There are excess holes, shown as small circles, in the P-section which exist in the covalent bond between the host atoms and the impurity atoms. The figure shows two impurity atoms of Aluminum near the junction.
There are excess electrons in the N section obtained from the pentavalent impurity atoms. The figure shows two Arsenic impurity atoms near the junction. Both N and P sections are electrically neutral.
The electron defuse from N to P section as the N section has excess of electrons as compared to P section. These electrons occupy holes of P side near the junction. A small amount of holes also diffuse from P to N section.
The adjoining figure shows the situation after some diffusion has occurred. Two electrons of Arsenic are shown to occupy the two holes near the Aluminum atoms. This leaves Arsenic atoms as positive ions and Aluminum atoms as negative ions. As the diffusion progresses, more and more Arsenic and Aluminum atoms become positive and negative ions respectively.
This result in the steady electric field near the junction due to the charges on the ions direction of which is from N to P region. The electrons have to overcome this increasing electric field to diffuse from N to P side. The diffusion of electrons stops when the electric field is sufficiently established to oppose the diffusion. This situation is shown in the following figure.Two points are noteworthy:
(1) Electrons are no longer the majority charge carriers in the small region of the N type material near the
junction and the holes are not the majority charge carriers in the small region of the P-type semiconductor near the junction. These regions are known as depletion region as they are deplete of their majority charge carriers. The width of the depletion region is approximately 0.5 μm.
(2) The varying electric potential at the region near the junction is called the depletion barrier. Its value is about 0.7 V for Si and 0.3 V for Ge.
It can be seen from the band diagram of the P-N junction shown that the charge carriers need about qVB energy to cross the junction and go into the other region of the diode.
Less the amount of impurity atom added to the P and N type semiconductors, wider is the depletion region and weaker the electric field intensity near the junction.
The depletion region contains immobile positive and negative charges which constitute a capacitor having depletion capacitance or transition capacitance, C d. The width of the depletion region increases with the increase in the reverse bias which decreases the value of the capacitance. Such diode in which value of the capacitance varies with voltage is known as varactor diode or variable diode.
