A p-n_junction consists of an n-type semiconductor in contact with a p-type semiconductor.

To a first approximation an n-type semiconductor only conducts electrons and a p-type semiconductor only conducts holes.

 

When forward biased above a certain voltage, current flows easily and electrons and holes move toward and combine at the junction.

In an LED the combining of electrons and holes produces photons of light.

 

When reverse biased, current does not flow even at relatively high voltages.

However when light falls on the junction of a reverse biased photodiode, electrons and holes are created and move in opposite directions away from the junction.

 

A photon-coupled transistor consists of an LED and a photodiode placed very close together and sharing a region of either n-type or p-type semiconductor.

Current injected into the LED creates photons which are absorbed by the photodiode which in turn creates more current through the LED which creates still more photons.

If 90 percent of the photons created by the LED are absorbed by the photodiode then the resulting transistor is capable of 10 fold amplification of the original current.

 

A normal transistor operates on the same principle but does not use or create photons of light.

 

All transistors are phototransistors, albeit not as good as those specially engineered for the job.

 

So what?

Edited January 5, 201511 yr by studiot

A photon-coupled transistor consists of an LED and a photodiode placed very close together and sharing a region of either n-type or p-type semiconductor.

Such element is called optoisolator/transoptor. I have couple.

http://en.wikipedia.org/wiki/Opto-isolator

 

Current injected into the LED creates photons which are absorbed by the photodiode which in turn creates more current through the LED which creates still more photons.

If 90 percent of the photons created by the LED are absorbed by the photodiode then the resulting transistor is capable of 10 fold amplification of the original current.

Not quite.

Job of transoptor is not amplification, but separation of circuits.

f.e. you have 1st circuit on computer side, sending data (0 no current, 1 current),

and 2nd circuit, using external power source,

when no current flows through computer circuit (0 signal), 2nd circuit has also no current flow.

when current flows through computer circuit (1 signal), 2nd circuit is passing current.

Failure of circuit in 2nd circuit won't ruin whole computer motherboard.

They're used f.e. in computer devices using old serial/parallel interfaces.

Edited January 6, 201511 yr by Sensei