The Field Effect Transistor is a special kind/type of transistor. Just like a ‘regular’ transistor (officially: Bipolar Transistor, or Bipolar Junction Transistor) it has three terminals. It is very easy to operate and therefore widely used in electronic circuits.
A FET uses the voltage that is applied to the input terminal, called the gate (similar to the base of a Bipolar Transistor) to control the current flowing through the output terminals called drain and source (similar to the collector and emitter of a Bipolar Transistor). The output current can be relatively high without influencing the Gate (control) voltage. So basically a FET behaves like a voltage controlled switch. FETs generally display very high input impedance (for DC: resistance) and are very well suited for (high frequency) switching applications.
The FET consists of a semiconductor channel with electrodes at either end referred to as the drain and the source. A control electrode called the gate is placed in very close proximity to the channel so that its electric charge is able to affect the channel. In this way, the gate of the FET controls the flow of carriers (electrons or holes) flowing from the source to drain. It does this by controlling the size and shape of the conductive channel. The semiconductor channel where the current flow occurs may be either P-type or N-type. This leads to two types or categories of FET known as P-Channel and N-Channel FETs. N-channel FETs are most commonly used.
There are various type of FET depending on the way they are built. The most common one is the MOSFET (Metal Oxide Semiconductor FET). The remainder of this text is about MOSFETs.
MOSFETs can operate in three regions:
- Cut-off region: In the cut-off region of the MOSFET is in a non-conducting state, i.e. turned OFF or channel current Ids = 0. The gate-source voltage Vgs is less than the threshold voltage Vgs(th) required for conduction.
- Linear region: In the linear region the channel is conducting and controlled by the gate voltage. The Vgs must be greater than the threshold voltage Vgs(th) and also the voltage across the channel, Vds must be greater than Vgs. Since in this region the relation between drain-source current Ids and gate-source voltage Vgs is linear, the FET can be used for amplification of signals.
- Saturation region: In this region the MOSFET is turned hard on. This is the region used for switching high currents.
How to connect a MOSFET
Below you can find the basics for connecting a MOSFET. For both circuits the voltage V+ may be, but does not need to be, different from the supply voltage of the controlling circuit. The diode is needed for protection when using inductive loads like coils, relays, or motors.
It is advisable to use a pull-down resistor from gate to ground (for N-FET) or pull-up resistor from V+ to gate (for P-FET). This will avoid problems related to the FET switching on because of disturbances in the environment or connecting wires.
Note: The N-FET is switched on by making the control output HIGH while the P-FET is switched on by making the control output LOW.
Most important specifications
- Vds,max: The maximum drain-source voltage when the MOSFET is closed.
- Id,max: The maximum drain current through the MOSFET.
- Rds,on: Static drain-source on resistance. The drain-source resistance in saturation (usually in the ohms range).
- Vgs(th): Gate-source threshold Voltage. The voltage at which the MOSFET starts conducting (usually around 2.5 to 3.5V). Note: this is not the minimum gate-source voltage to turn the MOSFET fully open which is usually somewhat higher.