P-N junction theory and formation

P-N junction Formation

As mentioned in the previous article, the P-N junction is two extrinsic (P-Type and N-Type) semiconductors combined to form the P-N junction by a special process fabrication. The properties of the P-N junction have a significant impact on industrial electronics and electronics manufacture like sensors where the current flows in one direction when it is forward bias and it can’t pass in the other direction in reverse bias.

How does the P-N junction work?

The P-Type.

The P-Type is an extrinsic semiconductor material (Si) doped by a trivalent acceptor atom (Boron). The three electrons from the Boron acceptor are bonded with three electrons from the Si by covalent bonds and there will be an empty positive charge position in the valance band (EV).

An electron from the nearby atom will jump and fill that vacant positive position leaving behind another empty positive position which will be filled by another nearby electron and so on. It will appear that a hole is moving through the crystal. So, the boron atom has a more negative charge and turns into a negative ion.

Each acceptor impurities atom creates a hole (large numbers) in the valance band and very few holes are created due to the jump of electrons from the valance band to the conduction band (EC) by thermal energy where the temperature is raised over 0 kelvin. The numbers for holes from acceptor impurities are higher than the number of electrons.

The majority carriers in the P-Type are holes and the minority are electrons but, the total numbers of the doped materials charges (positive and negative) are equal. In P-Type, the P letter denotes positive due to the positive charges of holes. The acceptor energy level (Fermi level EF) is slightly above and very close to the valance band.

The N-Type.

The N-Type is an extrinsic semiconductor material (Si) doped by a pentavalent donor atom (phosphorus). The four valance electrons of the SI atom make a covalent bond with four electrons of the five phosphorus electrons in its valance band and one electron will be free to move through the crystal.

As the fifth electron leaves the phosphorus atom, the phosphorus atom will be then a positive ion. Note that the number of the positive charges is equal to the numbers of the negative charges for the doped materials and the summation of positive and negative charges equals zero. The N letter denotes the Negative and references to the majority carriers.

The majority carriers in N-Type are electrons while the minority are holes. The donor energy band for N-Type is lying below and close to the conduction band so with a small amount of energy (at room temperature) the electrons will be excited and jump easily to the energy band level for the donor.

When the junction formed as below Figure, and according to the charge distribution and concentration in the P-Type which has a large number of holes and few numbers of electrons, and N-Type which has a large number of electrons and a few numbers of holes, there are two main processes executed, the diffusion process and the drifting process. The two processes are explained as follows:

1- Diffusion process:

When the P-N junction is formed, the holes are diffused (moved) from the P-Type which has a large number of holes (high hole concentration) inside the N-Type which has a low number of holes (low hole concentration), and the electrons diffused from the N-Type which has a large number of electrons (high electrons concentration) inside the P-Type which has a low number of electrons (low electrons concentration).

When holes and electrons are diffused in the N-Type and P-type, the holes will leave negative Boron ions behind at the P-Type near the junction between P and N and the electrons will leave positive phosphorus ions at the N-Type near the junction too as shown in the below figure.

Because of the negative Boron ions at the P side and positive Phosphorus ions, an electric field is built in at the junction between P and N. The region at the junction between P and N where the negative and positive ions exist is called the depletion layer. The generated current due to the difference of the majority carriers' concentration is called the diffused current.

2- Drifting process.

The drifting process is the process where electrons which are the minority carriers in the P-Type are passed from the P-Type to the N-Type and the passing holes which are the minority carriers in the N-Type to the P-Type and also due to the built-in electric field.

The direction of this electric field is the opposite direction of the diffused holes and electrons. Comparing the diffused and drift charges. The number of diffused charges is larger than the number of the drift charges.

The current due to the depletion layer, electric field, and minority carrier is called the drift current. The built-in potential across the P-N junction according to the accumulation of the positive and negative ions is called the barrier potential.

When the depletion layer (potential carrier) is large enough to make the drift current according to the electric field and the minority carriers equal to the diffusion current according to the majority charges, there will not be any charges moving across the junction. At that condition, the potential carrier will prevent the passing of the diffused current and the P-N is in the equilibrium condition.

Energy Band profile for P-N junction.

In P-Type, the fermi level is close to and above the valance band while in N-Type, the fermi level is below and close to the conduction band. According to the distribution of holes which are very large in P-Type and very low in N-Type and the electrons distribution where it is very large in N-Type and very low in P-Type, the holes diffused into the N-Type and the electrons diffused in the P-Type.

Because of that Fermi level of the P-Type increases because of increasing its energy according to the holes lost and the electrons gained and the fermi level of the N-Type will be decreased because of decreasing its energy due to the electrons lost and the holes gained till reaching the equilibrium case that the fermi level is constant for the P-N junction. The energy band profile for the P-N Junction is shown in the below figure.