When cathode is positive with respect to anode in an SCR, the number of blocked p-n junctions is
A
1
B
2
C
3
D
4

Question

When cathode is positive with respect to anode in an SCR, the number of blocked p-n junctions is

Examoogle · Accepted Answer

2

Explanation: In an SCR (Silicon Controlled Rectifier), which consists of a P-N-P-N structure with three junctions ($J_1$, $J_2$, and $J_3$), there are two junctions blocked when the device is in reverse-biased mode. Specifically, when the cathode is positive relative to the anode, junctions $J_1$ and $J_3$ are reverse-biased, effectively blocking current flow through the device.

Working Principle: An SCR is a four-layer semiconductor device. When the anode is negative and the cathode is positive, junction $J_2$ is forward-biased, while junctions $J_1$ and $J_3$ are reverse-biased. These two reverse-biased junctions block the flow of current, causing the SCR to operate in the reverse blocking region.

Key Formulas: $V_{AK}; $I_R = I_{S1} + I_{S3}$ — Leakage current in reverse blocking mode

Key Points: An SCR contains three p-n junctions: $J_1$ (Anode-P to Base-N), $J_2$ (Base-N to P-gate), and $J_3$ (P-gate to Cathode-N).; In reverse bias, junction $J_2$ is forward-biased, while $J_1$ and $J_3$ are reverse-biased.; The device only conducts when the anode voltage is sufficiently positive to cause avalanche breakdown in the reverse-biased junctions.

Advantages: High voltage blocking capability; Effective isolation of power circuits

Disadvantages: Significant power dissipation during breakdown; Potential for permanent device failure if excessive reverse voltage is applied

Applications: AC to DC rectification; Protection circuits against voltage spikes

Additional Info: The reverse leakage current ($I_R$) is very small and flows due to minority carriers in the depletion regions of $J_1$ and $J_3$.. Option B (2) is correct because $J_1$ and $J_3$ block the flow.

Answer: B is correct — When the cathode is positive with respect to the anode, junctions $J_1$ and $J_3$ are reverse-biased, effectively blocking the current.

Exam Tip: Always remember that in forward blocking (anode positive), only junction $J_2$ is reverse-biased; in reverse blocking, both $J_1$ and $J_3$ are reverse-biased.

Related Questions

Discussion (0)