+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Junior Member level 1
    Points: 1,489, Level: 8

    Join Date
    Apr 2005
    Posts
    19
    Helped
    0 / 0
    Points
    1,489
    Level
    8

    what forms the channel in a MOS

    Say V(S)=V(D)=0, VGS>VTH.
    My doubt is : What forms the channel? Are the electrons that form the inversion layer generated by EHP in the substrate? or they are injected from the source?

    I didnt find the exact answer in any text book. Please help.

    •   AltAdvertisement

        
       

  2. #2
    Member level 5
    Points: 2,727, Level: 12
    Achievements:
    7 years registered
    Willt's Avatar
    Join Date
    Mar 2006
    Location
    Hong Kong
    Posts
    83
    Helped
    5 / 5
    Points
    2,727
    Level
    12

    Re: what forms the channel in a MOS

    Provided that Vgs - Vth > 0, a channel is formed from the electrons in substrate which is connected to the ground for NMOS.
    When there is potential difference between drain and source, current flows through the transistor.



    •   AltAdvertisement

        
       

  3. #3
    Member level 1
    Points: 1,355, Level: 8

    Join Date
    Apr 2007
    Posts
    40
    Helped
    9 / 9
    Points
    1,355
    Level
    8

    Re: what forms the channel in a MOS

    Thats a good question.
    Well,I think when VDS=0 and VGS>0, then channel in an NMOS transistor is due to the e-s in the substrate.(minority carriers in the substrate.Hence off current of a transistor is pretty low,similar to that of a diode)
    However when VDS>0,then we have additional e-s from source contributing to current.Additonally we have a drift mechanism which accelerate holes from source to reach drain.

    So , conclusion:
    1)VDS<0 and VGS>0, e-s are from p-sub in NMOST
    2)VDS>0 and VGS>0. e-s are from both n-source and p-sub in NMOT.

    Hope this helps.
    PS: I do not have any concrete proof for this but I have discussed with other experienced guys and they feel above explanation is correct.



    •   AltAdvertisement

        
       

  4. #4
    Newbie level 4
    Points: 1,491, Level: 8
    Achievements:
    7 years registered

    Join Date
    Apr 2007
    Posts
    6
    Helped
    1 / 1
    Points
    1,491
    Level
    8

    Re: what forms the channel in a MOS

    Hi,

    When VGS is positive, the positive voltage on the gate attracts the electrons(minority carriers) from the substrate.Also because source and drain have plenty of electrons,and we have(Cgs and Cgd overlap caps), some of these electrons contribute to the channel formation.



  5. #5
    Full Member level 4
    Points: 3,495, Level: 13

    Join Date
    Nov 2006
    Location
    Cairo , Egypt
    Posts
    212
    Helped
    17 / 17
    Points
    3,495
    Level
    13

    Re: what forms the channel in a MOS

    For NMOS:

    Increasing VGS will attract electrons (subs. minority carriers)- by the electric field across the oxide- near the oxide surface , when the electrons concentration at the oxide si interface = the holes concentration at the substrate it is said that that this applied VGS = Vth.(this is Vth defin. independent of VDS)

    I think that what A_U_J said is right.

    But what about the LDD formation which is done to minmize carrier injection from the source/drain regions ?



    •   AltAdvertisement

        
       

  6. #6
    Member level 2
    Points: 2,482, Level: 11
    Achievements:
    7 years registered

    Join Date
    Dec 2006
    Posts
    44
    Helped
    2 / 2
    Points
    2,482
    Level
    11

    Re: what forms the channel in a MOS

    dear friend, I suggest you to study this book:

    1/analysis and design of analog integrated circuits by Gray



--[[ ]]--