P-channel Mosfet won't turn off in LTspice ?

Discussion in 'General Electronics Chat' started by agt128, Feb 26, 2013.

  1. agt128

    Thread Starter New Member

    Mar 3, 2012
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    In this circuit, a P-channel mosfet has gate & source both connected to V+ which is 9V. I expect the mosfet to be Off . However, LTspice shows current of 8.4mA (very large) in Resistor R1.
    Can you please explain why ?


    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You have the Mosfet inverted and the body diode is conducting.

    Put the source terminal on top and the gate to the left (rotate 180º and mirror), then try it again.
     
  3. agt128

    Thread Starter New Member

    Mar 3, 2012
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    Oh mind, how silly I was ... :confused:

    By any chance, could you please give me a hint as why the arrow points "out" in P-channel one , and points "in" in N-channel one.
    In BJT it is different, arrow points out for NPN , and points in for PNP.
     
  4. agt128

    Thread Starter New Member

    Mar 3, 2012
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    The picture from wikipedia is for N-channel one. The symbol is the P-mosfet. The notation of N, P, N doesn't match with P-mos at all (should be P-pocket , N-substrate(Pchannel) , P-pocket ).
    Any way, the arrow is clearly not for direction of current which is what I am confused.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    Which current are you thinking of?

    MOSFET symbols show "Electron Flow"

    BJT, diode symbols show "Hole Flow" aka "Conventional Flow"

    As long as you stick with one charge being defined positive and flow is toward "negative", the math works out at the schematic level.

    Circuits are easier to comprehend if using "Conventional Flow" to people new to electronics.
     
  6. agt128

    Thread Starter New Member

    Mar 3, 2012
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    I found an explanation which is very helpful, I suggest anyone take a look if interested :
    "Direction of \arrows" used to identify semiconductor types in a transistor may appear con-
    fusing. The arrows do NOT represent the direction of current
    ow in the device. Rather, they denote the direction of the underlying pn junction. For a NMOS, the arrow is placed
    on the body and pointing inward as the body is made of p-type material. (Arrow is not
    on source or drain as they are interchangeable.) In the simpli ed symbol for the case when
    body and source is connected, arrow is on the source (device is not symmetric now) and is
    pointing outward as the source is made of n-type materials. (i.,e. arrow pointing inward for
    p-type, arrow pointing outward for n-type)".

    Source : http://aries.ucsd.edu/najmabadi/CLASS/ECE60L/02-S/NOTES/FET.pdf
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yeah, I screwed my post so bad, I deleted it.:(
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    You can look at the arrow in the MOSFET symbol as the direction of forward bias current of the body diode. If you remember that you always want to keep that diode reverse biased under normal operating conditions then you can readily determine what the correct bias polarity is for both N and P MOSFETs.
     
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