Which of the following is a PNPN device?

A Silicon Controlled Rectifier (SCR) is constructed from four alternating layers of semiconductor material, forming a PNPN structure. This unique four-layer arrangement creates three internal PN junctions that give the SCR its characteristic latching ability. When a signal is applied to the gate, it triggers the inner junctions to conduct, allowing current to flow through the entire PNPN stack. The other options do not have a four-layer PNPN structure. For instance, a PIN diode has a P-type layer, an Intrinsic layer, and an N-type layer (PIN), while standard Zener or Schottky diodes rely on simple two-layer junctions or metal-semiconductor junctions.