INTEGRATED VACUUM TRANSISTORS AND FIELD EMITTER ARRAYS
Creators: name: Ghotbi, Shabnam
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Electrical engineering not elsewhere classified

description: The arrival of Si transistors and integrated circuit technology more than half a century ago made vacuum electronic technology almost extinct. Today, there are only a few niche applications for vacuum electronics. The main issues with this technology are its high voltage requirement and high-power consumption, difficult and costly fabrication technology, lack of integration capability, and poor reliability characteristics. Some of these issues may be addressed by going to nm scale fabrication that did not exist 60 years ago. Other problems such as reliability and lack of integration capability require alternative solutions to what has been proposed so far. Vacuum is the ultimate conduction media allowing electrons to reach the speed of light without any scattering. Consequently, a vacuum transistor, if designed correctly, can achieve THz frequency performance, while delivering Watt-level powers. No semiconductor technology can compete with vacuum technology to deliver such performance. In this work, novel methods for implementing nanoscale field emitter arrays used in vacuum electronics are proposed. Gated and ungated field emitters are fabricated with self-assembly technology and electron beam lithography. Different anisotropic dry etching recipes are developed to achieve emitters with different sharpness and aspect ratios. Our methods lead to field emitter array operation under low voltages (less than 20 V) and high current densities (around 50 A/cm2) using self-assembly and soft film anode-cathode isolator, and field emitter devices with ~4.5 A/cm2 current density with a turn-on voltage less than 50 V using electron beam lithography and oxide anode-cathode isolator. Making reliable field emitter devices is challenging. Due to Joule heating, ion bombardment, and geometrical variations for each tip in the field emitter arrays, emission current becomes nonuniform across the array. Sharper tips emit at a higher rate and eventually, the heat generated at the tip deforms the tips leading to electron emission at a lower rate. With ultra-low doped emitters, the current of each tip is limited to a few nano-amperes leading to a negligible current fluctuation at the tips. Our fabricated ultra-low doped devices with both self-assembly and electron beam lithography techniques presented constant emission current with almost no change over 24 hours of continuous operation. Such excellent reliability characteristics in vacuum field emitter devices have not been demonstrated to date. The screening effect in close-packed field emitter arrays which occurs by nearby conductive or semiconductive objects is thoroughly investigated and different solutions are proposed to reduce this effect between the emitters. Simulation studies using Sentaurus TCAD, MATLAB, and COMSOL Multiphysics simulators facilitated the design and optimization of gated and ungated field emitter arrays. These studies included the effect of sharpness, the distance between neighboring emitters, enclosing the emitters by a Si block around the emitters as well as anode-cathode separation on the electrical characterization of field emitter arrays. The optimum location and operating voltages which lead to a maximum gate control and emitter current density are also studied for gated field emitter arrays. Instead of individually gating each field emitter, it was found that controlling the emission of a sub-array with a metallic all-around gate is more efficient and it leads to higher current densities. Guided by simulations, gated field emitter arrays with 5×5 and 2×2 sub-arrays are developed. In terms of strength of the grid control (transconductance), turn-on voltage, maximum emission current, and field intensification factor, the device with the 2×2 sub-array was superior to the one with the 5×5 sub-array. The VFET with 5×5 sub-arrays achieved a higher current density due to a larger number of field emitters packed per active emission area. Finally, plans to further improve the technology and transitioning into the fabrication of vacuum integrated circuits are discussed.