Explanation for drift current

Question

My question is why the concept of drift current or movement of minority charges is necessary while the pn-junction is formed. So drift current was explained to me in more or less this way

As the holes diffuse from p to n side due to concentration gradient, it leaves behind an ionised acceptor which is immobile. Similarly when electrons diffuse from n to p side they leave ionised donors behind which too are immobile. As this continues the formation of the depletion region takes place, and due to the immobile charges an electric field sets in in the depletion region and this field leads to motion of charge carriers ie. the flow of electron from p to n and holes from n to p, this flow is regarded as drift current.

So let us assume there are two forces one that leads to diffusion of charge carriers which is due to the concentration gradient and other that arises due to the field generated, then why don't the diffusion simply stops when these forces become equal? Why do we need to introduce drift current?

Where did I go wrong or is there something I am missing? I've read answers on similar question that the diffusion is dominating but couldn't correlate with it my question if it is. Does this play a role somehow?

Answer 1

The world of electrons and holes is a very wiggly and jiggly world. Any thermal energy causes them to vibrate, impact each other and nucleuses and move pretty erratically (this is called the Brownian motion).

It doesn't really make sense at this point to talk about "single electron" movement, and instead we devised the models of drift and diffusion that describe their average behavior:

• Diffusion describes that on average electrons and holes will tend to spread out.
• Drift describes that on average electrons and holes will favor a direction when an electric field is described (probably not all of them, all the time, but most of them most of the time).

Diffusion does not mean that no two electrons can't stay together, just that it is very unlikely. Similarly, drift does not mean that electrons can't stay where they are momentarily, just that on average that is very unlikely.

So when you write, talk or read about drift and difussion, you should always keep in the back of your head that it is just an averaged behavior of a lot of holes and electrons. If, on average the electron and hole concentration stays constant throughout the device (but remember, individual electrons and holes will still be moving around erratically), then this is called thermal equilibrium. In a PN junction, this happens when the averaged effect of drift and averaged effect of diffusion balances out.

So try not to think about drift and diffusion as a nice single-minded stream of electrons or holes all moving in a single direction, and everything will likely make much more sense.

Answer 2

In zero bias the diffusion current is equal to the drift current. Majority charge carries diffuse from high concentration region to low concentration region and minority charge carriers inside the depletion region drift according to the electric field of the depletion region.

The tendency for diffusion never stops because there is always a concentration difference of electrons and holes between the 2 regions (N type and P type).In the beginning when the diode is first made , there isn't a depletion region .The depletion region is created due to the recombination of electrons and holes(majority carriers) at the middle of the junction. Inside the depletion region there is an electric field which opposes the diffusion.But it is not about balance of 'forces' because electrons and holes always want to diffuse from high concentration to low concentration regions. What's happening is while the electric potential of the depletion region is increased it opposes the diffusion of free majority charge carriers.At the same time due to imperfection in making of the diode there are some minority charge carriers(electrons in P type region and holes in N type region).If they become free inside the depletion region they will drift due to the electric potential of the depletion region. But for those minority charge carriers there isnt any 'diffusion force' which will oppose their motion because diffusion isn't a force.The only force both majority and minority charge carriers feel is the electrostatic force of the electric field of the depletion region. But majority charge carriers tend to move anyway opposite to the electric field because of the concentration gradient and the electric field resists that motion while on the other hand free minority charge carriers drift in the direction of the electric field.

Answer 3

why don't the diffusion simply stops when these forces become equal?

Because diffusion and drift are separate processes. One doesn't stop just because the other one balances it out.

Similarly, if you are standing on the ground, gravity doesn't stop because the normal force from the floor keeps you from falling. Gravity continues to act on you and the normal force of the floor continues to act on you, with the net result that you don't accelerate. But the fact that you're not accelerating doesn't mean that gravity has stopped working or that the floor has stopped exerting a force on you. Both forces are still present.

In the case of drift and diffusion, when the two currents both act the net current is zero, but both currents are still there, acting in opposite directions.