Question E6A12:
What is the purpose of connecting Zener diodes between a MOSFET gate and its source or drain?  

Choices:  
A: To provide a voltage reference for the correct amount of reverse-bias gate voltage  
B: To protect the substrate from excessive voltages  
C: To keep the gate voltage within specifications and prevent the device from overheating  
D: To protect the gate from static damage    

Correct Answer is  D: To protect the gate from static damage  

Correct answer: D — To protect the gate from static damage

Why D is correct
- A MOSFET gate is insulated by an ultra-thin oxide and has extremely high impedance. Static charges can easily raise $V_{GS}$ above the oxide breakdown limit (often around ±20 V), puncturing the oxide.
- Zener (or TVS) diodes placed from gate to source (or gate to drain, whichever is the RF/DC reference) clamp $V_{GS}$ to a safe value during ESD or transient events, shunting the surge away from the fragile oxide.

Why the other choices are incorrect
- A: “Provide a voltage reference for the correct amount of reverse-bias gate voltage”
  - MOSFET gates are not reverse-biased junctions like diodes/JFETs; the gate is insulated. Zeners here act as transient clamps, not precision references for setting gate bias.
- B: “Protect the substrate from excessive voltages”
  - The substrate (body) is usually tied to the source internally. The threat from ESD is to the gate oxide; clamping gate-to-source/drain specifically protects the gate, not the substrate.
- C: “Keep the gate voltage within specifications and prevent the device from overheating”
  - While clamping helps keep $V_{GS}$ within limits, “prevent overheating” is misleading. Overheating is primarily due to drain current/power dissipation, not the gate. The purpose is ESD/overvoltage protection of the gate oxide.

Useful rules of thumb and formulas
- Respect the absolute maximum gate voltage:
  $$|V_{GS}| \le V_{GS(\text{max})}$$
- Choose Zener clamp voltage with margin:
  $$V_Z \approx 0.6\text{ to }0.8 \times V_{GS(\text{max})}$$
  Example: if $V_{GS(\text{max})} = \pm 20\ \text{V}$, use $V_Z \approx 12\text{–}15\ \text{V}$.
- For bidirectional protection, use back-to-back Zeners or a bidirectional TVS.
- With a series gate resistor $R_G$, the surge current into the clamp is roughly:
  $$I_Z \approx \frac{V_{\text{surge}} - V_Z}{R_G}$$
- ESD energy that would otherwise stress the oxide scales as:
  $$E \approx \tfrac{1}{2} C_G V^2$$
  Lowering $V$ via clamping greatly reduces stress. A small “gate stopper” resistor (e.g., 50–200 Ω) also helps limit surge current and tame RF oscillations.