Understanding Drift and Diffusion Currents in PN Junctions

[PainterGuy]

hello everyone,

is this definition correct?
drift current is flow due to applied voltage across P-N junction & Diffusion is flow due to different concentration of hole & electron across P-N junction.

warning: [i read this definition somewhere, hence not my own work]

1:-- the definitions would means that diffusion current always exists between PN junction (say a diode) even when there is no applied voltage (means not connected to a power supply). but drift needs a power supply to exist.

2:-- while PN junction is having drift current, would there still exist diffusion current?

lead me to correct way please.

cheers

 

[Dickfore]

What is the direction of the diffusion current at a P-N junction?

 

[PainterGuy]

What is the direction of the diffusion current at a P-N junction?

hi,

will it not be from both sides? negative carriers from n-side diffusing into p-side and positive carriers diffusing into n-side?

cheers

 

[PainterGuy]

hello,

will someone comment on my first post please? it will be generous of you.

cheers

 

[Dickfore]

hi,

will it not be from both sides? negative carriers from n-side diffusing into p-side and positive carriers diffusing into n-side?

cheers

You just described the direction of diffusive flow of different carriers and that is correct. But, what is the (technical) direction of the electric current caused by the flow of negative carriers from the n-side to the p-side?

 

[PainterGuy]

You just described the direction of diffusive flow of different carriers and that is correct. But, what is the (technical) direction of the electric current caused by the flow of negative carriers from the n-side to the p-side?

many thanks Dickfore.

okay. let me try. suppose we have p material on right side of the screen and n material on the left side of screen. now we join them. now some of the electron from n material (from left side) will flow toward p material (on the right). overall no p carrier from p material will flow toward n material on the left.

as some of the electrons become part of p material, p material will become negative and as n material is somewhat deficient in electrons, therefore it becomes somewhat positive. and i think this what depletion layer is. at this point a potential exists between p material and n material but no overall charge exchange takes place. this is all i can think of. now please correct me and guide in proper direction. many thanks

cheers

 

[PainterGuy]

will someone comment on what i said in the previous posts please? it has been days.

cheers

 

[goodname]

All you have mentioned are approximations. Actual situation is different and complicated and what exactly do you want to clarify?

 

[PainterGuy]

Hello everyone,

[Yes, I made promise to someone that from now onward I will use proper capitalization ]

The points "1" and "2" in the below quoted post I need to clarify. Please shed some light.

Below is my edited version of first post:--

Is this definition correct?
Drift current is a flow due to applied voltage across P-N junction & Diffusion current is a flow due to different concentration of hole & electron across P-N junction.

1:-- Does the definition mean that diffusion current always exists between PN junction (say a diode) even when there is no applied voltage (means not connected to a power supply). but drift current needs a power supply to exist?

2:-- While P-N junction is having drift current, would there still exist diffusion current?

Cheers

 

[goodname]

Consider there are 6 electrons and are put in a separate cubic room(4 walls, 1 roof, 1 ground). Each electrons will exhibit their own electric field around it throughout the entire universe independent of other electrons. So each electron will try to be apart from each other geometrically(because of repulsion force between electrons). So now there is motion of electrons in different direction and finally each electron occupies center of each side of room (4 walls, 1 roof, 1 ground) at which the system attains equilibrium because the walls will not allow the electron to escape out of the room . This type of motion which is due to its own electric field(electric fields of 6 electrons alone) is called diffusion current.

Now you keep another 10 electrons outside the room nearby a wall and 10 protons outside the room nearby the wall which is exactly opposite to the first wall where 10 electrons were deposited.Now balance of the electric filed inside the room is disturbed and the 6 electron inside the room moves towards the wall which is nearer to 10 protons to attain new equilibrium. This type of motion caused by the external source of electric filed( not by the 6 electrons inside the room) is called drift current.

So answer for your questions depends on magnitude of applying voltage(external field source) and number of charge carriers inside the system.

1:-- Does the definition mean that diffusion current always exists between PN junction (say a diode) even when there is no applied voltage (means not connected to a power supply). but drift current needs a power supply to exist?

Answer:
Equilibrium will be reached quickly after the materials(p&n) are joined and diffusion current would tend to zero but never attains zero because new free electrons are always produced due to thermal agitation and very very small diffusion current continues forever.

Drift current presents only during the presence of external voltage.

2:-- While P-N junction is having drift current, would there still exist diffusion current?

Answer:

Yes, a component of it exists.

 

[PainterGuy]

Consider there are 6 electrons and are put in a separate cubic room(4 walls, 1 roof, 1 ground). Each electrons will exhibit their own electric field around it throughout the entire universe independent of other electrons. So each electron will try to be apart from each other geometrically(because of repulsion force between electrons). So now there is motion of electrons in different direction and finally each electron occupies center of each side of room (4 walls, 1 roof, 1 ground) at which the system attains equilibrium because the walls will not allow the electron to escape out of the room . This type of motion which is due to its own electric field(electric fields of 6 electrons alone) is called diffusion current.

Now you keep another 10 electrons outside the room nearby a wall and 10 protons outside the room nearby the wall which is exactly opposite to the first wall where 10 electrons were deposited.Now balance of the electric filed inside the room is disturbed and the 6 electron inside the room moves towards the wall which is nearer to 10 protons to attain new equilibrium. This type of motion caused by the external source of electric filed( not by the 6 electrons inside the room) is called drift current.

So answer for your questions depends on magnitude of applying voltage(external field source) and number of charge carriers inside the system.

1:-- Does the definition mean that diffusion current always exists between PN junction (say a diode) even when there is no applied voltage (means not connected to a power supply). but drift current needs a power supply to exist?

Answer:
Equilibrium will be reached quickly after the materials(p&n) are joined and diffusion current would tend to zero but never attains zero because new free electrons are always produced due to thermal agitation and very very small diffusion current continues forever.

Drift current presents only during the presence of external voltage.

2:-- While P-N junction is having drift current, would there still exist diffusion current?

Answer:

Yes, a component of it exists.

Many thanks goodname. It is very to the point and easy explanation. Very nice of you. Hope you will keep helping me in future the same way.

Cheers

 

[goodname]

Sure. Happy to help!

 

[Feldoh]

I would also like to note that drift current, is relatively independent of applied voltage on a pn-junction and because of such there will always be a small drift current due to minority depletion.