A MOSFET can have gate voltage above its threshold and still be a poor power switch. Threshold marks a small-current test condition, not a guarantee of low on-resistance. Gate-to-source voltage, drive speed, current, and cooling determine whether the device operates efficiently.
Threshold is not the fully-on specification
The gate-threshold voltage is specified at a particular drain current and test arrangement, often far below the intended power-switching current. It tells you about the onset of conduction under those conditions. To assess conduction loss, look for the on-resistance specification at the gate-to-source voltage your driver can actually provide. A part described as logic-level still needs its datasheet checked against the specific logic voltage. A guaranteed resistance at one gate voltage should not be assumed at a lower voltage merely above threshold.
Measure gate voltage relative to the source
The controlling quantity is gate-to-source voltage, not gate voltage relative to an arbitrary bench ground. If the source rises while the gate remains fixed, the effective drive can decrease. This matters in high-side arrangements and in circuits with source wiring resistance or inductance. A meter reading at one point can therefore look reassuring while missing the relevant relationship. Use an appropriate measurement method and reference arrangement; an ordinary grounded scope probe can create a short if connected to the wrong circuit node.
On-resistance turns current into heat
An illustrative 20mΩ on-resistance at 5A produces 0.5W of conduction loss using current squared times resistance. If effective resistance rises to 100mΩ, the same current produces 2.5W. Actual resistance also depends on junction temperature and gate drive. That feedback makes thermal design important even when the transistor appears to switch the load successfully. The package, board copper, heatsink arrangement, and ambient temperature determine how much temperature rise follows. A headline drain-current rating alone does not establish usable current in your layout.
Slow transitions add switching loss
The gate behaves as a charge-driven load during transitions. A weak driver may take too long to move through the voltage region where drain current and drain-to-source voltage overlap significantly. Increasing switching frequency repeats those losses more often. A DC meter can show a reasonable average gate voltage while hiding slow edges or ringing. Engineers use suitable waveform measurements and device data to assess that behavior. Gate-charge requirements, driver capability, resistance, layout, and switching frequency must be considered together.
Stay within the complete operating limits
Maximum gate voltage, drain voltage, safe operating area, temperature, and transient behavior all constrain a MOSFET application. More gate voltage is not an unlimited cure for heating, and excessive drive can damage the gate insulation. For a low-energy bench design, select a device with documented performance at the available drive, limit current appropriately, and evaluate temperature and switching behavior. Do not adapt this explanation into modifications of mains converters or high-energy motor drives without qualified design and testing procedures.
What to check before you act
- Use on-resistance specified at the actual gate drive.
- Reference gate voltage to the source terminal.
- Calculate conduction loss at realistic current and temperature.
- Check switching behavior and all absolute operating limits.
Common questions
Does exceeding threshold guarantee a low-resistance switch?
No. Threshold uses a limited test current; the on-resistance specification at the intended gate drive is the relevant starting point.
Can a multimeter miss a gate-drive problem?
Yes. An average reading can hide slow transitions, ringing, or an incorrect source reference.
The practical takeaway
A MOSFET needs the right gate-to-source drive, not merely a voltage above threshold. Match the datasheet conditions to the real circuit and evaluate both conduction and switching losses.
References and further reading
- Texas Instruments: Threshold versus guaranteed on-resistance
- Texas Instruments: MOSFET loss, ratings, and safe operating area
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
