Field-Emitter-Array Cold Cathode Arc-Protection Methods - A Theoretical Study
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Field-Emitter-Array Cold Cathode Arc-Protection Methods - A Theoretical Study L. Parameswaran, R.A. Murphy MIT Lincoln Laboratory, Lexington MA 02420-9108 ABSTRACT Field-emitter arrays (FEAs) are desirable for use as electron emitters in microwave-tube amplifiers because they can provide such advantages as higher efficiency and faster turn-on compared to their thermionic counterparts. Calculations have shown that Spindt-type metal and semiconductor emitters operate well below intrinsic current limits due to thermal effects, even for high-current applications such as klystrodes, twystrodes, and traveling-wave amplifiers. Nevertheless, the primary barrier to FEA utilization in such applications is premature failure due to arcing. These failures appear to be produced by ionization of gas molecules and/or desorbed contaminants, which are exacerbated by a poor vacuum environment. Lifetime and stability issues have been largely resolved for less stringent applications, such as flat-panel displays, through the use of integrated passive resistors that provide current limiting. However, such an approach is not directly compatible with operation at high frequency and current density. Other more complex approaches, such as the incorporation of active control in the form of integrated transistors, have also been demonstrated, but again, only for FEAs used in displays. This paper will review some of these schemes in the context of their efficacy in improving lifetime and stability of FEA cold cathodes in high-frequency applications. A theoretical analysis will be given of the effect on highfrequency performance of incorporating arc protection structures into Spindt-type metal FEAs. Specifically, two approaches will be considered: passive protection schemes such as the use of a modified thin film resistive layer, and active schemes such as FETs and saturated current limiters. INTRODUCTION Field emitter arrays have been the subject of interest as electron sources for several decades, and have been considered for a variety of applications. The most successful application to date has been their use in flat panel displays [1], but more recently other more stringent applications have been investigated, such as their use as ion sources for electric propulsion systems [2], electron-beam lithography [3] and vacuum pressure sensors [4]. A few of these applications are illustrated in Figure 1, which gives estimates of the ranges of current levels, operational frequencies and ambients in which the FEAs are required to function.
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Figure 1. Applications for field emitter arrays. Significant effort has been expended in developing FEAs as cold cathodes for high frequency inductive output amplifiers (IOAs) such as klystrodes, twystrodes, and travelling wave tubes. The gated FEA is capable of very high current densities, and can provide several advantages over its thermionic counterpart. Microfabrication techniques [5] have enabled the fabrication of extremely small devices with short gate-to-emitter lengths, resulting in low electron tra
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