Will Any SCR Trigger Optically If It Is Exposed To Light?

Question

I read somewhere that all SCR's are photosensitive, in the same way that all diodes emit light, but some are optimized to take advantage of this property while others are not.

There is an SCR I want to use, but I cannot find an optically triggered version of it. One idea I had is to buy the thing in die form and then stick a really bright LED next to it.

I assume that a device not made for this mode of triggering will require far higher optical energy to trigger, and if I need to deliver 100 W instead of 100 mW, that's okay.

Here are my questions:

1. Is it really true that all SCR's are photosensitive?
2. If so, are we only talking a 20-30 dB increase in light to make the thing trigger? Or does the chemistry of the thing need to be optimized during manufacturing in order to make practical use of this property?

Answer 1

All P-N junctions are light sensitive, so that includes diodes, transistors, and thyristors. surface metallisation my obscure part of the important junction, which may mean you need more light.

If a feature is not promised in the data-sheet you can't rely on it unless you test for it yourself. The manufacturer is free to change their process and product as they wish, and so long as the part they sell matches the data sheet it's not their problem.

but if you're buying dice you're probably making a life-time buy anyway so will be immune to changes.

Answer 2

I made a remote strobe trigger using an SCR that I had cut the top off and glued on a plastic window. But that was 1968 and I was working as a lab technician in semiconductor development, and I had access to clean room equipment. You need a device where the anode is attached to the base (that was standard) such that the cathode and gate are uppermost. The gate/cathode junction light sensitivity will vary device to device, and the voltage at the gate due to photons impinging on the junction can be manipulated - for example: a resistor clamping the gate to the cathode can set the light level for triggering; or a small inductor can be used to react to delta light change. If you are looking to trigger a thyristor from a light source, I would suggest more conventional means.