Abstract: Printed and flexible electronics enable interesting novel applications in the fields of sensors, bioelectronics and security applications. An interesting class of materials to realize this are metal oxides that are known for a variety of useful characteristics such as high electron mobility in thin films, printability, non-toxicity including environmentally friendly solvents, transparency, to name but a few. In addition, metal oxide based nanoparticle inks allow to print semiconducting thin films at low temperatures (around 100 °C). Equally important to the question of materials are the structuring techniques. A widespread digital printing approach is inkjet printing, which can be further combined with other techniques such as laser printing to yield even 3D device architectures.
In the talk, the plethora of inorganic materials and printing techniques used in the fabrication of multilayer devices such as transistors, memristors and even fully integrated circuits, will be discussed. A memristor is an electronic device in a simple sandwich-like structure containing two metals and a thin insulating layer. It exhibits a huge potential as a non-volatile data storage device and plays a vital role in novel paradigms of computing, such as neuromorphic computing. The fabrication and electrical performance of inkjet and laser printed ZnO digital memristors as well as WO analog memristors, which show “learning” behavior, will be presented. The conductance of analog type devices can be gradually manipulated to emulate different behaviors of the biological synapse, such as short time plasticity, long term potentiation, and long-term depression.
In addition, progress towards transistor architectures using nanoparticle-based In-oxide as the semiconductor and Pt or ITO as the electrode material will be presented. Along these lines, a transistor with In-oxide-semiconducting nanoparticles printed into a 3D laser written polymer reservoir with 10 μm height is presented. Electrical characteristics are discussed including transfer and output curves as well as FIB cuts and cross-sectional SEM images. Our devices show high ion currents in the range of ~mA for typical widths of few hundreds of micrometers. Last but not least, systems of integrated logic devices (transistor-resistor based inverters) are shown as applications for physical unclonable functions and basic building blocks to perform multiply and add functions.

Title: GaN RF HEMT: Some new approaches for Ohmic contacts and dopant free buffer layer
Abstract: GaN based High Electron Mobility transistors are used as RF power transistors in T/R modules and as switch in power converters. Even though these devices are commercially available, there is still scope for improvement in the process of fabrication which leads to better performance of the device. In this talk I will present some of the results of studies conducted on GaN based RF HEMTs. Epitaxial GaN HEMT stacks grown on silicon substrates were chosen for these studies. Optimization of ohmic contact resistance and improvement of surface morphology in highly scaled RF HEMTs is necessary to obtain high values of f t .L g product (f t is the cut off frequency and L g is the gate length). GaN buffer in the HEMT epitaxial stack is either Carbon or Iron doped to compensate the unintentional background carrier concentration to make the buffer insulating. This brings with it additional issues in growth of the epistack and buffer related current collapse. Our studies are an attempt to address both these issues.
InAlN/GaN hetero structures possess a very high two-dimensional electron concentration > 2E13 cm -2 and are attractive candidates for high power RF applications. Sc/Al/Ni/Au metal stack deposited on InAlN/GaN HEMT and annealed at 775C resulted in a contact resistance of 0.39.Ωmm. Detailed TEM investigations and EDS analysis were conducted to determine the mechanism of ohmic contact formation with low contact resistance. It was found that Scandium diffused through dislocations and there were ScGaN inclusions at the interface. In another study, we found that chemical surface treatment with TMAH before the deposition of the ohmic metal stack on InAlN/GaN HEMT, resulted in lower contact resistance. Here again TEM investigations were performed to determine the phenomena underlying the process of lower contact resistance.
Highly scaled InAlN/GaN HEMTs were fabricated with gate lengths ranging from 40nm to 200nm. Record values of f t -L g were demonstrated. Back gating experiments were conducted to investigate the role of buffer in the kink in the I D -V D characteristics.
CMOS like processing for fabrication of device is a path for reducing the cost of production of devices. Ti/Al/Ti/W was Sputter deposited as ohmic metal on recess etched AlGaN/GaN epitaxial stack and annealed at low temperature (550C). These resulted in low contact resistance with improved surface morphology. Detailed studies revealed the formation of TiN inclusions even at low annealing temperatures.
As an alternative to Carbon or Iron doped buffer, we investigated polarization graded undoped AlGaN layer as the buffer. This showed low leakage current and low buffer related current collapse. RF performance of HEMTs fabricated using these stacks were also studied.

Distinguished Lecture

Distinguished Lecture

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Chihaya Adachi

Title & Abstract