Supposedly, an SCR/thyristor is just a simple, four layer PNPN semiconductor.

If that's the case..

When a circuit calls for a SCR/thyristor, and there isn't one available, can it be substituted by (i.e. crafted from) two BJTs (or other discrete components, for that matter)?

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Here are a couple of examples I had in mind; anode = blue, gate = green, cathode = orange:
[![enter image description here](https://i.stack.imgur.com/USR7j.jpg)](https://i.stack.imgur.com/USR7j.jpg)
[![enter image description here](https://i.stack.imgur.com/LX5iT.jpg)](https://i.stack.imgur.com/LX5iT.jpg)