Do Fleming valves fail at high frequencies?

[awygle]

So I've been reading up on optical rectennas, and the sources I read say that the reason such devices are not yet efficient enough to be practical isn't the antenna, it's the rectifier, since solid-state Schottky diodes of the types normally used don't work in the THz range of AC frequencies generated by the antennas without large power losses. Apparently MIM diodes have been shown to work up to about 150THz, but the ideal wavelengths of light for solar power would produce 190 - 750 THz.

My question, then, is in the topic title - do Fleming valves (vacuum-tube diodes) fail at high frequencies as well? Also, what precisely causes the power loss in solid-state diodes to happen in the first place?

 

[Carl Pugh]

Fleming valves won't work when the frequency gets too high.
It takes some amount of time for electrons to go from the cathode to the anode. If the voltage reverses before the electron reaches the anode, there is no current, so the valve doesn't work.

The electrons reach some velocity and when they strike the anode, the kenetic energy is transferred to the anode.
Some of the power vacuum tubes operated with the anode cherry red or even white hot. Very impressive.

 

[sophiecentaur]

Some of the power vacuum tubes operated with the anode cherry red or even white hot. Very impressive.

Why the use of the PAST tense?
Big power triodes are still used in places - though the really big valves have water cooled anodes these days. Solid state does have its limitations, you know.

 

[vk6kro]

There are several reasons why normal vacuum tube diodes are not very good at high frequencies.

There is the transit time problem Carl has mentioned. Electrons take time to pass from Cathode to Anode and the voltage causing them to flow may reverse direction before they get there, if the frequency is high enough.

There is the capacitance across the diode which bypasses the diode effect at high frequencies because it appears in parallel with the diode.

There is the necessary wire length to connect the diode to the pins at the base of the vacuum tube. This has quite a lot of inductance and this becomes significant at high frequencies.

I mentioned "normal" vacuum tube diodes because Klystrons and Magnetrons are diodes and actually work very well at microwave frequencies.

 

[sophiecentaur]

Klystrons, TWTs and magnetrons work on a very different principle. Amplification is achieved by a combination of velocity modulation and a long drift time which gives 'bunching' (current modulation). Very different from the action of a grid in triode.