Why does a diode not conduct in reverse bias?

Answer 1

A diode cannot conduct in reverse bias because of the way it was made.

If the question is asking about a thermionic vacuum tube diode, the electrons produced by the heated cathode of the tube can only move in one direction: from the cathode to the anode - but only for as long as the anode is positively charged.

The reason is that, if the anode is made to be positively-charged, it ATTRACTS electrons - which are negatively charged - from the cathode, so a current flows from cathode to anode in the "normal" biased direction because of the physical fact that UNLIKE CHARGES ATTRACT ONE ANOTHER.

If the anode is made to be negatively charged, the electrons won't go to it because of the physical fact that LIKE CHARGES REPEL ONE ANOTHER, so no electrical current can flow from the cathode to a negatively charged anode.

(In fact no electrical current can ever flow from a negative anode to the cathode for an entirely different reason: no anode of any normal thermionic vacuum tube was ever designed and made to "produce" electrons in the way that a heated cathode produces them!)

To be able to "produce" electrons and let them fly to the positively charged anode, the cathode must be able to receive some more electrons to replace all the ones that have left the space around the cathode. That "supply of more electrons" is called an electric current.

That current does not come from nowhere - it has to comes from a source, such as a battery or a generator - and the electrons which reach the anode must then be conducted back to the battery or a generator so that they complete a full electrical circuit by traveling along the wires which connect the source to the diode.

Similarly, because of the way it was made, a semiconductor diode behaves in just the same way as a vacuum tube diode: electrons can flow from the negative end to the positive end of the semiconductor diode but not the other way around.

The actual reason why a semiconductor diode works like that is very much more complicated to explain than for a vacuum tube diode. Put very simply, a semiconductor diode has the kind of materials inside it that will only allow electrons to flow one way. If a voltage is applied to the diode the wrong way round (called reverse bias) practically no current can flow through the diode.