In a diode valve, the anode would not emit electrons when heated so when the polarity of the voltage across the tube was reversed, no current flowed. Notice how the solid state transistor works in a similar way.

  1. Why does the anode not emit electrons?

  2. Why does no current flow when the polarity of reversed? Why reverse the polarity?

  3. What is a solid state transistor? Just a normal transistor?

  4. What do they mean by solid state?

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    \$\begingroup\$ The phrase solid state transistor just means transistor. In the days when valves ruled the phrase solid state was widely used to distinguish silicon based devices from vacuum tubes. These days it's a historical relic. \$\endgroup\$
    – John Rennie
    May 9, 2015 at 5:50
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    \$\begingroup\$ Can I make a suggestion? Throw the book that this came from away as fast as you can. Do it now. Do not waste any more time on this book, unless, of course, it's a collectible textbook from the late 1940s or early 1950s, in which case I would seal it in plastic foil so that nobody gets to read the nonsense that is contained in it while the book preserves its maximum value. \$\endgroup\$
    – CuriousOne
    May 9, 2015 at 6:49
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    \$\begingroup\$ It's nonsense. The anode would emit electrons when heated. The point is that it is not heated, so it doesn't emit electrons. \$\endgroup\$
    – user207421
    May 9, 2015 at 8:26

1 Answer 1


The statement is a bit misleading. In vacuum tube diode the cathode is heated but the anode is not. That means the cathode emits electrons but the anode does not.

So if you apply a positive voltage to the anode and a negative voltage to the cathode the electrons emitted by the cathode move towards the anode and a current flows.

However if you apply a negative voltage to the anode and a positive voltage to the cathode no current flows because the (unheated) anode doesn't emit any electrons.

The comparison with a transistor seems odd, because a transistor is not like a diode. It's more like a triode. The obvious comparison is with a semiconductor diode, though this works by a completely different mechanism.

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    \$\begingroup\$ The anode can reach red heat, from the energy of collision of electrons into it. This often happens in a power amplifying valve, but may also occur in a power rectifier. So, electron emission may occur (if the polarities were briefly reversed) but at an extremely low level because the anode is made of steel, but the cathode is thoriated tungsten, where the thorium is an especially good emitter of thermal electrons. (There is also secondary emission, where electrons colliding with the anode dislodge more electrons directly). So, correct but "the anode is not heated" isn't the whole story. \$\endgroup\$ May 9, 2015 at 8:59

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