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Simple understanding of mosfets

  1. Mar 14, 2014 #1
    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

    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?
  2. jcsd
  3. Mar 14, 2014 #2


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    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.
  4. Mar 14, 2014 #3
    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.
  5. Mar 14, 2014 #4
    so when the gate voltage is a regular digital 5v, should I pretty much pretend it is a closed circuit?
    No catch?
  6. Mar 14, 2014 #5


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    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.
  7. Mar 15, 2014 #6
    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?
  8. Mar 15, 2014 #7


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    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)
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