Question

The two transistors in a SCR consists of following transistors

• A. one npn and other pnp
• B. both n-p-n
• C. both p-n-p
• D. one npn and other UJT

Hint:

SCR operation is explained using two-transistor model (npn + pnp).

Answer 1

The Correct Option is A

Step 1: Describe the four-layer PNPN structure of an SCR.
An SCR has four alternating semiconductor layers: p$_1$-n$_1$-p$_2$-n$_2$. This can be split into two overlapping transistors: p$_1$-n$_1$-p$_2$ forms a PNP transistor (T1), and n$_1$-p$_2$-n$_2$ forms an NPN transistor (T2). The two transistors share the middle n$_1$-p$_2$ region.

Step 2: Explain the regenerative latch mechanism.
The collector of T1 (PNP) feeds the base of T2 (NPN), and the collector of T2 feeds the base of T1. This cross-coupled connection creates positive feedback. The latch condition is $\alpha_{PNP} + \alpha_{NPN} \geq 1$.

Step 3: Identify the transistor types.
For this complementary regenerative feedback to work, the two transistors must be of opposite types. \[ \boxed{\text{One npn and one pnp}} \]