Component Theory - Field Effect Transistor


Field Effect Transistors are similar to bi-polar transistors in their component application but have the following connections -

  • Drain (d)  - which is the anode
  • Gate (g)    - which is the switching leg
  • Source (s) - which is the cathode

Unlike a bi-polar transistor they are completely digital components and can be found in their billions inside computing processor chips. Once a voltage of 2V is applied to the gate leg the transistor will conduct from the drain to the source (ON)

The MOSFET is an example of a field effect transistor and has the symbol shown on the left. It relies solely on the voltage level at the gate leg and not the current. For this reason they are ideal for low current applications such as computing and due to their digital nature they can be used to create Binary(0,1see video for more...


Often MOSFET transistors can be used to interface with microcontrollers for high current outputs such as motors & solenoids. Unlike a bi-polar transistor the MOSFET can cope with higher currents and can allow the addition of a heatsink to remove excess heat generated.

By using a MOSFET as an interface it will replicate the digital signal produced by the microntroller and fully turn the motor when a 1 is received (2V). At which point the motor will receive the full voltage of the power supply and a current increase from the transistor.



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