If you are an electronics hobbyist, you may have asked this question: Is a MOSFET easier to use as a switch than a BJT?
The short answer is yes—in many basic switching applications.
One reason is that a MOSFET is a voltage-controlled device. The voltage applied between the gate and source terminals, or VGS, controls whether the device turns on. In a simple switching application, the MOSFET can be turned on by applying a sufficient gate-to-source voltage.
A BJT, on the other hand, is generally treated as a current-controlled device. To drive a BJT into saturation, sufficient base current must be supplied. Determining the appropriate base current requires some analysis and calculation. The designer must select suitable base and collector resistors while also considering the transistor’s DC current gain (HFE or β).
This makes a MOSFET appear simpler to use as a switch than a BJT. However, there is an important point to remember:
Simply exceeding the MOSFET’s gate-to-source threshold voltage does not necessarily mean that the MOSFET is fully turned on. Here are the three important things to consider:
1. Consider the Gate-to-Source Threshold Voltage
The first parameter to consider is the gate-to-source threshold voltage, VGS(th).
The applied gate-to-source voltage must be sufficient to begin turning on the MOSFET. For a small MOSFET used in a simple switching circuit, setting the applied VGS just little above the threshold is enough to turn on the device under the intended operating conditions. For example, the datasheet of the IAUCN04S7N010G MOSFET from Infineon specifies its gate-to-source threshold voltage of 3V maximum. Setting the applied VGS to 20-30% higher than this will do.
It is important to understand that VGS(th) does not represent the gate voltage required to fully turn on the MOSFET. It is the voltage at which the device begins to conduct a specified small amount of drain current. This is very important fact for a MOSFET in high frequency switching converter. Therefore, if you are designing a practical switch, you should not simply look at VGS(th) and assume that the MOSFET will be fully saturated at that voltage.
2. Power MOSFETs Require More Than Just Exceeding VGS(th)
For power MOSFETs used in high frequency switching converters, simply exceeding the gate-to-source threshold voltage is not enough to guarantee that the MOSFET is fully turned on.
During the switching transition, the gate voltage may encounter the Miller plateau. The gate-to-source voltage temporarily remains relatively constant while the drain-to-source voltage changes significantly.
To achieve fast and efficient switching, the gate driver must be capable of supplying enough current to charge and discharge the MOSFET’s gate capacitances effectively. For power electronics engineers, setting the applied VGS higher than the miller plateau is a must.
After the Miller plateau has been overcome, the gate voltage can continue rising toward the driver’s final gate-drive voltage. A sufficiently high gate-drive voltage helps reduce the MOSFET’s RDS(on), which in turn reduces conduction losses. This is particularly important in high-frequency switching applications such as switching converters, where both switching losses and conduction losses can significantly affect efficiency.
3. Do Not Exceed the Maximum VGS Rating
The third consideration is the maximum gate-to-source voltage specified in the MOSFET datasheet.
Although the MOSFET needs sufficient gate voltage to turn on effectively, the applied VGS must remain below the device’s maximum rated value.
Exceeding the maximum VGS rating can damage the MOSFET’s gate material and will result in permanent device failure. For this reason, the gate-drive voltage should be selected carefully and should include an appropriate safety margin below the maximum VGS rating.
Conclusion
So, is a MOSFET easier to use as a switch than a BJT?
For many simple switching applications, yes. A MOSFET can be easier to drive because the gate is controlled by voltage, while a BJT requires an appropriate amount of base current to achieve saturation.
However, “voltage-controlled” does not mean “any gate voltage will work.”
When selecting a MOSFET for switching, you should consider at least three important parameters:
- VGS(th) – the gate-to-source threshold voltage.
- Gate-drive voltage and the Miller plateau – particularly important in power and high-frequency switching applications.
- Maximum VGS – the gate-to-source voltage that must not be exceeded.
The key takeaway is simple: Do not use VGS(th) as the target gate-drive voltage. Instead, check the MOSFET datasheet for the recommended gate-drive conditions and the RDS(on) specified at the intended gate voltage. A MOSFET may be easier to drive than a BJT, but choosing the correct gate voltage is still essential for reliable and efficient switching.