# How can depletion region be formed at the center?

Let's consider an electron at the junction in the n-type region. As it moves into the p-type we say it leaves behind a positive ion and hence in this way electrons at the junction in the n-type create a positive space charge region. But as an electron (at the junction) moves into the p-type, its position would be occupied by another electron waiting at the back (because all the electrons are in random motion).

So if we see in this way ultimately a positive space charge would be developed at the surface of the n region. so whats wrong with my understanding?

There is nothing wrong in your understanding. You are simply not extending it, that is how does the depletion region behave when it as grown sufficiently large?

In a PN junction, there is a majority of electrons at the n-side and holes at the p-side. Due to this concentration gradient, the electrons (say, E1) start diffusing into the p-side (leaving behind a bound positive charge). Similarly, the holes (H1) also start diffusing into the n-side, leaving behind a bound negative charge.

But as an electron (at the junction) moves into the p-type, its position would be occupied by another electron waiting at the back (because all the electrons are in random motion).

Yes, the electrons behind E1 (say, E2) would occupy the frontal position, leaving behind positive ions at their own (E2) position. However, this motion is not because the electrons are moving randomly but because they are trying to reduce the concentration gradient. This motion of charges is also known as diffusion current ($I_{d}$).

Now, since the gradient still persists, these electrons (E2) would then enter the p-side, again leaving bound positive charges. While the next layer (E3) occupies the initial position of E2 and the process carries on. A similar process occurs due to the motion of the holes - leaving bound negative charges in the p-side of the junction. Thus increasing the depletion region.

So if we see in this way ultimately a positive space charge would be developed at the surface of the n region.

No, the space charge does not keep increasing infinitely.

As the depletion region grows, a potential difference is created due to the bound charges, leading to the formation of an electric field inside the region. This electric field pushes electrons from n-side towards the p-side and holes from p-side towards the n-side (thus opposing the diffusion current). In fact, the larger the depletion region, the stronger is the electric field and consequently the drift current ($I_{s}$).

At a sufficient width (of the depletion region), equilibrium is achieved, as the diffusion current ($I_{d}$) equals the drift current ($I_{s}$), hence the net current in the PN junction tunes down to zero.

Now, since the depletion region was growing due to the motion of majority charge carriers (i.e. diffusion current), the depletion region consequently stops growing.

In reality, the width of the depletion region (in equilibrium) is generally in the range of micro-meters.

• thank you for your answer....electrons are in random motion,actually that randomness is the reason why charge gradient decreases(electrons are not living things to know that there is a charge gradient and they should decrease it)...if e1 position is occupied by e2 and e2's position by e3 how is that a positive ion is created ? Commented Aug 19, 2018 at 15:47
• First: Electrons do not need to be living to understand that. In nature low energy states ie stable configurations are always preferred over unstable ones. Water flows from high pressure to low, and so does wind, a body placed at the brim of a hemisphere falls and establishes itslef in the center (where gravitational potential energy is minimum - low energy state). In fact, living being are the ones capable of reducing concentration gradients, otherwise overpopulation in certain regions wouldn't be an existing problem :) Commented Aug 19, 2018 at 16:13
• Second: In an n-type atom there are five valence electrons, out of which one is loosely bonded. When it gains enough energy, it escapes, leaving a bound positively charged ion. Commented Aug 19, 2018 at 16:16
• once again thank u for your reply.....we cannot even say nature is preferring something because even nature isnt living...for instance u said water flows from high to low pressure...its not bcz nature is preferring it but bcz there is a net force in dat direction only...why things only fall in towards earth?not bcz nature is preferring it but bcz force of gravity is only of attractive nature..believing nature is preffering something is not science but mere philosophy:). Commented Aug 19, 2018 at 16:25
• I agree. I did word it wrongly :) Commented Aug 19, 2018 at 18:36