Question 8-18B6

Power MOSFETs are voltage-controlled devices, characterized by an insulated gate electrode separated from the semiconductor channel by a thin oxide layer. This insulating layer gives them a very **high input impedance**, meaning they draw minimal DC current from the driving circuit, primarily requiring voltage to turn on and off. A critical failure mode for MOSFETs, particularly in high-voltage or high-speed applications like RADAR switching supplies, is **gate punch-through** (also known as gate oxide breakdown or gate rupture). If the voltage applied between the gate and source exceeds the device's maximum specified rating, the thin insulating oxide layer can be permanently damaged, causing a conductive path (short circuit) between the gate and the channel, rendering the device inoperable. Therefore, high input impedance and gate punch-through as a failure mode are distinct characteristics of power MOSFETs. Options A and D are incorrect because MOSFETs have high, not low, input impedance. Option C mentions thermal runaway, which can occur in MOSFETs, but gate punch-through is a more specific and direct failure mode related to the gate's insulating structure.