Nano Vacuum Channel Transistors (NVCT)

The goal of Nano Channel Vacuum Transistor (NVCT) research is to develop and characterize three-terminal vacuum transistor devices that operate in ultra-high vacuum (UHV) and withstand temperatures up to 400 °C. The transistors consist of an insulated gate, an emitter array, and a collector. To avoid overheating the collector, the gate is pulsed from 0 to 40 V at a duty cycle of 10-20% while the emitter and collector are fixed DC voltages of 0V and 100 V, respectively. Current from emitter to collector is measured to obtain an output current – input voltage plot (I-V curve). The devices are heated using a molybdenum chuck inside the UHV chamber. After preliminary tests, the devices are moved to the UHV lifetime test chamber and run with pulsed gate voltage with fixed amplitude at constant temperature for hundreds of hours. Periodic IV sweeps are also conducted to observe changes. Factors such as overheating and arcing can lead to device degradation or failure. The goals of the project include designing driver systems for the devices, implementing automated Data Acquisition (DAQ) hardware to control and monitor testing systems, and using data to characterize the devices and determine approximate lifetime, maximum operating conditions, and failure conditions

Contract Farming and Food Security in Developing Economies: A Framework Model for Spillover Impact

Empirical literature on the effect of Contract Farming (CF) on economic development of a Less Developed Economy (LDC) is divided on the basic issue of concern for the policy makers in LDCs: should CF be encouraged, and if so, under what circumstances? Broadly, there are both intermediate (yield, price etc.) and ultimate (mainly household income and food security) benefits. However, the implication of the outcomes on welfare are not unidirectional. For instance, in most cases yield per hectare and household income of farmers increased along with rise in prices of crops. Das, Bhattacharya and Marjit (JRFM, 2023) builds a model to explore such adverse welfare impacts due to CF. This paper's focus is totally different. Also, there is no homogeneity in the sample of crops or the country of occurrence. Since most of these contracts are private in nature with a clear objective of profit maximization, the estimates could have self-section biases, which is rarely controlled for. Additionally, these are mostly in the nature of treatment/control group studies (though not RCTs). A fundamental issue is that spillover effects bias outcomes in these methods and it should be controlled for. This implies that there is virtually no empirical literature on spillover effects. Looking at it differently, these studies conclude that in the absence of spillover effects CF appears to be conditionally beneficial to LDCs. Given this background, this paper investigates: what are the nature of these conditions? To what extent do spillover effects relax them? Constructing a three-sector-four-factors general equilibrium model: agricultural with contract farming, traditional agriculture, and manufacturing, we derive the conditions under which it is conducive for low-income farmers. The objective is to prescribe a clear set of recommendations to the governments of the LDCs that are experimenting with CF on the nature of priors that they need to ensure for significantly increasing the probability of net benefit from CF

Improving Surface Finish of Nickel Bond Pads for Wire Bonding on High Temperature LTCC Devices

Low temperature co-fired ceramic (LTCC) is a material used primarily for packaging embedded circuits. It is resistant to high temperatures and can be used in harsh environments, as well as high vacuum applications. LTCC is designed to be cofired with silver conductive paste, however this paste can only withstand temperatures of up to 400C in vacuum. A solution to this is to cover exposed areas of silver paste with a high-performance nickel paste post fire. This paste can withstand higher temperatures in vacuum. High performance nickel paste is not designed to be cofired with LTCC, so a new fabrication process was developed. Furthermore, the bond pads created with the nickel paste have a higher surface roughness, making it difficult to perform wire bonding to them. A dry sanding technique was developed to prepare the new nickel pads for wire bonding. Surface roughness was then measured using atomic force microscopy. Further processing of the bond surface with ion milling would then be performed following the initial sanding

Simulation of a Radio-Frequency Wave Based Bacterial Biofilm Detection Method in Dairy Processing Facilities

This paper describes the principles behind the radio-frequency (RF) sensing of bacterial biofilms in pipes and heat exchangers in a dairy processing plant using an electromagnetic simulation. Biofilm formation in dairy processing plants is a common issue where the absence of timely detection and subsequent cleaning can cause serious illness. Biofilms are known for causing health issues and cleaning requires a large volume of water and harsh chemicals. In this work, milk transportation pipes are considered circular waveguides, and pasteurizers/heat exchangers are considered resonant cavities. Simulations were carried out using the CST studio suite high-frequency solver to determine the effectiveness of the real-time RF sensing. The respective dielectric constants and loss tangents were applied to milk and biofilm. In our simulation, it was observed that a 1 µm thick layer of biofilm in a milk-filled pipe shifted the reflection coefficient of a 10.16 cm diameter stainless steel circular waveguide from 0.229 GHz to 0.19 GHz. Further sensitivity analysis revealed a shift in frequency from 0.8 GHz to 1.2 GHz for a film thickness of 5 µm to 10 µm with the highest wave reflection (S11) peak of ≈−120 dB for a 6 µm thick biofilm. A dielectric patch antenna to launch the waves into the waveguide through a dielectric window was also designed and simulated. Simulation using the antenna demonstrated a similar S11 response, where a shift in reflection coefficient from 0.229 GHz to 0.19 GHz was observed for a 1 µm thick biofilm. For the case of the resonant cavity, the same antenna approach was used to excite the modes in a 0.751 m × 0.321 m × 170 m rectangular cavity with heat exchange fins and filled with milk and biofilm. The simulated resonance frequency shifted from 1.52 GHz to 1.54 GHz, for a film thickness varying from 1 µm to 10 µm. This result demonstrated the sensitivity of the microwave detection method. Overall, these results suggest that microwave sensing has promise in the rapid, non-invasive, and real-time detection of biofilm formation in dairy processing plants

Effect of Room Air Exposure on the Field Emission Performance of UV Light Irradiated Si-Gated Field Emitter Arrays

Field emission cathodes are promising candidates in nanoscale vacuum channel transistors and are used in microwave vacuum electron devices. Prior research has shown that UV light exposure as well as 350 °C vacuum bake can desorb water vapor from Si field emission tips, resulting in lower work function and improved emission performance. However, after long exposure to room air (greater than 24 h), the improved performance is lost as water adsorbs on the tips. In this study, experiments were carried on two sets of 1000 × 1000 Si-gated field emitter arrays to determine the length of time that emitters can be exposed to room air without degradation. First, the samples were exposed to UV light irradiation in vacuum, and the I–V curves were measured. Then, the samples were exposed to room air with a relative humidity ranging from 30% to 40% for varying times (5, 6, 8, 12, 24, and 48 h) and then tested again under high vacuum. It was found that the emission current did not degrade after room air exposure of 5 h. However, at 6 h of exposure, degradation started to occur, and after 24 h, the emission current went back to the original, pre-UV exposure case. In a separate experiment, UV irradiated samples were stored in nitrogen for 72 h, with a 10% degradation in current. These results demonstrate that field emission devices with improved performance resulting from water desorption can be handled in air up to 5 h, depending upon humidity and stored in nitrogen for 72 h while maintaining improved performance

LTCC MEMS Fabrication for High Temperature Vacuum Applications

Low temperature co-fired ceramic (LTCC) is a packaging material that has a strong resistance to high temperatures and harsh chemical environments. It is capable of creating a fully hermetic seal around any embedded circuitry, which makes it an ideal material for use in high temperature vacuum applications. However, the conductive pastes designed to be cofired with the LTCC are not rated for high temperature (up to 400°C) in a vacuum, so they are not suitable for designs that require conductors to be exposed to the external environment. The primary solution to this problem is the implementation of specialized conductive materials and sealants, but the use of these materials required the development of a new fabrication process because they are not designed to be directly compatible with LTCC

Complementary Vacuum Field Emission Transistor

A complementary vacuum field emission device structure is proposed, and its operation is analyzed by multiphysics simulation. A freely moving double-clamped cantilever, which balances the electrostatic attraction force and elastic restoration force, is used as the source electrode while the drain electrode is fixed. The electron-emitting cathode is formed on the source for the n-type device and on the drain for the p-type. Thus, complementary current–voltage characteristics are obtained only with Fowler–Nordheim tunneling electron transport. The impact of various design parameters such as vacuum gap, cantilever beam thickness, gate width, and tip offset on the drain and gate leakage currents is investigated. Inverter logic operation is verified successfully using complementary devices

LTCC Fabrication and Cold Plasma Application

Low Temperature Co-Fired Ceramics (LTCC) are a multilayer glass ceramic substrate that are especially useful in creating microelectronic devices. By combining individual layers that have different functionalities, LTCC technology offers many useful features such as low dielectric loss, multi-functional features through high integration, and the ability to be co-fired with highly conductive metals such as silver and gold. Using these properties our lab has been focusing on cold plasma production

Temperature Effects on Gated Silicon Field Emission Array Performance

Silicon field emitter arrays (Si FEAs) are being explored as an electron source for vacuum channel transistors for high temperature electronics. Arrays of 1000 × 1000 silicon tip based gated field emitters were studied by measuring their electrical characteristics up to 40 V of DC gate bias with a 1.3 mA emission current at different temperatures from 25 to 400 °C. At ∼350 °C, residual gas analyzer measurements show that water desorption and carbon dioxide partial pressures increase significantly, the gate to emitter leakage current decreases by more than ten times, and the collector current increases by more than ten times. These improvements remained after heat-treatment but were then lost once the device was exposed to the atmosphere for several days. The improvements could be recovered upon additional baking suggesting that adsorbates (primarily water) on the surface affected field emission and surface leakage. It was also found that after heat-treatment, the electrical characteristics of the devices exhibited \u3c 3% variation in collector current at 40 V, which (without exposure to the atmosphere) can be termed as a weak temperature dependence. These results suggest that Si FEAs could be viable as a high temperature transistor

Effect of Water Vapor Desorption on the Performance of Gallium Nitride Field Emitter Array

We are exploring the potential of gallium nitride (GaN) field emitter arrays in vacuum channel transistors. This study investigated the impact of ultraviolet (UV) light on the emission properties of large arrays of GaN field emitters. Arrays of 150 × 150 emitters were analyzed before and after UV exposure. With a constant collector voltage of 200 V DC, gate voltage sweeps from 0 to 60 V were applied. The initial I–V measurements showed a rapid increase in emission current, indicating a conditioning effect, settling at a stable value of 1.25 μA after three to five sweeps. Remarkably, exposure to UV light resulted in a fivefold increase in the maximum field emission current, reaching an impressive 6 μA. This significant enhancement highlights the potential of UV treatment for improving the performance of GaN-based field emitters. This surge in current can be attributed to the desorption of water vapor caused by the UV light. To compare with the heating-based water desorption technique, another array of 150 × 150 emitters was characterized before and after heating at 400 °C. While the collector voltage remained constant at 200 V DC, the gate voltage was systematically increased from 0 to 75 V in this experiment. This controlled sweeping of the gate voltage provided a precise method for characterizing the field emission properties of the GaN emitters. The I–V measurements revealed that, similar to the UV exposure case, collector current increased by approximately four times after heat treatment at 400 °C for 10 min. This resulted in a maximum field emission current of around 10 μA at 75 V. As with the UV case, this increase can also be attributed to surface desorption, primarily of water