The Broadband Field-emission Microtube Incorporating A Virtual Cathode

Abstract

There has been considered the possibility of the use of the well-known techniques of increasing broadbandness of electronic tubes for vacuum microelectronic devices incorporating Gel-emission matrix cathodes. It has been shown, that the application of the virtual cathode and focusing lenses makes it possible to increase the transconductance of anode-to-grid characteristics (dozen of times), and to lower the control electrode capacity (a few times), if enables the increase in the broadbandness of the microtubes on the field-emission matrix cathodes by an order of magnitude. Broadbandness is one of the most important parameters of the tube. It determines the possibility to amplify an elecrtic signal by the tube within a frequency band and defines the ultimate frequencyfT, where the tube gain becomes equal to 1. The study of the tube operation in various circuits of it connection has shown, that its broadbandness mainly depends on the structural design and modes of the supply. Generally, the broadbandness is astimated as follows: fT=S/27cC where S is the transconductance of anodeto grid characteristic S-dJJdU, (la is the tube anode current; Ug is the control grid vo1tage);C is the contol grid capacitywith respect to the electrodes around it. From the approximate theory of the electronic grid tubes, based on the planar electrode models, is follows [I that the tube broadbandness is estimated as follows: frM (jc)1'3/(dcg) where M is cathode (JJ-to-anode (Ja) current passage factor : M= JJJc; jc is the cathode current density; dcg is the grid-to-cathode dostance. The above formula shows the possibility of using a number of techniques to increase broadbandness. The first techniques represents the reduction of distance between the cathode and the control grid. The principal concept of the implementation of the technique is that the actual cathode is moved into some other olace in order to create a virtual cathode as a cloud of the spatial charge. In this case the distance between the control electrode and the virtual cathode can be rather small. The technique of virtual cathode has been tested in the electronic tubes and has brought positive results. In practice, the above