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Understanding N channel and P Channel FETs

Discussion in 'General Electronics Discussion' started by Rajinder, Jul 30, 2018.

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  1. Rajinder

    Rajinder

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    Jan 30, 2016
    Hi all
    I have always had difficulty in understanding how FETs work. N channel and P channel also all the variants such as enhancement and depletion types. Also the operation modes ie. Linear mode triode mode etc
    I would like to learn this practically.
    Are there any good resources where i could learn practically and with easy examples.
    Thanks in advance.
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    As ever, I always refer such queries to the Horowitz and Hill book "The Art of Electronics".

    There are, no doubt, many other books on the subject (and websites) but for sheer completeness you really should invest in a copy of that book. It will help you in all areas of electronic knowledge and, much as I like e-books etc, a good hold-in-yourhand book is always the best solution.
     
  3. Rajinder

    Rajinder

    531
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    Jan 30, 2016
    Thanks will get a copy.
     
  4. Minder

    Minder

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    Apr 24, 2015
    Having come up through the valve era, I embraced the Mosfet because it resembled the old Valve (tube) technology I was used to, I rarely use any bi-polar devices now unless I have to.
    It might even be an asset to go back and review the old Triode valve if not familiar.
    And other trans conductive devices.
    Max.
     
  5. WHONOES

    WHONOES

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    May 20, 2017
    I agree with you on the book. I have recommended it several time. Also, like you, I prefer something I can hold in my hand. Good stuff.
     
  6. WHONOES

    WHONOES

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    May 20, 2017
    In a nutshell, MOSFETS are enhancement mode and JFets are depletion mode.
     
  7. Bluejets

    Bluejets

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    Oct 5, 2014
    As far as I know, mosfets have both enhancement and depletion types......?????
     
  8. Ratch

    Ratch

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    Mar 10, 2013
    A BJT is also a transconductive device.

    Ratch
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yes, if you consider the Vgs(th) as a parameter that can be altered by the designer of the mosfet, roughly speaking an enhancement mode mosfet has a Vgs(th) greater than zero and a depletion mode device has one less than or equal to zero. This represents a continuum where we place enhancement vs depletion devices either side of zero and logic level devices arbitrarily close to zero.

    It is theoretically possible to make an enhancement mode jfet, but it would be about as useful as equally possible (enhancement or depletion mode) mosfets where the Vgs(th) was greater than Vgs(max).
     
  10. WHONOES

    WHONOES

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    May 20, 2017
    Which is why I have always regarded them as I described earlier.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's fair enough to describe jfets as depletion mode.

    Maybe describe mosfets as "most often enhancement mode"

    See BSS169H6327XTSA1CT-ND
     
  12. BobK

    BobK

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    Jan 5, 2010
    That does not make sense to me. Higher source to gate voltage produces more current. That sounds like enhancement. The difference is that the gate threshold is below zero.

    Bob
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm not sure what you're referring to, but if it is my comment about what could be manufactured (including an enhancement mode jfet), they were examples of things that would be about as useful as soluble fish.
     
  14. BobK

    BobK

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    Jan 5, 2010
    What I am saying is that I cannot understand the why an N-channel MOSFET is called enhancement mode and an N-channel JFET is called depletion mode when, in both cases, raising Vgs increases the current in the channel. In both cases there is a threshold voltage, and when Vgs is below Vgsth, current is cut off. The only difference being that Vgsth is negative for the JFET and positive for the MOSFET.

    Bob
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What you need to consider is the channel state when Vgs = 0. If there is conduction, and you have to change the Vgs to turn the device off, then your action to use the device is by depleting the channel, and this the device is depletion mode.

    In the reverse case, where Vgs = 0 results in no conduction, and you need to change Vgs in order to make it conduct, then the device is an enhancement mode device.

    In both cases you can change Vgs in the opposite sense to turn an enhancement device off harder, or a depletion mode device on more.

    For all intents and purposes most enhancement mode mosfets are effectively completely off well before Vgs = 0, so we rarely pull the gate lower. However, you'll find that Vgs(max) is typically defined symmetrically around zero, and placing a negative bias on an N Channel MOSFET can be used to increase gate current and also reduce effects where coupling of gate capacitance to the device's output capacitance can induce gate voltages which turn a device back on.
     
  16. BobK

    BobK

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    Jan 5, 2010
    Thanks, now I understand.

    Bob
     
  17. Bluejets

    Bluejets

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    Me too... Thanks
     
  18. Ratch

    Ratch

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    You have a right to be confused. There is a lot of confusing information about transistors everywhere.

    Let's start with the enhancement type MOSFET, also known as the insulated gate FET or IGFET. Below is a picture of the structure of a N-channel enhancement MOSFET. A P-channel enhancement MOSFET acts the same, but the voltages, charge carriers and semiconductors are reversed.

    Rajinder1.JPG

    Notice it has a thin layer of silicon oxide insulating material under the gate lead. When no voltage is applied to the gate, the FET acts like two back-to-back diodes, and a high resistance exists between the source and drain. Applying a positive voltage with respect to the source causes the holes to move further into the p-type substrate leaving behind a depletion region beneath the silicon oxide layer consisting of bound negative charges created by the uncovering of the acceptor atoms. Increasing the gate voltage attracts electrons from both the source and drain N-regions so that a electron rich or N-channel is formed. This is shown below.
    Rajinder2.JPG
    So the enhancement type MOSFET creates a channel by electrical induction. No current will exist between the source and drain until the gate voltage is at a sufficient level and a voltage difference between the source and drain is present.

    The deplection-type MOSFET operates the same way except the N-channel is physically implanted between the source and gate. This means that current will exist between the source and drain even if no gate voltage is present, provided there is a voltage difference between the source and drain. A forward gate voltage will increase the source to drain current and a reverse gate voltage is need to deplete the N-channel and stop the current. So this FET type can be operated in either the enhanced mode by a forward gate voltage or depletion mode by a reverse voltage.

    The JFET is shown below.
    Rajinder3.JPG

    This simple structure will allow current between the source and drain even if no voltage is applied to the gate. To reduce current, a reverse voltage has to be applied to the gate which causes the p-depletion region to expand and make the N-channel narrower and restrict current.

    To summarize: FET,s, TUBE,s, AND BJT,s are all transconductance devices. The TUBE,s and FET,s control current by electric fields and BJT,s control current by diffusion. FET,s manage current by sending a single polarity charge flow along depletion regions in channels, and BJT's do it by sending charges of both polarities through depletion regions by diffusion. The charge flow is a 90 degree difference in direction between FET's and BJT's.

    Ratch
     
  19. Ratch

    Ratch

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    Mar 10, 2013
    Wrong, wrong, and wrong.

    Ratch
     
  20. Ratch

    Ratch

    1,094
    334
    Mar 10, 2013
    Correct.

    Ratch
     
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