Behavioral modeling of zinc-oxide, thin-film, field-effect transistors and the design of pixel driver, analog amplifier, and low-noise RF amplifier circuits

Zinc-oxide (ZnO) is of great interest due to transparent properties, high breakdown voltages, and low cost. Behavioral modeling is presented in this dissertation to model ZnO thin-film field-effect transistor (FET) drain current versus gate-source overdrive voltage. Initial findings show that in “strong inversion,” saturation, the drain current equation reveals a quartic-law dependency on gate-source overdrive voltage instead of square-law dependency seen in complementary metal-oxide semiconductor (CMOS) with no mobility reduction effects. This is postulated to result from the ZnO mobility showing a square-law increase with gate-source overdrive voltage. A “strong inversion,” saturation model having ±1.6% deviation from measured data is created in verilog-A to simulate and design circuits. Circuits include a fabricated and measured pixel driver circuit sinking 28 µA of current while only having a gate area of 20 µm2. This ZnO thin-film FET pixel driver is believed to have the highest current density reported at the time of this writing. Also, the first known ZnO thin-film FET analog amplifier is analytically designed for a gain of 3 V/V at 10 kHz while drawing only 8 µA of supply current. Finally, the first known ZnO thin-film FET low-noise RF amplifier is designed, utilizing scattering parameters measured at the Air Force Research Laboratory on a device with minimum channel length of 1.25 µm. This amplifier has a small-signal gain of 12.6 dB at 13.56 MHz, and a current drain of 268.4 mA at a drain voltage of 13 V

Active-matrix GaN micro light-emitting diode display with unprecedented brightness

Displays based on microsized gallium nitride light-emitting diodes possess extraordinary brightness. It is demonstrated here both theoretically and experimentally that the layout of the n-contact in these devices is important for the best device performance. We highlight, in particular, the significance of a nonthermal increase of differential resistance upon multipixel operation. These findings underpin the realization of a blue microdisplay with a luminance of 10⁶ cd/m²

Wide Bandgap Based Devices

Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits

Chip- and System-Level Reliability on SiC-based Power Modules

The blocking voltage, switching frequency and temperature tolerance of power devices have been greatly improved due to the revolution of wide bandgap (WBG) materials, such as silicon carbide (SiC) and gallium nitride (GaN). Owing to the development of SiC-based power devices, the power rating, operating voltage, and power density of power modules have been significantly improved. However, the reliability of SiC-based power modules has not been fully explored yet. Thus, this dissertation focuses on the chip- and system-level reliability on SiC-based power modules. For chip-level reliability, this work focuses on on-chip SiC ESD protection devices for SiC-based integrated circuits (ICs). In order to develop SiC ESD protection devices, SiC-based Ohmic contact and ion implantation have been studied. Nickel/Titanium/Aluminum (Ni/Ti/Al) metal stacks were deposited on SiC substrates to form Ohmic contact. Circular transfer length method (CTLM) structures were fabricated to characterize contact resistivity. Ion implantation was designed and simulated by Sentraurus technology computer aided design (TCAD) software. Secondary-ion mass spectrometry (SIMS) results show a good match with the simulation results. In addition, SiC ESD protection devices, such as N-type metal-oxide-semiconductor (NMOS), laterally diffused metal-oxide-semiconductor (LDMOS), high-voltage silicon controlled rectifier (HV-SCR) and low-voltage silicon controlled rectifier (LV-SCR), have been designed. Transmission line pulse (TLP) and very fast TLP (VF-TLP) measurements were carried out to characterize their ESD performance. The proposed SiC-based HV-SCR shows the highest failure current on TLP measurement and can be used as an area-efficient ESD protection device. On the other hand, for system-level reliability, this dissertation focuses on the galvanic isolation of high-temperature SiC power modules. Low temperature co-fired ceramics (LTCC) based high-temperature optocouplers were designed and fabricated as galvanic isolators. The LTCC-based high-temperature optocouplers show promising driving capability and steady response speed from 25 ºC to 250 ºC. In order to verify the performance of the high-temperature optocouplers at the system level, LTCC-based gate drivers that utilize the high-temperature optocouplers as galvanic isolators were designed and integrated into a high-temperature SiC-based power module. Finally, the high-temperature power module with integrated LTCC-based gate drivers was characterized by DPTs from 25 ºC to 200 ºC. The power module shows reliable switching performance at elevated temperatures

Chip- and System-Level Reliability on SiC-based Power Modules

The blocking voltage, switching frequency and temperature tolerance of power devices have been greatly improved due to the revolution of wide bandgap (WBG) materials, such as silicon carbide (SiC) and gallium nitride (GaN). Owing to the development of SiC-based power devices, the power rating, operating voltage, and power density of power modules have been significantly improved. However, the reliability of SiC-based power modules has not been fully explored yet. Thus, this dissertation focuses on the chip- and system-level reliability on SiC-based power modules. For chip-level reliability, this work focuses on on-chip SiC ESD protection devices for SiC-based integrated circuits (ICs). In order to develop SiC ESD protection devices, SiC-based Ohmic contact and ion implantation have been studied. Nickel/Titanium/Aluminum (Ni/Ti/Al) metal stacks were deposited on SiC substrates to form Ohmic contact. Circular transfer length method (CTLM) structures were fabricated to characterize contact resistivity. Ion implantation was designed and simulated by Sentraurus technology computer aided design (TCAD) software. Secondary-ion mass spectrometry (SIMS) results show a good match with the simulation results. In addition, SiC ESD protection devices, such as N-type metal-oxide-semiconductor (NMOS), laterally diffused metal-oxide-semiconductor (LDMOS), high-voltage silicon controlled rectifier (HV-SCR) and low-voltage silicon controlled rectifier (LV-SCR), have been designed. Transmission line pulse (TLP) and very fast TLP (VF-TLP) measurements were carried out to characterize their ESD performance. The proposed SiC-based HV-SCR shows the highest failure current on TLP measurement and can be used as an area-efficient ESD protection device. On the other hand, for system-level reliability, this dissertation focuses on the galvanic isolation of high-temperature SiC power modules. Low temperature co-fired ceramics (LTCC) based high-temperature optocouplers were designed and fabricated as galvanic isolators. The LTCC-based high-temperature optocouplers show promising driving capability and steady response speed from 25 ºC to 250 ºC. In order to verify the performance of the high-temperature optocouplers at the system level, LTCC-based gate drivers that utilize the high-temperature optocouplers as galvanic isolators were designed and integrated into a high-temperature SiC-based power module. Finally, the high-temperature power module with integrated LTCC-based gate drivers was characterized by DPTs from 25 ºC to 200 ºC. The power module shows reliable switching performance at elevated temperatures

EFFECTS OF INTERNAL FIELDS IN QUANTUM DOTS

In this work we study the effect of built in electrostatic fields in Quantum Dots. Built-in electrostatic fields in Zincblende quantum dots originate mainly from--(1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. We also study the geometric dependence of built in fields on 3 shapes namely Box, Dome and Pyramid. The main objectives are 3 fold they are (1) Explore the nature and the role of crystal atomicity at the interfaces and built-in fields (strain-field, and piezoelectric polarization) in determining the energy spectrum and the wave functions. (2) To identify the shift in the one-particle energy states, symmetry-lowering and non-degeneracy in the first excited state and strong band-mixing in the overall conduction band electronic states. (3) Finally geometric dependence of the above-mentioned phenomena. We discuss the importance atomistic effects and the need for 3 dimensional atomistic simulator NEMO 3D. We also discuss the effect of built in fields in HEMT (High Electron Mobility Transistor)

Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

High Performance Micro-scale Light Emitting Diode Display

Micro-scale light emitting diode (micro-LED) is a potentially disruptive display technology because of its outstanding features such as high dynamic range, good sunlight readability, long lifetime, low power consumption, and wide color gamut. To achieve full-color displays, three approaches are commonly used: 1) to assemble individual RGB micro-LED pixels from semiconductor wafers to the same driving backplane through pick-and-place approach, which is referred to as mass transfer process; 2) to utilize monochromatic blue micro-LED with a color conversion film to obtain a white source first, and then employ color filters to form RGB pixels, and 3) to use blue or ultraviolet (UV) micro-LEDs to pump pixelated quantum dots (QDs). This dissertation is devoted to investigating and improving optical performance of these three types of micro-LED displays from device design viewpoints. For RGB micro-LED display, angular color shift may become visually noticeable due to mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts. Based on our simulations and experiments, we find that the mismatched angular distributions are caused by sidewall emission from RGB micro-LEDs. To address this issue, we propose a device structure with top black matrix and taper angle in micro-LEDs, which greatly suppresses the color shift while keeping a reasonably high light extraction efficiency. These findings will shed new light to guide future micro-LED display designs. For white micro-LEDs, the color filters would absorb 2/3 of the outgoing light, which increases power consumption. In addition, color crosstalk would occur due to scattering of the color conversion layer. With funnel-tube array and reflective coating on its inner surface, the crosstalk is eliminated and the optical efficiency is enhanced by ~3X. For quantum dot-converted micro-LED display, its ambient contrast ratio degrades because the top QD converter can be excited by the ambient light. To solve this issue, we build a verified simulation model to quantitatively analyze the ambient reflection of quantum dot-converted micro-LED system and improve its ambient contrast ratio with a top color filter layer

High-Temperature Optoelectronic Device Characterization and Integration Towards Optical Isolation for High-Density Power Modules

Power modules based on wide bandgap (WBG) materials enhance reliability and considerably reduce cooling requirements that lead to a significant reduction in total system cost and weight. Although these innovative properties lead power modules to higher power density, some concerns still need to be addressed to take full advantage of WBG-based modules. For example, the use of bulky transformers as a galvanic isolation system to float the high voltage gate driver limits further size reduction of the high-temperature power modules. Bulky transformers can be replaced by integrating high-temperature optocouplers to scale down power modules further and achieve disrupting performance in terms of thermal management, power efficiency, power density, operating environments, and reliability. However, regular semiconductor optoelectronic materials and devices have significant difficulty functioning in high-temperature environments. Modular integration of optoelectronic devices into high-temperature power modules is restricted due to the significant optical efficiency drop at elevated temperatures. The quantum efficiency and long-term reliability of optoelectronic devices decrease at elevated temperatures. The motivation for this study is to develop optoelectronic devices, specifically optocouplers, that can be integrated into high-density power modules. A detailed study on optoelectronic devices at high temperature enables us to explore the possibility of scaling high-density power modules by integrating high-temperature optoelectronic devices into the power module. The primary goal of this study is to characterize and verify the high-temperature operation of optoelectronic devices, including light-emitting diodes and photodiodes based on WBG materials. The secondary goal is to identify and integrate optoelectronic devices to achieve galvanic isolation in high-density power modules working at elevated temperatures. As part of the study, a high-temperature packaging, based on low temperature co-fired ceramic (LTCC), suitable to accommodate optoelectronic devices, will also be designed and developed