#Mosfet meaning how to#

Ī variating voltage on the gate will give a variating Vds due to the changing resistance of the mosfet channel (higher gatevoltage > lower Rds > higher current possible).Īs you can see in the graph a higher Vgs allows a higher Id cause there is a lower Vds.

When you want to amplify (class B) or dim you need to keep the mosfet in the lineair region. (its a crappy source of info but this graph is standard(for n-mosfet) Have a look at the graphic from wikipedia , In itself i think there was no confusion last night ,there were 3 people saying the same in their own words. Threshold voltage (Vtp) is typically negative in the case of P-MOS.Ī P-MOS switch will have about three times the resistance of an N-MOS device of equal dimensions because electrons have about three times the mobility of holes in silicon. The P-MOS switch passes all voltages higher than (Vgate+Vtp). In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. When the switch is conducting, it typically operates in the linear (or Ohmic) mode of operation, since the source and drain voltages will typically be nearly equal.

An N-MOS switch passes all voltages less than (Vgate–Vtn). The higher the voltage, the more the MOSFET can conduct. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. In the case of an N-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control. This analog switch uses a four-terminal simple MOSFET of either P or N type. Those guys have high Rds(on) so they are typically NOT used for linear applications. In saturation, Vds is very low because Rds(on) typically is very low so the mosfet dissipates little power.Ī typical vertical mosfet turns on at Vgs=3-4v, and goes into saturation after Vgs > 5 - 6v - we typically drive them at Vgs=10v.Ī logical mosfet turns on at Vgs=2v, and goes into saturation at Vgs > 4v.Ī lateral mosfet usually goes on at Vgs=1v. Mosfet dissipates considerable amount of heat in this region, and this is where you use a mosfet for amplification.Īs you continue to increase Vgs, the mosfet goes into saturation where it functions more or less like a resistor between its D and S pins - thus the Rds(on) parameter dominates. Vgs relationship can be 'linearized' via a source resistor, for example. This is called the linear region because the mosfet's Id vs. Vgs chart (with Vds fixed but Vds isn't that big of a factor here). you can see that in your chart, between the traces.Īs Vgs goes up, the mosfet starts to conduct, as you can see in a typical Id vs. typically you will see a Vgs(th) parameter in the datasheet. The transition from the 'off' state to the 'on' state is gradual. Usualy most of those factors are of no concern ,but this datasheet does answer all your questions about Rdson and what it consists of. The last datasheet describes in detail what composes your Rdson (and other parameters ).

#Mosfet meaning how to#

It describes how to measure your Rdson and also how they specify it. Have a look at page 5 of this datasheet and it should take your doubts away. (Rdson stated in the datasheet is at 25 degree celcius and is usualy also noted at 75 or 100 degree) ĭo keep in mind to fully open a mosfet a gatevoltage of 10 to 15 volt is needed (logic mos excluded). In the switch state you either have very high resistance(closed) or Rdson like stated in the datasheets when its fully opened. In the *triode* region you can regulate the *resistance* with Vg ,your resistance depends on Vg.Īssuming you can keep the mosfet temperature stable ,then your Rds will be as stable as your gatevoltage. (hot mosfets have a slightly higher resistance) The only change in resistance you will wittnes is due to the positive temperature coefficient of the mosfet. From what i know Rdson is measured when the mosfet is fully opened.