Hi,

I was reading the PN junction. During the formation of PN junction, the diffusion process takes place in which the free electrons cross the boundary of the P-N and joined with the holes in the P-type to form the negative ion and holes on the N side cause positive ions. So, the P-side has negative ions and N-side has positive that constitutes the depletion region.

From doping we know there are other bulk free electrons in the N-type (Due to added impurity atom) and holes in the P-side. What I am not able to understand that why not the electron can join the N-side positive ions to make it neutral atom and

secondly why not on P side electrons from negative ions electrons drift further away from the boundary to join with other holes and more electrons cross the depletion region. Why they always stay near the boundary (Depletion Region).

Best Regards,

 

Don't think of individual electrons moving from one site to another. Think in terms of an electron cloud and the probability that electrons will migrate from one area to another.

Before the P and N doped material come together, they are both neutral in charge.
When you bring the P and N materials together, electrons will drift from the N material to the P material. As they do this, and electric field is created which would oppose the migration of more electrons, i.e. the probability of more electrons migrating diminishes. There reaches a stage where the electric field is so large that the net migration diminishes to zero. Remember that electrons are still moving around but it is the net movement that we observe.

 

Thanks Mr Chips but there is some thing which I dont understand when the ionization takes place (Lets say in N-type Material) and the depletion region formed, why not more electrons comes and join positive ion and neutralize it.

 

At the initial stage (zero bias voltage) there is no ionization. Both P and N materials are neutrally charged.
In N-type material, there are impurities such has phosphorus with 5 valence electrons.
In P-type material, there are trivalent dopants such as boron. When the N-type and P-type material come together, the electrons in the N-type material have a tendency to migrate to the P-side.

Why more electrons do not continue to migrate? The electric field created stops more migration. Remember, this is a probability problem. The more electrons migrate, the more difficult it becomes for more migration to occur against the increasing electric field.

 

Ok this is understood when the charge carriers are trying to cross the PN junction border line but what I dont understand here that Why not the ionized atoms which are positive charge on N-side attract electrons and get neutralized because there are number of free electrons on N side. Thanks

 

The junction is an energy barrier and the electrons do not have enough energy to climb the hill and discharge across the junction.
Not unless they are pushed by the additional force from an external an electric field. Even then, nothing is really moving in the conventional sense. Its the barrier to movement of energy (not electrons or holes per se) which is being affected.

Interestingly the charge carriers move at snail like velocities (uM/s) in any conductor- energy is transferred the back of the train to the front yet the wagons stay in the same configuration. With a few rattles of course.