diode basic

[timkuc]

My question is about drift and diffusion current.In a semiconductor diode no "bias" condition diffusion is present and due to transfer of charges due to diffusion depletion region created and then this region has ions which produce electric field which results in drift current due to force the minority charges from both the sides.So the diffusion current and after that drift current comes into picture .In forward bias there will be potential difference across the diode so there must be electric field in the diode.So in forward bias current should be due to the battery voltage .What I don't understand is ,is there any phenomenon of diffusion after applying the battery (because battery produces electric field in material,that means only drift current should be there as the drift current definition says "Drift current is the movement of electrons in electric field) ? Some says that drift current only depends on temperature (Why ?). I think I am not getting the drift current what it really is . I am in really in trouble.

 

[Ratchit]

My question is about drift and diffusion current.In a semiconductor diode no "bias" condition diffusion is present and due to transfer of charges due to diffusion depletion region created and then this region has ions which produce electric field which results in drift current due to force the minority charges from both the sides.

In a junction diode, there are three mechanisms which move the charge carriers. They are drift, diffusion, and recombination-generation (RG). As soon as a diode is manufactured, the electrons from the n-material and the holes from the p-material diffuse into each other's material, neutralize each other's charge carriers, and set up a depletion region. This leaves behind charged immovable ions on the boundary of the depletion region which oppose and eventially stop any further diffusion. These charged ions do this by setting up an opposing barrier voltage caused by their charge. Applying a forward voltage lowers the barrier voltage and allows further diffusion to take place and a current to exist. Applying an opposing voltage increases the barrier value and retards or stops diffusion from taking place.

In a forward-biased junction diode, diffusion is the primary mechanism for moving charge carriers. Drift is inconsequential and R-G is always inconsequential at room temperatures. In a reverse-biased junction diode, drift is the primary mechanism for moving charge carriers. The reverse current on a diode voltage-current graph is composed mostly of drift current, which shows how small it is.

So the diffusion current and after that drift current comes into picture .In forward bias there will be potential difference across the diode so there must be electric field in the diode.So in forward bias current should be due to the battery voltage .

As I said, in a forward-biased diode, diffusion is the primary mechanism for moving charge carriers. The voltage is used to lower the barrier voltage so that more diffusion can take place.

What I don't understand is ,is there any phenomenon of diffusion after applying the battery (because battery produces electric field in material,that means only drift current should be there as the drift current definition says "Drift current is the movement of electrons in electric field) ?

Diffusion always takes place in a forward-biased diode and is the primary mechanism for moving charge carriers. Drift current also applies to holes in a electric field. Drift is not significant in a forward-biased diode.

Some says that drift current only depends on temperature (Why ?). I think I am not getting the drift current what it really is . I am in really in trouble.

False. You cannot heat a semiconductor slab and expect to observe drift current. Drift current is the movement of charge carriers due to an electric field. The temperature affects the resistivity of the material, so expect temperature to modify the value of the drift current somewhat.

Ratch

 

[timkuc]

Sir you told that "In a forward-biased junction diode, diffusion is the primary mechanism for moving charge carriers." That means there are two things which is making charges to move from n to p (or p to n ) 1. Diffusion (Natural) 2.Electric field of battery (As we know that lowering the barrier potential means giving enough energy to the carriers to pass the junction ) ? Sir ,am I right?

 

[Ratchit]

Sir you told that "In a forward-biased junction diode, diffusion is the primary mechanism for moving charge carriers." That means there are two things which is making charges to move from n to p (or p to n ) 1. Diffusion (Natural) 2.Electric field of battery (As we know that lowering the barrier potential means giving enough energy to the carriers to pass the junction ) ? Sir ,am I right?

I said there were 3 mechanisms for moving charges in a diode. In a forward-biased junction only diffusion is significant. Lowering the barrier voltage takes energy. If the charge carriers could diffuse before when the barrier voltage was low or nonexistent, why can't they do it again when the barrier voltage is lowered? Why would they need any extra energy?

Ratch

 

[timkuc]

ohh.. sir I got it. But what about drift current .I understood that in no bias condition drift current was due to the immobile ions' electric field .Is there any drift current exists in forward bias situation sir?

 

[Ratchit]

ohh.. sir I got it. But what about drift current .I understood that in no bias condition drift current was due to the immobile ions' electric field .Is there any drift current exists in forward bias situation sir?

If the P and N leads of the diode were open, then obviously no current, whether drift or diffusion could exist. If the P and N leads were shorted to provide a conduction path, would there be any current? In other words, would a P-N junction provide an everlasting source of current from the barrier voltage? Where would the electrical energy to do that come from? In the forward bias situation, I did say that drift current exists, but it is insignificant.

Ratch

 

[steveB]

"Some says that drift current only depends on temperature ..."

False. You cannot heat a semiconductor slab and expect to observe drift current. Drift current is the movement of charge carriers due to an electric field. The temperature affects the resistivity of the material, so expect temperature to modify the value of the drift current somewhat.

The reverse current on a diode voltage-current graph is composed mostly of drift current

Ratch, can you clarify what you are trying to say here.

Something seems inconsistent in what you said. You mentioned that a reverse biased diode has current that is dominantly drift current. However, we all know that the reverse saturation current of a diode is highly dependent on temperature. Typical numbers for silicon diodes is a doubling of reverse leakage current for every 10 to 15 deg. C rise in temperature, which most would consider to be very temperature dependent. If reverse saturation current is mostly drift current and reverse saturation current is highly temperature dependent, then it follows that when people say that drift current is temperature dependent, it is an accurate statement.

 

[Ratchit]

Ratch, can you clarify what you are trying to say here.

Something seems inconsistent in what you said. You mentioned that a reverse biased diode has current that is dominantly drift current. However, we all know that the reverse saturation current of a diode is highly dependent on temperature. Typical numbers for silicon diodes is a doubling of reverse leakage current for every 10 to 15 deg. C rise in temperature, which most would consider to be very temperature dependent. If reverse saturation current is mostly drift current and reverse saturation current is highly temperature dependent, then it follows that when people say that drift current is temperature dependent, it is an accurate statement.

Look at the last question and answer of post #2. The OP asked if the drift current was wholly dependent on temperature. I said no it was not, but it was affected by temperature, and I gave a reason why that was so. The forward current is also affected by temperature, by the way. If fact, Schockley's equation has a name for the temperature dependent voltage. It the called the "thermal voltage".

Ratch

 

[steveB]

OK, so you are objecting the the word "only" in the statement "Some says that drift current only depends on temperature ...". Of course, absolute statements like that tend to be wrong.

Yes, both the thermal voltage Vt and the reverse saturation current Io are temperature dependent in the Shockley equation i=Io ( exp(v/Vt) -1 )

Thermal voltage varies linearly with temperature T in Kelvin (Vt=m K T / q), and reverse saturation current is exponentially related to T with roughly a doubling in value for every 10 deg. C temperature rise.

 

[timkuc]

I know sir now I will be irritating you but these are the last points I want to clarify/understand 3 doubts.

1.
We say that forward bias voltage lowers the potential barrier at depletion region (which I understood clearly),but what I understanding from the answers you have given that after lowering the potential diffusion phenomenon happening .But I also studied that

==>> If we apply V voltage in forward
potential energy will be transformed into kinetic energy
qV = 1/2[mv^2]
v = (2qV/m)^1/2
So the electrons and holes reach the depletion region ,actually will cross the depletion region .So here by this concept we can see that external electric field (battery V volt) moving the electrons still you are saying that diffusion is in the action . Which one is true (little explanation will be helpful)?

2. I understood the concept of drift current in no bias and reverse bias condition but there is any drift current in forward bias?. If yes then there should be some internal electric field in the diode which is forcing carriers in reverse of applied external electric field which I think can't happen in forward bias.
3. I understood that the drift current should depend on temperature because this current is due to minority carriers . (Increase in temperature increases the no of electron-hole pair) .But actually these carriers are also charges (this sounds stupid I know) so these charges should be affected by the externally applied voltage (electric field) according the point no. 2 I have mentioned above (If that point was correct ).
I will be very very thankful to you if you clear my doubts . THANK YOU

 

[NorthGuy]

The OP asked if the drift current was wholly dependent on temperature.

I think the difference is in words. Ratchit reads "dependent on" as "caused by, cannot exist without". OP and Steve read it as "affected by, varying in value with".

 

[steveB]

I know sir now I will be irritating you but these are the last points I want to clarify/understand 3 doubts.

1.
We say that forward bias voltage lowers the potential barrier at depletion region (which I understood clearly),but what I understanding from the answers you have given that after lowering the potential diffusion phenomenon happening .But I also studied that

==>> If we apply V voltage in forward
potential energy will be transformed into kinetic energy
qV = 1/2[mv^2]
v = (2qV/m)^1/2
So the electrons and holes reach the depletion region ,actually will cross the depletion region .So here by this concept we can see that external electric field (battery V volt) moving the electrons still you are saying that diffusion is in the action . Which one is true (little explanation will be helpful)?

2. I understood the concept of drift current in no bias and reverse bias condition but there is any drift current in forward bias?. If yes then there should be some internal electric field in the diode which is forcing carriers in reverse of applied external electric field which I think can't happen in forward bias.
3. I understood that the drift current should depend on temperature because this current is due to minority carriers . (Increase in temperature increases the no of electron-hole pair) .But actually these carriers are also charges (this sounds stupid I know) so these charges should be affected by the externally applied voltage (electric field) according the point no. 2 I have mentioned above (If that point was correct ).
I will be very very thankful to you if you clear my doubts . THANK YOU

I'm not sure I understand your comment #1. Diffusion is the action in the pn junction, even if drift current is the overall cause of current in the wires and ohmic regions due to applied voltage.

#2 As you say, drift current is free charges moving due to the electric field. There is still electric fields present in forward biasing, so I would say drift currents exist in general.

#3 I think you have this correct, if I'm understanding you correctly.

 

[Ratchit]

I know sir now I will be irritating you but these are the last points I want to clarify/understand 3 doubts.

1.
We say that forward bias voltage lowers the potential barrier at depletion region (which I understood clearly),but what I understanding from the answers you have given that after lowering the potential diffusion phenomenon happening .But I also studied that

==>> If we apply V voltage in forward
potential energy will be transformed into kinetic energy
qV = 1/2[mv^2]
v = (2qV/m)^1/2
So the electrons and holes reach the depletion region ,actually will cross the depletion region .So here by this concept we can see that external electric field (battery V volt) moving the electrons still you are saying that diffusion is in the action . Which one is true (little explanation will be helpful)?

What does an equation that relates the potiential energy to the kinetic energy have to do with diffusion? The electrons and holes will diffuse even without any forward or reverse voltage, and create a depletion layer until the bound ions left behind halt the process by repelling the charge carriers. If a forward voltage is applied across the depletion region, it will become thinner and sustain a current at an equilibrium value because the barrier voltage is lowered and the voltage source supplies more charge carriers.

2. I understood the concept of drift current in no bias and reverse bias condition but there is any drift current in forward bias?.

I said before that there was an insignificant amount.

If yes then there should be some internal electric field in the diode which is forcing carriers in reverse of applied external electric field which I think can't happen in forward bias.

I don't understand what you are saying. A forward voltage will produce a forward drift current. A reverse voltage will produce a reverse drift current.

3. I understood that the drift current should depend on temperature because this current is due to minority carriers . (Increase in temperature increases the no of electron-hole pair) .But actually these carriers are also charges (this sounds stupid I know) so these charges should be affected by the externally applied voltage (electric field) according the point no. 2 I have mentioned above (If that point was correct ).
I will be very very thankful to you if you clear my doubts . THANK YOU

The drift current in semiconductors is limited by the thermally generated carriers. It has nothing to do with whether the carriers are majority or minority. No matter how high you crank up the voltage, the drift current will be constant, because only a limited number charge carriers are generated at a particular temperature, and drift current is made up of thermally generated carriers. That is why you can see on a diode voltage-current curve that the reverse current is constant until the breakdown limit. No more charge carriers are available at that temperature no matter what the reverse voltage is. In the forward direction, the diffusion current increases exponentially with the applied forward voltage until the diode blows out, and the forward drift current will be insignificant because it is limited to the thermally generated charge carriers. Drift current increases linearly with voltage and is composed and limited by the thermally generated carriers.

Ratch

 

[Ratchit]

I think the difference is in words. Ratchit reads "dependent on" as "caused by, cannot exist without". OP and Steve read it as "affected by, varying in value with".

After reading post #8, I don't see how anyone can intrepret what I said as "cannot exist without."

Ratch

 

[NorthGuy]

After reading post #8, I don't see how anyone can intrepret what I said as "cannot exist without."

Here's what you said in post #8:

"The OP asked if the drift current was wholly dependent on temperature. I said no it was not, but it was affected by temperature".

Clearly, the meaning that you assign to "dependent on temperature" is different from the meaning that you see in "affected by temperature". Others seem to regard these as synonymous. That was the reason for the confusion.