Christopher E. Holland

Field-emitter arrays for vacuum microelectronics

An ongoing program on microfabricated field-emitter arrays has produced a gated field-emitter tip structure with submicrometer dimensions and techniques for fabricating emitter arrays with tip packaging densities of up to 1.5*10/sup 7/ tips/cm/sup 2/. Arrays have been fabricated over areas varying from a few micrometers up to 13 cm in diameter. Very small overall emitter size, materials selection, and rigorous emitter-tip processing procedures have contributed to reducing the potential required for field emission to tens of volts. Emission current densities of up to 100 A/cm/sup 2/ have been achieved with small arrays of tips, and 100-mA total emission is commonly produced with arrays 1 mm in diameter containing 10000 tips. Transconductances of 5.0 mu S per tip have been demonstrated, indicating that 50 S/cm/sup 2/ should be achievable with tip densities of 10/sup 7/ tips/cm/sup 2/. Details of the cathode arrays and a variety of performance characteristics are discussed. >

100 W Operation of a Cold Cathode TWT

Recent demonstration of low-voltage high-transconductance field emitter array operation holds promise for the successful development of reliable cold cathode vacuum electronics device technologies. This paper reports on the experimental studies of implementation of such field emitter arrays as the electron source for a moderate power traveling wave tube (TWT) operating in the C-band frequency regime. The cold cathode TWT has operated for over 150 h at duty factors up to 10%, beam currents up to 121 mA, and RF powers up to 100 W at 5 GHz. High cathode current densities of 15.4 A/cm2 were achieved concurrent with excellent beam control, resulting in 99.4% beam transmission under zero-RF-drive operating conditions and 97.3% transmission at maximum RF output power. The cathode is shown to operate with a 72% reduction in the operating voltage from the previous generation of emitters fabricated by SRI International, bringing the operating voltage for full current operation well below 100 V. Extensive device characterization and life testing has been performed, and interesting variation in cathode performance was observed during the high-duty high-current portion of the testing program. The results presented here represent the highest current, highest power, and highest duty factor ever reported for an RF vacuum device employing a field emission cold cathode electron source.

Field emitter array development for microwave applications. II

Microfabricated field emitter arrays are being used in an ongoing DARPA/NRL program as a means for gating or prebunching electrons in a microwave amplifier tube. The goals of the program are to demonstrate 10 dB gain at 50 W and 10 GHz in a gated Klystrode amplifier tube with 50% efficiency. The proposed cathode specifications call for 160 mA peak emission and 10 GHz emission modulation from an annular emitter array having a 600 μm outer diameter and an inner diameter to be determined by transconductance and capacitance requirements. Experimental results have shown an average array capacitance of 6 nF/cm2, and that a transconductance of 1 μS/tip can be achieved at emitter–tip loadings of 10 μA/tip. Calculations based on these results show that emitter arrays having 0.4 μm diam gate apertures on 1 μm centers, a 600 μm outer diameter, and a 560 μm inner diameter should meet the tube specifications. Such arrays have been fabricated and shown to have essentially the same characteristics (Fowler/Nordheim coeff...

Field-emitter arrays to vacuum fluorescent display

A thin vacuum-fluorescent display utilizing a matrix-addressable array of groups of Spindt-type field-emission emitter tips is described. Development of the display required expanding the cathode fabrication technology to cover 5-in-diameter areas uniformly with field-emitter arrays. Each color element of the display is addressed by a group of a few hundred emitter tips operating in parallel. Electrons emitted from the tips are proximity-focused on each color element. Because each pixel is essentially a miniature cathode-ray tube (CRT), the device has the potential to produce brightness similar to those on a CRT in a panel that is 3.3 in square (8.4 cm) in area and 0.15 in (4 mm) thick. >

Field‐emitter‐array development for high‐frequency operation

Power amplification at high frequencies is one of many applications under investigation using vacuum microelectronics technology. The results of an ongoing program to develop a gigahertz amplifier using Spindt‐type field‐emitter arrays are reported. The maximum frequency at which a field‐emission source in the form of a Spindt‐type cathode array can be operated is determined by the capacitance of the structure and the transconductance of the array. It has been shown that by fabricating arrays with high emitter‐tip packing densities and small total areas it should be possible to achieve operation in the gigahertz range. Structures having a capacitance in the range of 0.1 pF have been fabricated, and total emission currents of 25 mA with current densities of 1000 A/cm2 have been demonstrated with a 0.1 pF structure having 625 emitter tips (40 μA/tip). The transconductance under these operating conditions was 500 μs or 0.8 μS/tip. Simultaneous experiments with our standard low‐frequency Spindt‐type cathode g...

Experimental demonstration of an emission-gated traveling-wave tube amplifier

This paper reports the results of the development of a traveling-wave tube (TWT) amplifier designed and operated using a high-frequency emission-gated field emitter array (FEA) cold cathode. The TWT was conservatively designed to operate with only 1% cathode current modulation but results show that 30% modulation of the current was achieved in the C-Band frequency range. The emission-gated TWT prototype was operated up to a current of 5 mA and RF output power of 280 mW using a 300-/spl mu/m diameter FEA cathode having 10 000 emitter tips with testing performed in single-pulse mode using 100-/spl mu/s pulses. Excellent beam control was demonstrated under all experimental conditions tested. Simulation shows that, with the same TWT circuit and demonstrated cathode modulation level, a 1-mm diameter cathode would generate /spl sim/60 W of output power in the same frequency band and /spl sim/80 W if the circuit were optimized for the measured level of modulation. Measurements also show that performance of the device does not degrade with frequency up to at least 7.0 GHz, which is the maximum operating frequency of the TWT. Cold measurements of the FEA electron gun alone indicate operation of the cathode up through 20 GHz might be possible. These results represent the first operation of an emission-gated cathode in a TWT and the highest power operation ever recorded in a microwave vacuum device using an emission-gated electron source.

Field emission cathode array development for high-current-density applications

Abstract This paper reports on the present status of the field emission cathode array [1] development program at SRI International. Recent efforts have been directed toward improving the fabrication technology with regard to uniformity and yield, studying means of increasing the packing density of the emitter tip arrays, and investigating high-current-density operation. The results show that cathodes of this type can achieve current densities of more than 50 A/cm 2 , and that lifetimes of well in excess of 50,000 h can be expected under controlled conditions.

High current, high current density field emitter array cathodes

Microfabricated field emitters have shown the potential for very high current densities (>100A∕cm2) and total emission currents (>1A). However, realizing this potential has been elusive, primarily because these cathodes exhibit insufficient emission uniformity over an emitter array. In this article we report the development of an in situ processing method based on emitter tip self-heating during operation that is shown to improve emission uniformity between emitter tips. Two tips differing in emission current by three orders of magnitude for a given voltage as fabricated are shown to be essentially identical in their emission characteristics after controlled pulsing to very high emission current. When the method was applied to a 50 000 tip array, it produced 300 mA of emission (40A∕cm2). The experimental arrangement prevented advancing to higher emission levels due to space charge limitations. It is expected that 1 A of emission at ∼100A∕cm2 is possible with appropriate modifications to the experimental a...

Cesiated thin-film field-emission microcathode arrays

The effect of adsorbed cesium on the performance of thin‐film field‐emission microcathode arrays with molybdenum tips is discussed. A reduction in the voltage required to extract an average of 1 nA per tip from 50 to 15 V is reported. Cesiated arrays have exhibited stable performance when operated continuously for up to 650 h at array current densities up to 100 A/cm2, with minimal leakage current. Using the Fowler–Nordheim theory, analysis of current‐voltage characteristics of a heavily cesiated emitter array gives values for the emitting area, work function, and geometrical factor.

A study of field emission microtriodes

Structures simulating integrated field emission microtriodes have been successfully fabricated and tested. These microtriodes use Spindt-type molybdenum field emission cathodes with the anodes in three different configurations. The anodes are located at a distance d>5 mm, approximately 1.25 mm, and approximately 2-20 mu m from the gate electrode and the emitter tip. The effects of device operation on the performance of the Mo field emission tips operating in close proximity to anodes were presented. A transconductance of 0.5 mS (5 mu S/tip or 12 S/cm/sup 2/) and a cutoff frequency of 0.8 MHz have been measured. Extension of these results indicates that transconductances of 250 mS/mm and cutoff frequencies of 100 GHz may be possible. >