Amorphous Silicon Thin Film Transistor Fabrication and Models

One of the primary purposes of this research was to develop techniques to improve the quality of vacuum evaporated amorphous silicon (a-Si), i.e. lower the density of localized states in the mobility gap. The electron beam evaporation of amorphouss silicon and hydrogenation by ion implanting has proved promising. This technique permits independent control of amorphous silicon disorder and the hydrogenation level, thereby separating the process of hydrogenation from that of film deposition. Electrical measurement of field effect conductance changes was used as a probing tool to monitor changes in the properties of a-Si before and after hydrogenation. Field effect data was transcribed by a computer program to determine the density of localized states. Amorphous silicon films were prepared by electron beam evaporation of a high purity silicon onto the surface of a thermally oxidized crystalline silicon substrate. The films were deposited at a fixed rate in a high vacuum. Immediately after deposition, some films were subjected to in situ thermal anneal and some films were not. A Comparison of the results of these two eases revealed the porous nature of evaporated a-Si. Hydrogen incorporation into a- Si films was performed by ion implantation followed by a low temperature thermal activation of the hydrogen. After hydrogenation, a field effect conductance change of four orders of magnitude was observed on the devices which were not in situ thermally annealed. A comparison before and after hydrogenation demonstrates that almost three orders of magnitude reduction (from about 1022 to about 1019/cm3-eV) in the density of localized states near the Fermi level (N F/T) was achieved. Varying the hydrogen implantation dosage between lxlO16 to 1.5xl017/cm2, with all other sample preparation procedures fixed, caused a decrease in NF/T from 8.6xl020 to ixl019/cm3-eV. The effect of in situ thermal annealing prior to hydrogen implantation was also investigated. By performing a 400°C anneal for four hours immediately following film deposition the film porosity was greatly reduced. The film was then implanted with hydrogen to a total dose of lxl017/cm2. A field effect Conductance change of six orders of magnitude was observed which yielded a N|F/T of 4xl017/cm3-eV, approaching that of glow discharge produced films. The second purpose of the research was to develop modeling techniques for the a-Si:H TFT. Despite rapid progress in the TFT performence, [performance] the theoretical basis to determine static- and dynamic-characteristics of TFTs has not yet been determined mainly because the influence of the localized states on TFT operation is very complicated. The theoretical expression of drain current as a function of gate bias and drain voltage was derived. To use the theoretical expressions, the localized state density distribution N(E) must be known, A derived yet practical formula for the N(E) did not exist. A common way is to use the experiment of field effect conductance change to determine the N(E), With the data theoretical expressions the localized state density N(E) could be calculated by using a numerical technique, but it is cumbersome and connot [cannot]be determined uniquely. As a design tool for devices and circuits, a simple theory which can express concisely the TFT characteristics is very important. In this report, several models for N(E) are listed:. Approximate analyses for characteristics pf a-Si:H TFT are derived. In two special cases, i.e. uniform localized state density distribution and exponential localized distribution, some useful approximate expressions was obtained. Compared with the experiment data, the uniform density distribution of localized state model is a good approximate expression for a large density discribution [distribution] of localized states near Fermi level. The exponential model is a good approximate expression for lower density distribution of localized states near Fermi level

Intelligent Chemical Sensor Systems for In-space Safety Applications

Future in-space and lunar operations will require significantly improved monitoring and Integrated System Health Management (ISHM) throughout the mission. In particular, the monitoring of chemical species is an important component of an overall monitoring system for space vehicles and operations. For example, in leak monitoring of propulsion systems during launch, inspace, and on lunar surfaces, detection of low concentrations of hydrogen and other fuels is important to avoid explosive conditions that could harm personnel and damage the vehicle. Dependable vehicle operation also depends on the timely and accurate measurement of these leaks. Thus, the development of a sensor array to determine the concentration of fuels such as hydrogen, hydrocarbons, or hydrazine as well as oxygen is necessary. Work has been on-going to develop an integrated smart leak detection system based on miniaturized sensors to detect hydrogen, hydrocarbons, or hydrazine, and oxygen. The approach is to implement Microelectromechanical Systems (MEMS) based sensors incorporated with signal conditioning electronics, power, data storage, and telemetry enabling intelligent systems. The final sensor system will be self-contained with a surface area comparable to a postage stamp. This paper discusses the development of this "Lick and Stick" leak detection system and it s application to In-Space Transportation and other Exploration applications

Silicon Carbide-Based Hydrogen and Hydrocarbon Gas Detection

Hydrogen and hydrocarbon detection in aeronautical applications is important for reasons of safety and emissions control. The use of silicon carbide as a semiconductor in a metal-semiconductor or metal-insulator-semiconductor structure opens opportunities to measure hydrogen and hydrocarbons in high temperature environments beyond the capabilities of silicon-based devices. The purpose of this paper is to explore the response and stability of Pd-SiC Schottky diodes as gas sensors in the temperature range from 100 to 400 C. The effect of heat treating on the diode properties as measured at 100 C is explored. Subsequent operation at 400 C demonstrates the diodes' sensitivity to hydrogen and hydrocarbons. It is concluded that the Pd-SiC Schottky diode has potential as a hydrogen and hydrocarbon sensor over a wide range of temperatures but further studies are necessary to determine the diodes' long term stability

SiC-Based Gas Sensors

Electronic grade Silicon Carbide (SiC) is a ceramic material which can operate as a semiconductor at temperatures above 600 C. Recently, SiC semiconductors have been used in Schottky diode gas sensor structures. These sensors have been shown to be functional at temperatures significantly above the normal operating range of Si-based devices. SiC sensor operation at these higher temperatures allows detection of gases such as hydrocarbons which are not detectable at lower temperatures. This paper discusses the development of SiC-based Schottky diode gas sensors for the detection of hydrogen, hydrocarbons, and nitrogen oxides (NO(x)). Sensor designs for these applications are discussed. High sensitivity is observed for the hydrogen and hydrocarbon sensors using Pd on SiC Schottky diodes while the NO(x) sensors are still under development. A prototype sensor package has been fabricated which allows high temperature operation in a room temperature ambient by minimizing heat loss to that ambient. It is concluded that SiC-based gas sensors have considerable potential in a variety of gas sensing applications

A Hazardous Gas Detection System for Aerospace and Commercial Applications

The detection of explosive conditions in aerospace propulsion applications is important for safety and economic reasons. Microfabricated hydrogen, oxygen, and hydrocarbon sensors as well as the accompanying hardware and software are being, developed for a range of aerospace safety applications. The development of these sensors is being done using MEMS (Micro ElectroMechanical Systems) based technology and SiC-based semiconductor technology. The hardware and software allows control and interrocation of each sensor head and reduces accompanying cabling through multiplexing. These systems are being, applied on the X-33 and on an upcoming STS-95 Shuttle mission. A number of commercial applications are also being pursued. It is concluded that this MEMS-based technology has significant potential to reduce costs and increase safety in a variety of aerospace applications

A Hazardous Gas Detection System for Aerospace and Commercial Applications

The detection of explosive conditions in aerospace propulsion applications is important for safety and economic reasons. Microfabricated hydrogen, oxygen, and hydrocarbon sensors as well as the accompanying hardware and software are being developed for a range of aerospace safety applications. The development of these sensors is being done using MEMS (Micro ElectroMechanical Systems) based technology and SiC-based semiconductor technology. The hardware and software allows control and interrogation of each sensor head and reduces accompanying cabling through multiplexing. These systems are being applied on the X-33 and on an upcoming STS-95 Shuttle mission. A number of commercial applications are also being pursued. It is concluded that this MEMS-based technology has significant potential to reduce costs and increase safety in a variety of aerospace applications

Chemical Gas Sensors for Aeronautic and Space Applications 2

Aeronautic and space applications require the development of chemical sensors with capabilities beyond those of commercially available sensors. Areas of interest include launch vehicle safety monitoring, emission monitoring, and fire detection. This paper discusses the needs of aeronautic and space applications and the point-contact sensor technology being developed to address these needs. The development of these sensors is based on progress in two types of technology: 1) Micromachining and microfabrication technology to fabricate miniaturized sensors. 2) The development of high temperature semiconductors, especially silicon carbide. Sensor development for each application involves its own challenges in the fields of materials science and fabrication technology. The number of dual-use commercial applications of this microfabricated gas sensor technology make this area of sensor development a field of significant interest

Ambipolar charge transport in organic field-effect transistors

Published versio

4H-SiC JFET Multilayer Integrated Circuit Technologies Tested Up to 1000 K

Testing of semiconductor electronics at temperatures above their designed operating envelope is recognized as vital to qualification and lifetime prediction of circuits. This work describes the high temperature electrical testing of prototype 4H silicon carbide (SiC) junction field effect transistor (JFET) integrated circuits (ICs) technology implemented with multilayer interconnects; these ICs are intended for prolonged operation at temperatures up to 773K (500 C). A 50 mm diameter sapphire wafer was used in place of the standard NASA packaging for this experiment. Testing was carried out between 300K (27 C) and 1150K (877 C) with successful electrical operation of all devices observed up to 1000K (727 C)

Low Earth Orbit Space Environment Testing of Extreme Temperature 6H-SiC JFETs on the International Space Station

This paper reports long-term electrical results from two 6H-SiC junction field effect transistors (JFETs) presently being tested in Low Earth Orbit (LEO) space environment on the outside of the International Space Station (ISS). The JFETs have demonstrated excellent functionality and stability through 4600 hours of LEO space deployment. Observed changes in measured device characteristics tracked changes in measured temperature, consistent with wellknown JFET temperature-dependent device physics