Mosfet

Discussion in 'General Electronics Chat' started by vustudent, Jun 27, 2010.

  1. vustudent

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    Mar 11, 2009
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    A MOSFET is a symmetric device, therefore the terminal which has the higher voltage would be the drain and the lower would be the source?
     
  2. jpanhalt

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    Look up the datasheet for the NTD20P06L (or any P-channel mosfet) and see if you reach the same conclusion.

    John
     
  3. vustudent

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    Mar 11, 2009
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    [​IMG]

    If I am understanding this symbol correctly, holes would flow to the drain only when the zener voltage has been reached, so voltage at the source is greater in this case, and it is the case for all pmos I just realized.
     
    Last edited: Jun 27, 2010
  4. kubeek

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    That is not a zener diode, nor a schottky. It is an intrisic body diode which always is inside of a mosfet, but not on purpose.
     
  5. beenthere

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    Some confusion may arise from the similarity of names. What was originally meant by "mosfet" was a device much like the ubiquitous MPF102 - a small, very low power amplifier with a large input impedance.

    The power variations actually use thousands of paralleled cells like those individual mosfets, and might be more accurately referred to as "hexfets" - http://en.wikipedia.org/wiki/Power_MOSFET
     
  6. Ron H

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    I'm missing your point.:confused:
     
  7. beenthere

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    From the original post -
    The MPF102 is symmetric as indicated. Voltage on the gate will promote conduction between the other terminals, and polarity is not significant. An IRF510 (for instance) is not like that, as the drain must be positive with respect to the source.
     
  8. Jony130

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    But isn't MPF102 a JFET not a MOSFET?
    And only JFET are symmetrical.
    All discrete MOSFET have built-in diode
     
  9. Ron H

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    MPF102 is a JFET. All MOSFETs have a substrate (body) which must be connected to some voltage. Most are 3 pin devices, with the body tied to the source. Four pin devices such as SD210 bring out the body on a separate pin.
     
  10. beenthere

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    Well, I used to know that....
     
  11. jpanhalt

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    I have read that once on, a mosfet conducts in either direction. For example:
    Of course, when off, there is also reverse conduction through the intrinsic diode. A circuit that used the "on" reverse conduction property to advantage would be interesting. What would be required to keep the gate turned on during reverse conduction? Perhaps there is a compilation of odd circuits for mosfets that someone knows about.

    John
     
  12. vustudent

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    Mar 11, 2009
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    I am not sure what do you mean by reverse conduction, an nmos for example when not "turned on" would still conduct slight current in the subthreshold range given the fact that the nmos does not physically "turned on"like a switch once the threshold voltage has been reached, but is a graduate process.

    Or are you referring to the leakage current from the source to the gate?
     
  13. vustudent

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    "If the MOS structure is symmetric, why do we call n region the source and the other drain? This becomes clear if the source is defined as the terminal that provides the charge carriers (electrons in the case of NMOS devices) and the drain as the terminal that collects them. Thus, as the voltages at the three terminals of the device vary, the source and the drain may exchange roles." quoted from "Design of Analog CMOS Integrated Circuits" by Behzad Razavi.


    So it would be absurd to say that current flows from the source to the drain in an nmos, since saying so is without realizing how the terms drain and source are defined in the first place; where in an nmos the source is where electrons source out to the drain and thus current flows from the drain to source. For an nmos if current is flowing from left to right, the left would be your drain and right the source, vice versa.

    And I have just solved my own question.

    So it would be yes for an nmos since with how drain and source are defined, electrons will flow from the source to the drain only when an E field is established directing from the D to the S in terms of a higher voltage at the drain.

    And for pmos the source would always have a higher voltage.
     
  14. jpanhalt

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    I can't tell if both of the above posts refer to my comment about conduction. I used conventional current and for simplicity refer only to the N-channel mosfet. When the mosfet is turned on, current normally flows from Drain to Source. When it is off, the intrinsic diode will conduct from Source to Drain. However, it is stated in many places, including the Microsemi link, that when On, the mosfet conducts equally in both directions (Drain-to-Source = "forward" and Source-to-Drain = "reverse"). That is consistent as well with your quote from Razavi.

    I have never studied that reverse conduction and was wondering if it had some practical usefulness.

    John
     
  15. vustudent

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    Mar 11, 2009
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    wait a minute, I guess I was refering to an nmos you used as a switch in cmos technology, with p substrate and n wells.

    but this thing here : [​IMG]

    this power mosfet is no where near what i have learned. What is an APT n channel power mosfet anyway?
     
  16. RRITESH KAKKAR

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    Jun 29, 2010
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    I want to know that i have IRF 3205 mosfet and It's working But some Dout is there that is this dep. or ench. MOSFET & If i change conn. across source & Drain It is not Working I have listen there is no imp. of It only in transister.......?
     
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