# Simple understanding of mosfets

In summary, mosfet gate to source voltage controls the resistance between the source and drain. Increasing the gate to source voltage will allow more current to flow.
Ok so I am pretty experienced, I've done tons of analog and digital circuitry but mostly programing microcontrollers. But I never really got to understanding mosfets too well, so this is what I know,

N-channel are turned on by posotive voltage
p-channel are turned on by negative voltage
Depletion are also on from 0v
(uses)

But how does the gate voltage change the Vds, like does the output from source voltage = the gate voltage, like a bipolar transistor +-0.6v? What happens when you increase the gate voltage?
Or is it essentially a closed circuit with no voltage drops?

The gate is essentially electrically isolated from the rest of the device. So the gate modulates the resistance between the source and drain, but the gate voltage has no direct impact on the source and drain voltages. It is very different from a bipolar transistor in this respect.

A BJT is a current driven device and a MOSFET is a voltage driven device. An applied voltage to the gate of a MOSFET will attract charge carriers. That creates a channel between the source and drain. Increasing (or decreasing) the voltage on the gate will make the channel larger allowing more current to flow.

I just looked at the wiki for MOSFETS and it seems pretty good. It has some nice graphs and diagrams. A picture is worth a thousand words as you know.

so when the gate voltage is a regular digital 5v, should I pretty much pretend it is a closed circuit?
No catch?

Let's talk n-channel. What controls the resistance between the source and drain is the gate to source voltage. If the gate to source voltage is above the device's threshold, current can flow from drain to source.

MOSFETs are also have a constant current characteristic. At a given Vgs they behave resistive up to a certain current, then behave as constant current devices.

5V Vgs is above the threshold voltage of most but not all MOSFETs. (for example http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00066266.pdf has max 5V Vgs just to conduct 250uA. The graphs show 10V Vgs for 250Amps. (look at figure 4 to see the nearly constant current characteristic at Vgs = 6V)

So, you need to look at the datasheet for the device you are interested in.

Below the threshold voltage MOSFETs exhibit exponential sub-threshold characterisics which some designers exploit.

I think I was just interpreting it wrong,
Is by saying it puts out that 250a with 10vgs at 5vds
Pretty much saying that at 5vds and 10vgs it offers
0.02 ohms of internal resistance between drain and source?

You terminology is a bit off, but you got the idea.

Again (from figure 4), note that for 6V Vgs the Ids stays about 30A for Vds between 10 and 20V. That means the resistance changes as the current changes.

Note that at 7V Vgs the resistance is pretty constant up to 150A. (the slope of the curve is the dynamic conductance, opposite of resistance)

All MOSFETs have that behavior. At any Vgs above threshold there is a range of constant resistance as the Ids rises, then the channel becomes saturated and no more current can pass through.

In the data tables there is a rating of 0.015 ohms max at 10V Vgs and 69A. (Static drain source on resistance)

## 1. What is a MOSFET?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor that is used in electronic devices to control the flow of current. It is a three-terminal device that can be switched on or off by applying a voltage to its gate terminal.

## 2. How does a MOSFET work?

A MOSFET works by using an electric field to control the flow of current through its channel. When a voltage is applied to the gate terminal, it creates an electric field that either allows or blocks the flow of current through the channel. This makes it a voltage-controlled device.

## 3. What are the advantages of using MOSFETs?

MOSFETs have several advantages over other types of transistors, including high switching speeds, low power consumption, and the ability to handle high currents and voltages. They also have a simple structure, making them easy to manufacture and integrate into electronic circuits.

## 4. What are the different types of MOSFETs?

There are several different types of MOSFETs, including enhancement-mode and depletion-mode MOSFETs. Enhancement-mode MOSFETs require a positive voltage at the gate terminal to turn on, while depletion-mode MOSFETs require a negative voltage. There are also different types of MOSFETs based on their construction, such as planar, vertical, and power MOSFETs.

## 5. What are some common applications of MOSFETs?

MOSFETs are used in a wide range of electronic devices and circuits, including power supplies, amplifiers, motor control circuits, and digital logic circuits. They are also commonly used in switching applications, such as in computer processors and LED lighting. Additionally, MOSFETs are used in high-frequency circuits, such as in radio and communication systems.

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