Two transistors arranged as a single thyristor (aka SCR - Silicone Controlled Rectifier). This is an alternative to the first circuit in this small series of basic thyristor demonstrations.
The thyristor is really just a 4 layer (PNPN) semiconductor component. It's not a component made available by the developers of this simulator, so I've assembled two transistors (P[NP+NP]N) back to back, like so: P[NP]N.
Just to be clear: Notice how the NP pins of the former (P[NP]) become common to the NP pins of the latter ([NP]N).
A small, temporary current pulse to the gate will activates the thyristor. It latches (continues to conduct after the trigger current has gone). One way to unlatch a thyristor is to break the circuit (with an NC switch) before the anode; as demonstrated in this circuit.
Please feel free to elaborate, or correct me in the comments.
