+ Post New Thread
Results 1 to 7 of 7
  1. #1
    Member level 2
    Points: 323, Level: 3

    Join Date
    Feb 2019
    Posts
    44
    Helped
    0 / 0
    Points
    323
    Level
    3

    PN junction in thermal equilibrium

    With zero bias, due to concentration gradient, the diffusion of majority carriers happen. Because of this we have built in E which favors the flow of minority carriers... on the n side we have uncovered positive ions and these ions oppose the motion of holes from p side but helps the thermally generated holes on the n side to flow across the junction. The thing that I do not follow, these positive ions should repel the minority holes on the n side as well, instead minority carrier holes get swept across the junction.. Same phenomenon on the p side , the minority e should experience a repulsive force from uncovered negative ions on p side but why doesn’t this happen ?

    •   AltAdvertisement

        
       

  2. #2
    Full Member level 6
    Points: 3,068, Level: 13
    Achievements:
    7 years registered

    Join Date
    Mar 2012
    Posts
    380
    Helped
    74 / 74
    Points
    3,068
    Level
    13

    Re: PN junction in thermal equilibrium

    Quote Originally Posted by Robotduck View Post
    With zero bias, due to concentration gradient, the diffusion of majority carriers happen. Because of this we have built in E which favors the flow of minority carriers... on the n side we have uncovered positive ions and these ions oppose the motion of holes from p side but helps the thermally generated holes on the n side to flow across the junction. The thing that I do not follow, these positive ions should repel the minority holes on the n side as well, instead minority carrier holes get swept across the junction.. Same phenomenon on the p side , the minority e should experience a repulsive force from uncovered negative ions on p side but why doesn’t this happen ?
    Concentration gradients cause carriers to leave where they are not wanted and go where the are welcomed. Because they are charged particles, they cause ions to form when they leave and also form when they arrive at another location. Thermal conditions cause carriers to separate/attract to/from their parent atoms and do the same thing. When no voltage is applied across the PN junction, the thermal current exists is the opposite direction and is equal to the junction current. Therefore, the total current through the PN junction is balanced to zero when when the junction is shorted.

    Ratch
    Hopelessly Pedantic



  3. #3
    Advanced Member level 5
    Points: 17,223, Level: 31
    Achievements:
    7 years registered

    Join Date
    Nov 2012
    Posts
    3,147
    Helped
    777 / 777
    Points
    17,223
    Level
    31

    Re: PN junction in thermal equilibrium

    With zero bias, due to concentration gradient, the diffusion of majority carriers happen. Because of this we have built in E which favors the flow of minority carriers
    With ZERO external bias. There is an internal bias due to the junction potential.

    Yes, there is a continuous diffusion current and that is exactly balanced by the current due to the minority carriers.

    This is a dynamic equilibrium process. We call a process to be in dynamic equilibrium when two opposite rate processes cancel exactly.

    The junction potential is set up or produced by the diffusion of the majority carriers across the pn junction. The junction potential opposes diffusion of majority carriers.

    If there are NO minority carriers, this is going to be a static equilibrium process.



    •   AltAdvertisement

        
       

  4. #4
    Member level 2
    Points: 323, Level: 3

    Join Date
    Feb 2019
    Posts
    44
    Helped
    0 / 0
    Points
    323
    Level
    3

    Re: PN junction in thermal equilibrium

    But my question is : The built in E field in the Depletion region favors the flow of minority carriers. The thing that is bothering me is - See you have uncovered positive ions on n side. this net positive charge and holes in N region should have repulsive forces.. How come then the holes in N region are able to cross the junciton.. you see what I am saying ?



  5. #5
    Advanced Member level 5
    Points: 17,223, Level: 31
    Achievements:
    7 years registered

    Join Date
    Nov 2012
    Posts
    3,147
    Helped
    777 / 777
    Points
    17,223
    Level
    31

    Re: PN junction in thermal equilibrium

    The built in E field in the Depletion region favors the flow of minority carriers
    Right. AND this tends to lower the junction potential. As the junction potential decreases, diffusion of majority carriers take place- that increases the junction potential.

    This can be seen when you change the temp because temp change affects the dynamic equilibrium - the junction potential changes. Temp change affects minority carriers and majority carriers differently.



  6. #6
    Member level 2
    Points: 323, Level: 3

    Join Date
    Feb 2019
    Posts
    44
    Helped
    0 / 0
    Points
    323
    Level
    3

    Re: PN junction in thermal equilibrium

    I think I probably was not clear about my question. Now I have attached a figure with this message. Can you please look at the figure and please tell me what am I missing Click image for larger version. 

Name:	PN.jpg 
Views:	4 
Size:	494.4 KB 
ID:	155375?
    What you said is absolutely right but this one thing is bothering me and I am sure it is related basics of electrostatics....
    Thank you



    •   AltAdvertisement

        
       

  7. #7
    Advanced Member level 5
    Points: 17,223, Level: 31
    Achievements:
    7 years registered

    Join Date
    Nov 2012
    Posts
    3,147
    Helped
    777 / 777
    Points
    17,223
    Level
    31

    Re: PN junction in thermal equilibrium

    What you said is absolutely right but this one thing is bothering me and I am sure it is related basics of electrostatics....
    The majority carriers are coming from doping. On the p-side, the holes are mobile but the electrons are not; on the n-side the electrons are mobile but the holes are not. Why? The impurity levels are not continuous. They are shown as thin lines close to the conduction or valence bands.

    The minority carriers come from the intrinsic character of the semiconductor; both electrons and holes are mobile.

    The distinction is notional but convenient. Because holes are nothing but missing electrons in the structure.

    The concentrations of intrinsic and extrinsic conductions (carriers) are order of magnitude different. That is the reason reverse current is so small.

    This is a qualitative description but is essentially correct.



--[[ ]]--