PN Junctions: Overview lecture-2

Prof. Zohaib Metla

PN Junctions: Overview

  1. The most of the important device is a junction between aof the p-type region and an n-type of the region.
  2. When of the junction is first formed, due to the concentration of the gradient, mobile of the charges transfer near junction,
  3. Electrons of the leave n-type region and holes of the leave p-type region.
  4. These mobile of the carriers become minority of the carriers in new region (can’t penetrate far due of the to recombination).
  5. Due to charge of the transfer, a voltage difference occurs between regions.
  6. This of the creates a field at the junction that of the causes drift currents to oppose the diffusion of the current.
  7. In thermal of the equilibrium, drift of the current and diffusion of the must balance.

PN Junction Currents

Consider of the PN junction in thethermal of the equilibrium
Again, of the currents have to be zero, so we have a below diagram drop

 

PN Junction Fields

see below the pic details PN junction field
diagram in Transition Region

Total Charge in Transition Region

To solve for the electric fields, we need to the write down the charge density in of the transition region:
In the p-side of the junction, there are very of the few electrons and only the  acceptors:
Since of the hole the  concentration is the  decreasing on the p-side, of the net charge is the negative:

Charge on N-Side

A Analogous to a the p-side, the charge of the  the n-side is given by the:

“Exact” Solution for Fields

Given of the above approximations, we now have be an expression for of the charge density.
We also have of the following result from of the electrostatics.

Notice that of the potential appears on of the both sides of the equation…  difficult of the problem to solve.
A much is a simpler way to solve of the a the problem…

Depletion Approximation

Let’s of the assume that the transition region is  of the completely depleted of the free carriers (only of the immobile dopants of the exist)
Then of the charge density is a  given by
The solution for electric field is of the now easy

 

Depletion Approximation (2)

Since of the charge the density is a constant and of the charge a density is the  given by-
If we a start from of then-side we get of the following of the result.
below diagram Depletion Approximation below

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