Which semiconductor device requires an external trigger pulse for operation?

The silicon controlled rectifier (SCR) is a semiconductor device that indeed requires an external trigger pulse to operate. It is a type of thyristor that can be turned on and off by applying a gate pulse. When a positive voltage is applied to the gate terminal, it allows current to flow between its anode and cathode, turning the SCR on. Once the SCR is conducting, it continues to conduct even if the gate pulse is removed, but it will stop conducting only when the current through it falls below a certain threshold.

This characteristic makes the SCR particularly useful in applications such as power control, where it can manage high voltages and currents, making it a popular choice in AC power control and motor speed control circuits. The need for a trigger pulse distinguishes the SCR from other semiconductor devices like transistors and diodes, which operate based on different principles.