What Is a MOSFET?

A MOSFET is a voltage-controlled semiconductor device commonly used for switching and power control. The three main terminals are the gate, drain and source.

Drain-Source Voltage

The drain-source voltage rating, commonly written as V_DS, specifies the maximum voltage the device is designed to withstand under the conditions stated in the datasheet.

The rating should not be selected exactly at the circuit voltage. Transient voltage spikes can exceed the normal supply voltage.

Drain Current

The drain current rating indicates how much current the MOSFET can handle under specified thermal and electrical conditions.

The published current rating should not automatically be treated as a guaranteed continuous operating current in every application. Heat dissipation and the mounting conditions are important.

RDS(on)

R_DS(on) is the resistance between the drain and source when the MOSFET is turned on under specified gate conditions.

Lower R_DS(on) generally means lower conduction losses. The conduction loss can be estimated using:

P = I² × R

Gate Threshold Voltage

The gate threshold voltage, V_GS(th), is often misunderstood. It does not normally represent the gate voltage required to turn the MOSFET fully on.

For power switching applications, always check the R_DS(on) specification at the actual gate voltage available in the circuit.

Power Dissipation

MOSFET power dissipation depends strongly on temperature and the thermal environment. A device mounted to a suitable heatsink can dissipate considerably more heat than the same device without adequate cooling.

Gate Charge

Gate charge is important when a MOSFET is switched rapidly. A MOSFET with high gate charge requires more charge to be moved by the gate driver during switching.

This can affect switching speed and driver losses.

Important Specifications

Specification	Meaning
VDS	Drain-source voltage rating
ID	Drain current rating
RDS(on)	On-state drain-source resistance
VGS(th)	Gate threshold voltage
Qg	Total gate charge
PD	Power dissipation rating

Conclusion

Selecting a MOSFET requires looking at several specifications together. Voltage rating, current, R_DS(on), gate drive voltage, gate charge and thermal performance should all be considered for the intended application.