# What exactly does a FET do?

What does a FET actually do?

I thought I knew the answer to this question, however upon researching further about the function of a FET I became unsure whether I had the right idea.

I thought that when the gate of an N-channel FET is positive and current flows towards the gate, another current flows from the drain to the source. And that a P-channel FET has current flowing from the source to the drain when the gate is negative.

What I really want to know is what happens when current is supplied to the gate of an N-channel FET and a P-channel FET?

If I were to connect an LED in the path, current follows from the drain to source or source to the drain...

a) Is the LED on or off when a current flows towards the gate of a P-channel?

b) Is the LED on or off when current flows towards the gate of an N-channel?

c) What would the difference be between PNP/NPN transistors and N/P-channel FETs?

If my initial ideas about what a FET does were right, then surely a FET is basically the same as a PNP/NPN transistor, apart from the way they toggle the ‘switch’, like how a FET uses an electric field to do this.

I don’t have a prototyping breadboard or any other components with me to test my ideas.

Sorry if my question sounds dumb, I’m a beginner in electronics.

Any help would be greatly appreciated.

• Are you talking about JFETs or MOSFETs? They are very different beasts. Apr 9, 2021 at 18:54
• @ElliotAlderson it doesn't really matter JFETS with MOSFETS are both voltage controlled devices so we can group them while BJTs are current controlled devices which is what the OP is asking about. Apr 9, 2021 at 19:00
• @MissMulan I think it does matter if the OP is asking about current flowing toward the gate. It is possible to have current flowing through the gate of a JFET. Also, depletion mode JFETs are common but depletion mode MOSFETs are not. Apr 9, 2021 at 19:10
• No in a JFET the Gate-Source junction is always reverse biased. Apr 9, 2021 at 19:12
• @MissMulan You can forward bias a jfet, if only slightly (without damaging it, anyway). Apr 9, 2021 at 19:31

What does a FET actually do?

A FET is a transistor which uses the field effect to control current flow.

The trick is that applying a (constant) electric field does not need any current and does not require a flow of energy (power) to maintain that field. So when using a FET you only need a voltage to control the flow of current in a steady (DC) situation.

Current is only needed when you want to change that voltage. The input of a FET behaves like a capacitor, you need current to charge/discharge it but once the voltage you want has been reached, no more current is needed. This is true for JFETs and MOSFETs.

There are depletion type FETs, these are "normally on", when their gate-source voltage $$\V_{GS}\$$ is zero, current can still flow. Nearly all JFETS are of this type. Depletion MOSFETs exist but are rare.

There are also enhancement type FETs, these are "normally off", when their gate-source voltage $$\V_{GS}\$$ is zero, no current can flow. Only when a certain $$\V_{GS}\$$ is applied, current can flow. Most MOSFETs are of this type.

N-channel MOSFETs need a positive $$\V_{GS}\$$ to allow current flow (switch on).

P-channel MOSFETs need a negative $$\V_{GS}\$$ to allow current flow (switch on).

What I really want to know is what happens when current is supplied to the gate of an N-channel FET and a P-channel FET.

As I wrote: these devices are voltage controlled so you don't "supply a current". Instead, you apply a voltage.

If I were to connect an LED...

Go search for how to control an LED with a MOSFET, there are plenty of examples that show how to do this. With the information above you should be able to figure out how this works.

What would the difference be between PNP/NPN transistors and N/P-channel FETs?

Bipolar transistors (NPN, PNP transistors) have a very different working principle. These devices do need a current to control the current flow.

Both MOSFETs and bipolar transistors can be used to switch on/off LEDs. In general when using a bipolar transistor, you need a resistor to limit the base current and you need to supply a (small) current to the transistor. When using a MOSFET a resistor in series with the gate is generally not needed as no current will flow anyway. Some MOSFETs do need a large $$\V_{GS}\$$ (several volts)to fully turn on, that is sometimes an issue.

If my initial ideas about what a FET does were right then surely a FET is basically the same as a PNP/NPN transistor...

Yes/No. Both devices are VERY different. However, both can be used to switch a load. Both can be used as a signal amplifier. The fact that both bipolar transistors and MOSFETs are still used in large quantities means both types have their place. JFETs are used much less though, only in certain niche applications.

I suggest that you look at example circuits that show how all these transistors are used. You don't have to fully understand these devices to be able to use them.

• Thanks, this really helped me. Lots of the other information on the internet just confused me more but now I think I understand. Apr 9, 2021 at 22:07

In a JFET, which it sounds like you're asking about, you have a piece of P- or N-type semiconductor that forms a channel between the source and drain terminals. Since it's doped, you have free charge carriers which are free to move when you apply a voltage between the source and drain, so current flows. The gate terminal is connected to a section of oppositely-doped semiconductor somewhere in the middle of the channel.

With no voltage applied to the gate, this section of oppositely-doped semiconductor will create a depletion region where there are no free charge carriers that extends into the channel (simplification, I know). This reduces the effective cross-section of the channel, so its resistance goes up and current between the source and drain will drop (assuming you don't change the voltage between the source and drain).

If you apply a voltage to the gate relative to the source that makes the depletion region expand (i.e. reverse bias), you will reduce the cross-section of the channel further and less current will flow. If you apply a voltage to the gate relative to the source that makes the depletion region shrink (forward bias), the cross-section of the channel will increase and more current will flow.

However, JFETs aren't really designed for forward biasing the gate-channel junction and you'll probably burn up your device if you do that.

• Thanks, I was wondering what people were talking about when they mentioned terms like ‘forward bias’ and ‘reverse bias’. Apr 9, 2021 at 22:14

For completeness I’d like to add that when a MOSFET is switched on (by applying a voltage to the gate relative to the source) then current can flow in either direction between the drain and source. There’s a caveat that the body diode is always present and so a MOSFET in the ‘off’ state presents as a diode between drain and source while in the ‘on’ state is behaves more or less like a (low-value) resistor.

The main difference between FETs and BJTs is they control current flowing through them differently.In FETs the drain current depends on the voltage of the gate and the voltage between drain and source while in BJTs the emitter current depends on the base current.

In FETs no current flows through the gate while in BJTs there is current flowing through the base.

FETs use 100% of the available current while in BJTs some current "goes to waste" in order to turn on the BJT.

• Comments are not for extended discussion; this conversation has been moved to chat. Apr 10, 2021 at 17:05