How many PN junctions are present in the structure of a Silicon Controlled Rectifier (SCR)?
Three
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.
Similar Questions
The transition of an SCR from the forward blocking state to the conduction state is called:
Assume an SCR is operating in the forward blocking state. Once a sufficient positive current pulse is applied to the gate terminal, the SCR triggers into the conduction state. What happens if the gate signal is completely removed immediately after conduction starts?
The holding current of an SCR is defined as the:
Important Questions from Thyristors
_______ is the device which acts like an N-P-N and a P-N-P transistor connected base-to-base and emitter-to-collector.
SCR stands for
A conducting SCR can be opened by reducing __________to zero.
Which of the following statements correctly describes the structure of a Silicon Controlled Rectifier (SCR)?
The thyristors which have a turn-off time less than _________ are called inverter grade thyristors.
