PN JUNCTION DIODE(unbiased)

To understand PN junction diode , consider two separate pieces of semi conductors (p-type and n-type )  as shown in below figure

Bring p-type and n-type semi conductors together and join them to make pn junction diode as shown in figure below.

There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:

• 1. Zero Bias(UNBIASED) – No external voltage potential is applied to the PN junction diode.
• 2. Forward Bias – The voltage potential is connected positive to the P-type material and negative to the N-type material across the diode.

• 3. Reverse Bias – The voltage potential is connected negative to the P-type material and positive to the N-type material across the diode.

• PN Junction Diode in Equilibrium with No Applied Voltage(UNBIASED)

1.A diode is made of semiconductor material (such as crystalline silicon) with impurities added in a process called doping. Different impurities are added to the anode and cathode sides to make the diode electrically asymmetric.

2.The semiconductor on the anode side is doped with atoms having 3 valence electrons each (such as indium, aluminum or gallium). Relative to pure silicon which has 4 valence electrons, it has a deficit of electrons and these missing electrons are called holes. A semiconductor doped in this way is called p-type, where the “p” stands for the positively-charged holes

3.The semiconductor on the cathode side is doped with atoms having 5 valence electrons each (such as phosphorus or arsenic) and, thus, has an excess of electrons relative to pure silicon. This kind of doped semiconductor is called n-type because the excess electrons are negatively charged.

5. If one half is doped by P-type impurity and the other half is doped by N-type impurity, a PN junction is formed. 

6. The semicondctor dividing the two halves is called PN junction. The N-type material has high concentration of free electrons, while P-type material has high concentration of holes. 

7. Therefore, at the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side.This process is called diffusion. 

8. As the free electrons move across the junction from N-type to P-type, the donor ions become positively charged. Hence a positive charge is built on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filling in the holes. 

9. Therefore, a net negative charge is established on the P-side of the junction.

10. This net negative charge on the P-side prevents further diffusion of electrons into the P-side. Similarly, the net positive charge on the N-side repels the hole crossing from P-side to N-side.

11. Thus a barrier is set-up near the junction which prevents further movement of charge carriers. i.e. electrons and holes. 

12.  As a consequence of the induced electric field across the depletion layer, an electrostatic potential difference is established between P-and N-regions, which is called the potential barrier, junction barrier, diffusion potential, barrier potential,space charge region or contact potential. 

13. The magnitude of the contact potential V varies with doping levels and temperature. V, is 0.3 V for germanium and 0.7 V for silicon.

UNBIASED PN JUNCTION DIODE