A Silicon Controlled Rectifier (SCR) is the most common member of the thyristor family. It is built as a four-layer PNPN semiconductor sandwich, which is the structural key to counting its junctions: with four alternating layers there are exactly three internal PN junctions (each boundary between adjacent, oppositely doped layers forms one junction).

Structure and junctions:

• Layers: P₁ – N₁ – P₂ – N₂
• J₁: between P₁ and N₁ — the anode-side junction
• J₂: between N₁ and P₂ — the middle junction that blocks forward voltage until the device is triggered
• J₃: between P₂ and N₂ — the cathode-side junction

The device has three terminals: Anode (A), Cathode (K) and Gate (G). In the forward-blocking state J₁ and J₃ are forward biased while J₂ is reverse biased and holds off the voltage; a small gate current injects carriers that break down J₂ and latch the SCR into conduction. A helpful memory aid is the two-transistor model, where the PNPN is seen as interleaved PNP and NPN transistors sharing these same junctions.

Why the other options are wrong: Four counts the layers, not the junctions — four layers give only three boundaries. Two and One undercount; a two-junction device would be an ordinary transistor and a one-junction device a simple diode, neither of which has the latching PNPN behaviour of an SCR. Hence the correct count is three.