P-N Junction:
When a P-type semiconductor is connected to a N-type semiconductor in atomic level in such a way that their connecting surface maintains the crystal construction continuously; then this connection surface is called P-N junction and the device so formed is called PN junction diode. Generally, by special techniques in single crystal of silicon and germanium, the impurities are so mixed that its one part becomes P-type semiconductor and the second part becomes N-type semiconductor, and the boundary between these two regions is called P-N junction. A P-N junction cannot be formed by pressing a P-type semiconductor over a N-type semiconductor because in this condition the connecting surface on the crystal construction is not continuous.
As the P-N junction starts, due to the concentration gradient across P and N-sides, holes diffuse form P-side to N side (P → N) and electrons diffuse form N-side to P-side (N → P). This motion of charge gives rise to diffusion current across the junction. When an electron diffuses from N → P, it leaves behind an ionised donor on N-side. This ionised donor (positive) charge is immobile as it is bounded to the surrounding atoms. As the electrons continue to defuse form N → P, a level of positive charge (or positive space-charge-region) on n side of the junction is developed. Similarly when a hole diffuses form P→ N due to concentration gradient, it leaves being an ionised acceptor (negative charge) which is immobile. As the holes continue to diffuse,a layer of negative charge (or negative space-charge region) on the P-side of the junction is developed. This space-charge region of either side of the junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction depleted the region of its free charges. (Figure).
Forward Biasing:
When an external voltage V is applied across a semiconductor diode such that P-side is connected to the positive terminal of the battery and N-side to the negative terminal (figure); then P side is at higher voltage than N-side. This is said as forward biasing. In this situation, an external electric field, E is established at the junction which is directed in the direction, of P to N. It is opposite to the internal electric field, Ei in the junction. Due to this the value of barrier potential at the junction reduces and now holes of P-region and electrons of N region move towards the junctions. As a result, more number of holes and electrons reach the depletion layer and its width decreases. Due to reduction of the value of potential barrier on either side of the junction, more and more charge carriers are distributed. Holes of P-side move towards the direction of E and electrons of N-side move opposite to the direction of E. In this situation, diffusion current is more than drainage current.
In the P-region near the positive end of the battery if an electons reaches this place by breaking the covalent bond then as a result of this process a hole is generated which moves towards the junction region. That electron moving through the connecting wires reaches the positive end of the battery. At the same time an electron starts its motion form the negative end of the battery and enters N-region and does motion towards the junction. In this way, there is flow of current in the junction. In P-region it is due to electrons; which are the majority-charge carriers for these regions. Therefore, this flow of current is due to the diffusion of majority charge carriers, clearly due to this biasing there is help in the flow of current in the junction, hence it is called forward biasing. In the external circuit the flow of current is only due to electrons.
If the voltage used by the battery increased the potential barrier will reduce, more majority charge carriers will diffuse in the junction regions and the value of electric current will increase. Since, the flow of current is easily flowing through the junction, hence, the resistance of the junction will be very small in this case.
