This work reviews the progress in the area of high current density electron emitters, specifically for use in high frequency and high-power microwave vacuum electron devices. The review is divided into two subsections: 1) thermionic cathodes and 2) field emission cathodes. The thermionic cathode section includes the discussion on the M-type, MM-type, Scandate and the CPD cathodes. CPD cathode promises better usability in a practical device as compared to the Scandate cathodes which are limited by emission nonuniformity, poor resistance to poisoning/ion bombardment and low life. With the increasing demand for miniaturized and high power VEDs in the near future, the development of high current density field emission cathodes is of immediate interest to the VED community. The Spindt, CNT, and graphene-based emitters are the potential candidates in the field emitters section. Graphene-based film cathodes, when compared to the Mo/Si FEAs and the CNTs, offer larger emission area as well as high current carrying capability. This review includes the evolution of the emission capabilities of the emitters, state-of-the-art performances, and possible future developments.

A Review of Electron Emitters for High-Power and High-Frequency Vacuum Electron Devices

The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals

Scandate cathode made out of tungsten nanoparticles have several advantages such as high current density, uniform emission, and long life over conventional cathode. Fabrication of porous tungsten pellet out of tungsten nanoparticles by pressing and sintering is a critical challenge. Cold pressing followed by sintering of nanoparticles results shrinkage of pellet size and uncontrolled porosity, which motivate us to work on sintering of nanoparticle. A new chemical synthesis approach was adopted for precisely controlling the powder particle parameters. The present technology relates to the preparation of an ultra-fine nano-scandate powder using chemical route, dissolving tungsten, and rhenium powder in special solvent under ultra-agitation, wherein the other constituents (such as Ba, Ca, Sc, and alumina) are uniformly distributed throughout the powder. Dissolving tungsten and rhenium in the solution, and further sol–gel process, resulted in the formation of spherical shaped particles. Nanoparticles were characterized using: (1) X-ray diffraction (XRD) for phase purity of powder, (2) Field Emission Scanning Electron Microscopy (FE-SEM) for particle size and distribution, (3) Scanning Electron Microscopy (SEM) to study the surface micro-features such as pore dimension and pore distribution and (d) Energy Dispersive Analysis of X-rays (EDAX) for elemental composition. The results showed that the powder has uniform grain size with an average particle diameter of ~100 nm. In this proposed work, rhenium has been co-doped with scandia doped tungsten nanoparticles in order to improve shrinkage. Addition of rhenium in scandia doped tungsten nanoparticles showed about 10% less shrinkage compared with scandia doped tungsten nanoparticle.

Synthesis of Rhenium–Scandia doped tungsten nanoparticles for shrinkage investigation

Design and development of thermionic emission microscope for the characterization of multi-beam cathode

Fabrication and enhancement of a new high-current density scandate electron emitter for a pulsed Pierce electron gun

Nanoscandate cathode (NSC) has several advantages over a conventional scandate cathode in terms of emission uniformity, resistance to ion bombardment, and long life In the present technology of “Yttrium-doped NSC,” yttrium oxide (Y2O3) is codoped with tungsten powder, scandium oxide, and other impregnant materials to make nanopowder using the solgel technique The pellets, made out of powder, were integrated with potted heaters The cathodes were analyzed in an analytical chamber containing: 1) Auger electron spectroscopy for surface analysis; 2) anode for emission measurements; and 3) thermionic emission microscope (THEM) for studying emission uniformity It was found that by adding a small amount of yttrium oxide of 3%–5% by weight increases the emission uniformity of cathode significantly as observed through THEM Emission current density of more than 80 A/cm2 was drawn from these cathodes under deep space charge conditions

Study and Development of Yttrium-Doped Nanoscandate Cathode for High-Power VEDs

Design and Analysis of a Field Plate Engineered High Electron Mobility Transistor for Enhanced Performance

This study investigates the feasibility of Nano gap embedded AlGaN/GaN HEMT to sense antibody-antigen reaction. The sensor can detect the binding response of a typical antibody Ixodes ricinus immunosuppressor (anti-Iris) protein in Nano cavity. The sensor performance is analyzed by the change in electrical characteristics by the anti-iris antibody and iris antigen interaction. The device performance is optimized independently by varying cavity thickness, cavity length, biomolecule length, and Al composition. The output drain current recorded at V DS = 10V was linearly incremented with the length of the bio layers. The increase in cavity thickness and length will reduce the sensitivity due to a reduction in the cavity fill-in factor. The maximum sensitivity obtained is 169% at a cavity thickness of 10 nm and cavity length of 100 nm with 30% Al content. Silvaco Atlas simulation software has simulated and optimized the proposed device structure. • The antigen-antibody reaction with thiol modified surface has been analyzed using AlGaN/GaN HEMT. • The effect of cavity thickness, cavity length, and biolayers length has been analyzed. • The effect of cavity fill factor has been analyzed by changing the biolayers length to analyze sensor's performance. • The maximum drain current sensitivity obtained is 169%. 

A highly sensitive Nano Gap Embedded AlGaN/GaN HEMT sensor for Anti-IRIS antibody detection

AlGaN/GaN high electron mobility transistors (HEMTs) demonstrate exceptional properties desired for sensing regime applications due to their extraordinary chemical stability, non-toxicity, surface amenable to functionalization, high surface charge sensitivity and high temperature endurance. This report provides a comprehensive review on the AlGaN/GaN HEMT based solid-state microsensors used for detection of gases, biomarkers, ions and high energy radiation which has application in environmental, clinical and water quality monitoring besides medical research. The material properties and fabrication aspects are highlighted first and then sensing mechanisms for different applications and various device design advancements during past decades in literature are discussed along with the modelling and simulation perspectives. There are some reports which discuss AlGaN/GaN HEMT for a particular application only and thus there was a need for a review which discusses various aspects and applications in one study to render an overall bird's eye view of potential of this material system. 

R. K. Poonia

Papers

Study and Development of Yttrium-Doped Nanoscandate Cathode for High-Power VEDs

Design and Analysis of a Field Plate Engineered High Electron Mobility Transistor for Enhanced Performance

A highly sensitive Nano Gap Embedded AlGaN/GaN HEMT sensor for Anti-IRIS antibody detection

AlGaN/GaN high electron mobility transistor for various sensing applications: A review