Conventionally, MOSFET integrated circuits using Si substrates are employed for logical operation. Since these are solid-state devices, they are easy to handle and GHz operating frequencies have been achieved by the development of miniaturization techniques. Also, logical operation elements such as NOT, NOR and NAND gates and logical operation circuits by coupling these have been conceived and designed by use of MOS devices mainly.
To operate at higher frequencies than MOSFETs using Si substrates, chemical compound semiconductors such as GaAs are employed, and for specific applications, high-frequency amplification, vacuum valves are employed. Use of vacuum valves utilizes the fact that the mobility of carrier electrons is greater in vacuum than in the solid state.
Vacuum valves mostly utilize thermionic emission but Spindt type field emission type cold cathodes, in which a high current density can be obtained with low current consumption, may also be employed. High-frequency operation of such Spindt type field emission type cold cathodes is disclosed for example in: Journal of Vacuum Science and Technology B, 14 (1996) 1986. An example of use of such field emission type cold cathodes in integrated circuits is also disclosed in for example Japanese Patent Publication No. H. 11-329263.
However, there are restrictions on electron/hole mobility in solid-state devices made of for example Si that are currently widely employed in logical operation. Although progress has been made in achieving higher speeds by development of miniaturization techniques such as lithography, there are still limits on operating frequency.
On the other hand, there are problems regarding the degree of integration of vacuum valves capable of high-frequency operation. Also, since a Spindt type microfabricated cold cathode having a large number of elements in a small area has the property of large current density, applications have concentrated on for example microwave tubes or displays rather than logical operation.
Also, the operating voltage of these was high. This therefore led to the problems of large power consumption and withstanding voltage when reverse bias was applied. In addition, they could not be battery-driven and so could not be made portable.
When the field emission type cold cathode described in the aforementioned Japanese Patent Publication No. H11-329263 is employed in an integrated circuit, the element is of a planar form, so considerable area is required for the element layout. Japanese Patent Publication No. H11-329263 doesn't refer to a specific method of achieving integration when electron emission elements are connected for multi-stage operation.
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