p n junction (depletion region and energy bands)

[@neerajpremadhuri5636]

Sir, why energy band of p type sc is at upper level than n type?

[@EngineeringPhysicsbySanjiv]

To make Fermi level constant

[@neerajpremadhuri5636]

Why not vice- versa?

[@passionateLearner-0910]

is Nd=Na in pn junction diode? what Ni there in formula of Vo is? it is of p or of n?

[@EngineeringPhysicsbySanjiv]

Nd is donor concentration in n type semiconductor and Na is acceptor concentration in p type semiconductor.. these can be different. Ni is intrinsic concentration. Means it is concentration of electrons in conduction band and holes in valence band without doping..

[@AnshulSharma1997]

what are these positive and negative charges you have shown. Are they immobile ions or what. And also as electrons and holes already are free and immobile ions remains there only. so what is the actual cause of electron and holes not moving further at certain time.

[@EngineeringPhysicsbySanjiv]

Electrons diffusing from n-side to p-side leave behind donor ions (Nd+) and holes diffusing from p-side to n-side leave behind acceptor ions (Na-). So, the positive and negative charges shown near the junction are these immobile ions. The diffusion of electrons and holes across the junction occurs till the equilibrium is reached. At equilibrium, the negative charge near the junction on p-side, will repel the incoming electrons from n-side and the positive charge near the junction on n-side, will not allow the incoming holes from p-side and the diffusion stops.

[@AnshulSharma1997]

Sir my question is when large number of hole and electron diffuses in both the region, are tnere enough immobile ions or equal number of holes and electron moves such that when they recombine, do they recombine with the acceptor and donor ion or electron and hole recombine to form an mobile ion....also sir when they again recombine do they again regain their form as they are represented in n and p type... Also sir why do they fermi level must be same in unbiased case...i am not getting the logic of it

[@EngineeringPhysicsbySanjiv]

ions are immobile. Electrons can cross the junction. And Electrons will cross the junction only if the fermi levels on the two sides are different. In this case, the electrons on one side of the junction have a higher energy, so electrons will flow from the higher energy side to the lower energy side. When p-n junction is formed, this transfer will occur till the equilibrium is reached. At equilibrium, the diffusion stops that means the fermi levels are aligned.