Secondary emission

Secondary emission is a phenomenon that occurs in electron tubes where electrons impact an electrode with sufficient energy to 'knock' additional electrons from the surface of that electrode. Generally, one electron gives rise to several secondary electrons.

This effect can be undesireable such as in the tetrode thermionic valve (tube). In this instance the positively charged suppressor grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive suppressor grid current. It is also partly responsible for this type of valve (tube) exhibiting a 'negative resistance' characteristic.

The effect can also be exploited to advantage such as in the photomultiplier tube. In this instance the electrons (or indeed electron) emitted from a photocathode are accelerated towards an electrode (called a dynode). This electron or electrons strike with sufficient energy to 'knock' many more electrons from its surface. These new electrons are then accelerated towards another dynode where even more electrons are emitted. This process occurs (typically) 10 or so times. The result is that the tiny and normally undetectable current from the photocathode becomes a much larger and easily measurable current flowing in the final anode circuit. The current gain is typically many hundreds of millions.