What protective device is commonly built into the gate of MOSFETs?

The commonly built protective device in the gate of MOSFETs is a Zener diode. The primary function of this Zener diode is to protect the gate oxide layer from overvoltage conditions. MOSFETs are sensitive to voltage levels applied to their gates, and exceeding the maximum gate-source voltage rating can lead to gate breakdown, which can permanently damage the device.

By incorporating a Zener diode, any excessive voltage that exceeds the threshold will cause the Zener to conduct, thereby clamping the voltage at a safe level. This helps maintain the integrity of the MOSFET during operation and protects it from failure due to transient voltage spikes.

The other options, while they have their own applications in electronics, do not serve the same specific purpose in the context of protecting the gate of a MOSFET. For instance, while a diode may protect against reverse voltage, it cannot clamp voltage levels in the same way a Zener diode does. Fuses are used for overcurrent protection and resistors are typically for current limiting or biasing, neither of which addresses overvoltage at the gate directly.