GATE 2024 EE Q.27: solution and answer

Official answer: DSource: Official answer key, GATE 2024 EE (IISc Bengaluru)

1. The question asks which of four devices of similar power rating has the shortest switching times, so compare how each one turns on and turns off.

2. SCR: a latching, current-triggered device that cannot be turned off from the gate. It needs commutation, with turn-off times of tens of $\mu \text{s}$ or more, so it is limited to a few kHz. It is the slowest of the four.

3. GTO: it can be turned off through the gate, but turn-off needs a large negative gate current and the tail current is long. Typical switching frequency is about 1 to 2 kHz.

4. IGBT: a voltage-controlled device with a bipolar output stage. Minority-carrier charge gives a current tail at turn-off. Typical switching frequency is up to about 20 to 50 kHz.

5. Power MOSFET: a majority-carrier, voltage-controlled device with no minority-carrier storage, so there is no tail current. Switching times are tens of $\text{ns}$ and frequencies reach hundreds of kHz, up to the MHz range.

6. Order of switching speed: $\text{SCR}<\text{GTO}<\text{IGBT}<\text{Power MOSFET}$, so the fastest is the Power MOSFET, option (D).