Switching Components

FETs - Field Effect Transistors

  • MOSFETs Summary.
  • A solid state switch (with some quirks). We control the switch with a voltage source (gate voltage) proportional to the FET’s drain or source voltages. We can’t (necessarily) provide a MOSFET with a logic 1 to switch on and a logic 0 to switch off.
  • To turn a FET off and on, we are effectively what is effectively a capacitor across a metal electrode and some N or P-type silicon. As we saw earlier, charging a capacitor takes time, and because FETs are frequently used in applications with high switching frequencies (see next section), we want to ensure that they switch as quickly as possible. Also note that R_dson is proportional to the amount of charge you can build up across that capacitor, as shown in the “typical output characteristics plot” below.
  • Because of this FET drivers are highly recommended in most power electronics applications! See why we use FET drivers (a driver for your driver)). Drivers also can also be purchased which simultaneously drive high and low side N-FETS without worrying about a floating source voltage, and automatically prevent shoot through!
  • FETs are constructed such that they have body diodes which is important to be aware of.


  • N-Channel Mosfets - Off when gate voltage is at source voltage, and on when gate voltage is at the drain voltage.
  • P-Channel Mosfets - Off when gate voltage is at source voltage, and on when gate voltage is at the drain voltage. Good for simple high side switching, but generally more expensive + larger for the same performance as an N-FET.
  • GaN-FET - super high switching speeds, efficiency, and peak power throughput. Still comparatively new technology that’s slowly becoming more readily available.
  • Lots more nuance than what we’ve discussed, but if you are using an appropriate FET and driver, it becomes easier to think of FETs as simple switches.
  • Let’s hop into a datasheet to discuss: IRF540, an N-type MOSFET:
    • Drain-Source Voltage
    • Continuous Drain Current
    • However maximum ratings are not operating ratings - see typical output characteristics.



BJTs - Bipolar Junction Transistors

  • Good summary of BJTs and their uses
  • Very similar function and applications to a MOSFET, but they require non-negligible amounts of current to be kept on, and so are less efficient and rarely used in modern switching power electronics applications.