The use of vacuum tubes as power amplifiers in microwave systems may be on their way out following the development by researchers from Agere Systems Inc. of a solid-state microtriode that can be combined with conventional silicon ICs.
At very high frequencies, conventional transistors are limited by the speed at which charge carriers move through their semiconducting materials. That is not the case for vacuum tubes, leading to their continued use in microwave frequency systems.
The Agere device promises to combine the high frequency and power characteristics of vacuum tubes with the reliability and ease of production of silicon transistors.
Traditional vacuum tubes rely on thermionic electron emission from hot emitters. This limits their operational lifetimes and makes them difficult to miniaturize.
Some researchers have looked at smaller scale devices where the electron supply comes from 'Spindt tips,' arrays of metallic or silicon needle-like structures that emit electrons through geometric field enhancement as a result of their small tip radius.
But these devices have poor long-term emission stability because the tips spoil after long periods of use. To overcome these problems, the Agere team used carbon nanotube emitters, which offer high electron emission currents with stability superior to Spindt tip emitters.
These microtriodes are three-terminal devices with a similar design to three-terminal vacuum tubes, but they are created on a silicon substrate using micro-electromechanical systems (MEMS) techniques. The lateral arrangement of the gates results in a simpler and more flexible design than previous vertical structures.
Although the initial results are modest, the team believes its work represents a first step in the integration of modern MEMS fabrication techniques, new nanotube growth processes and mature silicon processing technologies. They expect improvements in device characteristics and stability to enable these devices to move into high-frequency, high-power electronics.