P-Channel                                                    N-Channel


MOSFET or simply FET (MOS = metal-oxide semiconductor and FET = field effect transistor) is a type of transistor, a component used as a switch or an amplifier of electric signals.



A MOSFET usually has threee terminals: Gate, Source and Drain. There are two essential types: N-channel and P-channel, which are basically different because of the polarization. The current to be supplied to the circuit, which will circulate between Source and Drain of the FET, is controlled by the voltage applied on the Gate terminal. This last one has a dielectric separation from the other two, generating therefore an almost-null current value on the Gate, and an electric field which influences in the Source and in the Drain. Below, we have these two basic types of FETs and their corresponding usual symbologies:

Concerning Polarization:





Another more known type of transistor, the BJT (bipolar junction transistor),  also has three terminals: Base, Collector and Emitter. The current to be supplied to the circuit, which will circulate between Collector and Emitter of the BJT, is controlled by the current of the Base terminal, unlike a MOSFET, which is controlled by the voltage. This is one of the main differences between them, causing a TBJ to be generally applied in circuits with a low value of current, and a FET not only for these ones but also for applications with higher values of power/current.

Application Examples of MOSFETs

One of the most common applications for MOSFETs is on CMOS circuits (see the reference link at the end of this tutorial for more details). However, there are also others, for example:

- Voltage Controlled Resistance

- Power Switching Circuits

- Frequency Mixers

- Etc.


Example: Turning a DC motor on with a MOSFET (MOSFET as a switch) and the Arduino.


1 Arduino Board 

MOSFET Transistor (N-channel) 

- 1 MOSFET Transistor (P-channel)

- 1 DC motor (5V - 1A)

Jumpers for Connection

- 1 Resistor of 200 Ohms

- 1 5V - 1A Source



With a N-channel MOSFET:


With a P-channel MOSFET:

Firmware for both kinds of MOS (Enter this code in the Arduino's IDE):

// Controlling a DC motor with a MOSFET transistor (basic application) for P-channel and N-channel
// The motor used in this Firmware is of 5V - 1A
// This hardware was not projected to control the sense of rotation of the motor (unless it inverts its polarity), only its speed.
// Nevertheless, it can be controlled by a more elaborated hardware (an H bridge of MOSFETs for example)

#define GatePin 6 // Defines the pin to receive the signal in the Gate of the MOS
int GateSignal = 0; // Value that will be set in the PWM pin (it may vary form 0 to 1023) for controlling the speed of the motor.

void setup () {
pinMode (GatePin, OUTPUT); // Defines Arduino's pin 5 as an output

void loop () {
while ((GateSignal <0) || (GateSignal >1023)) { // While the value of "GateSignal" is not between 0 and 1023,
continue; // the Arduino continues without setting anything up in the "GatePin".
analogWrite (GatePin, GateSignal); // But if it is in this range, the Arduino sends the signal to the "GatePin".

Pay attention to the blue line of the code. In N-channel MOS polarization, the maximum rotation of the motor will be when the value of "GateSignal" is 1023 (therefore in N-channel the Gate terminal conducts with logic level "1"). However in P-channel, this rotation will be the maximum if the logic level of "GateSignal" is "0" (therefore in P-channel the Gate terminal conducts with logic level "0"). And the same happens with the minimum rotaion.

And this is the basic idea of a MOSFET!!! We hope you have enjoyed. Any doubts, post here, we will be available.

Reference Links:

P-Channel MOS Datasheet

N-Channel MOS Datasheet

CMOS Technology



Views: 28569

Comment by Shannon Bradley on October 30, 2012 at 10:13am


MOSFETs are low noise and offen used in guitar amps, only they do cook-out easy. I got to get a small Kiln & mod. it.

Comment by Shannon Bradley on October 30, 2012 at 12:24pm

Sorry, Cook-out means burn-out easy. Nothing to do with making MOSFETs. I may need to make a greater foward than germanium. Tho I think China has one of Ge. Just a bit more conductivity. No two in paralled doesn't do it. Offen things deify math. Any-ways now I'm here.

Question : If Germanium-on-insulator substrates are seen as a potential replacement for silcon on miniatursed chips,,,, What is the above last FET ? does it replace some-thing ? My books miss that.

Comment by Geoffrey Hunter on October 30, 2012 at 3:27pm
causing a TBJ to be generally applied in circuits

BJT spelt wrong :-0

Perhaps worth mentioning MOSFETs can fail due to gate punch-through and need protecting in certain situations, this is a common occurance when using MOSFETs to switch motors.

Comment by Shannon Bradley on October 31, 2012 at 7:50am

Thanks Geoffrey,

 Ya, they here showed TO-200 MOSFET's but didn't say a mfd.# so I don't know for certain what it was used for. I have seen and readed about failures as op-amps. Hard to remember but I recall touching two speaker wires to-gether by accident with so vol. up and needing to replace them or may-be it came that way. I can't remember what they were in... But at high current I guess it comes with the territory. I tryed to fine that MOSFET with the 'on-insulator' I'm guessing its more rugged and is used for switching or a rectifier. I have to wonder if one was ever made but I do have info on Ge-on-insulator. Please don't tell me these are common.

Comment by Jonathan Dean on November 5, 2012 at 10:16am

What are factors to consider when deciding between using a BJT or MOSFET?

Geoffrey, could you expand some on what gate punch-through is?

Comment by Geoffrey Hunter on November 6, 2012 at 1:04pm

Gate punch-through: Occurs when a large voltage spike appears on the gate that exceeds the maximum gate-source voltage (typically 10-20V). It punches a hole in the weak oxide layer, normally causing the MOSFET to fail ON. See (http://cladlab.com/electronics/components/mosfets) for more info.

Comment by Shannon Bradley on November 7, 2012 at 7:19am

Yes Mr. Hunter,

 Just an large static discharge at the gate is all it will take.

 Well, I'm still dumfounded why Professor Rodrigues would bring up such an MOSFET. It must have been on the bottom of his bag of tricks. I had to dig deep into my archives but I found a little on it if any-one wants more. I found it in a 1972 Physical and Solid State Electronics (Kang) book a Addison-Wesley Series In Electrical Engineering. I do remember some-thing of the sort in my transistor class but I got to tell you. I only see it on paper. Kind of like an thermistor voltage regulator vacuum tube that Zeinth used for there Transoceanoics radio from the 1940. Any-ways there should be a fig of both silicon types of this be-low. I have no Ratings or characteristics on this.

Comment by Marcelo Rodrigues on November 8, 2012 at 4:11am


The images are not appearing. Probably they are in some restrict server or folder.

Can you check that?


Comment by Shannon Bradley on November 9, 2012 at 8:21am

I was up-set I didn't know what an Insulated FET was. I all-so though it was some kind of a test or that the author didn't. I couldn't understand why such a device would be pointed out of what was shown. I hit my books 1060's to to-days. It had to be some-where. I frist found it in a 1972 book and then saw that it was a IGFET a much more common name. I then found a short 1/2 page on them in a 1989 book. As I readed about them I learn that one could be used both as a PNP and NPN. Then the flood gates open and yes I did know about them. I could even recall covering them in transistor class. It was 1972, I was 22yrs old My teacher was 72yrs old and was sooo respected and loved by all. He only spent about one hour on them. Just to show us about them and then told us that he didn't like them, he said they have problems and he didn't want us to employ them. Then he moved on ... If I thiught this thread was going to end I could tell you how wonderful this old fellah was. Any-ways top fig. IGFET and be-low two types of insulated. I would like at this time to point out the - thin-film transistor (TFT)

Comment by Shannon Bradley on November 9, 2012 at 8:23am


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