What is the MOSFET threshold voltage and how does it affect switching characteristics?

• A. Vth is the gate-source voltage where drain current begins to flow; determines whether device is on; leakage current below threshold; In switching, Vgs controls switching speed and conduction.
• B. Vth is drain-source breakdown voltage;
• C. It's the maximum gate voltage allowed.
• D. Vth is the input impedance.

Answer: (B)

The MOSFET threshold voltage is the gate-source voltage at which a conductive channel forms between drain and source, allowing significant current to flow. This turning-on point matters for switching because the amount of current and how quickly the device can switch depend on how far the gate voltage is above that threshold—the overdrive (Vgs minus Vth). Below threshold, the transistor is essentially off with only very small subthreshold leakage; once Vgs exceeds Vth, current rises and the on-resistance decreases as you increase the overdrive, speeding up charging and discharging the gate and the drain, which improves switching speed. However, too much overdrive wastes power and stresses the device. Vth itself shifts with temperature and process variations, so designers must account for worst-case turn-on margins. The breakdown voltage (the maximum Vds the device can withstand before avalanche breakdown), the maximum gate voltage rating, and input impedance are different specs and do not define the turn-on point.