Question

The SCR triggers when:

• A. Gate pulse with forward bias
• B. Reverse voltage
• C. Temperature rises
• D. Current zero

Hint:

An SCR can be turned on by several methods (such as high forward voltage, high $dv/dt$, temperature rise, or light illumination), but applying a gate pulse under forward bias is the only controlled and standard method used in power electronic circuits.
Once the SCR is on, it can only be turned off by reducing the anode current below the holding current ($I_H$).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the primary operating condition required to trigger (turn on) a Silicon Controlled Rectifier (SCR).

Step 2: Key Formula or Approach:
The question is qualitative.
The solution requires an understanding of the PNPN junction structure of a thyristor and its gate-control characteristics.

Step 3: Detailed Explanation:

• A Silicon Controlled Rectifier (SCR) is a four-layer, three-junction (PNPN) semiconductor device with three terminals: Anode ($A$), Cathode ($K$), and Gate ($G$).

• When the anode is made positive with respect to the cathode, the outer junctions $J_1$ and $J_3$ are forward-biased, while the inner junction $J_2$ is reverse-biased.

• This state is called the forward-blocking state, where only a tiny forward leakage current flows, and the SCR remains turned off.

• To turn the SCR on (trigger it into the forward-conduction state), the reverse-biased depletion region at junction $J_2$ must be broken down.

• The most reliable way to initiate this breakdown is to apply a positive gate voltage pulse between the gate and cathode terminals while the SCR is forward-biased.

• The positive gate pulse injects electrons into the inner P-layer (near the cathode), which increases the carrier concentration and triggers a regenerative (latching) process across all three junctions.

• Once this regenerative feedback begins, the SCR turns on rapidly, and the gate loses control over the anode current.

• Applying a gate pulse when the SCR is reverse-biased (anode negative with respect to cathode) does not turn the device on; instead, it increases the reverse leakage current, which can cause thermal runaway and destroy the device.

Step 4: Final Answer
Thus, the SCR triggers when a gate pulse is applied under forward-bias conditions, matching option (A).