Jawaharlal Nehru Technological University Hyderabad B.Tech III POWER ELECTRONICS Previous Question Paper (October - 2020 - R13)

A thyristor (Silicon Controlled Rectifier) is a four-layer, three-junction semiconductor device consisting of alternating p-type and n-type materials arranged as p-n-p-n. The three terminals are the Anode (A) connected to the outer p-layer, the Cathode (K) connected to the outer n-layer, and the Gate (G) connected to the inner p-layer.

The construction involves a silicon wafer where the junctions J1, J2, and J3 are formed by diffusion. The outer p-layer (anode) and outer n-layer (cathode) are heavily doped, while the inner p and n layers are lightly doped to support high voltage blocking. The gate terminal is located near the cathode junction to facilitate charge injection.

Schematic structure: A p-n-p-n stack with Anode at top, Cathode at bottom, and Gate attached to the inner p-layer. The circuit symbol is a diode symbol with an additional terminal (Gate) emerging from the p-layer near the cathode.

For an SCR to turn on using a gate signal, the following conditions must be met:

1. The SCR must be forward-biased, meaning the anode potential must be positive with respect to the cathode.
2. A positive gate current must be applied between the gate and cathode terminals.
3. The gate current must exceed the minimum gate trigger current (Igt) required for the specific device to initiate the regenerative feedback process.
4. The anode current must reach or exceed the Latching Current (Il) within the duration of the gate pulse to ensure the device remains in the ON state after the gate signal is removed.

When these conditions are met, the gate current injects charge carriers into the inner p-layer, which reduces the width of the depletion region at junction J2. This triggers a regenerative action where the internal transistors (Q1 and Q2) drive each other into saturation, causing the device to latch into a low-impedance conducting state.