Diode and it's behaviour in forward and reverse biased condition

What is Diode?

 A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. 

Although in the real world, diodes can not achieve zero or infinite resistance. Instead a diode will have negligible resistance in one direction (to allow current flow), and very high resistance in the reverse direction (to prevent current flow). A diode is effectively like a valve for an electrical circuit. Semiconductor diodes are the most common type of diode. These diodes begin conducting electricity only if a certain threshold voltage is present in the forward direction (i.e. the “low resistance” direction). The diode is said to be “forward biased” when conducting current in this direction. When connected within a circuit in the reverse direction (i.e. the “high resistance” direction), the diode is said to be “reverse biased”

The p-n Junction

When we place p-type and n-type semiconductors in contact with one another, a p-n junction is formed. p-n junctions are basic components of most common electrical devices. While semiconductors doped with either n-type dopants or p-type dopants are better conductors than intrinsic semiconductors, interesting properties emerge when p- and n-type semiconductors are combined to form a p-n junction.

Diagram of the diffusion across a p-n junction, with the resultant uncovered space charges, the electric field and the drift currents.

Diagram of a p-n junction under reverse bias, showing conduction and valence bands, the depletion zone, the potential barrier, the resultant electric field and the types of semiconductor. The p-n junction forms between juxtaposed p- and n-type semiconductors. The free electrons from the n-type semiconductor combine with the holes in the p-type semiconductor near the junction. There is a small potential difference across the junction. The area near the junction is called the depletion band because there are few positive holes and few free electrons in this region.

If no electricity is being passed through the system, then no current passes through the junction between n- and p-type semiconductors. In this scenario, the surplus of electrons from the n-type semiconductor and the deficiency in electrons from the p-type semiconductor combine to create a depletion region. In this state, the system is said to be at equilibrium. However, if the cathode of a battery is connected to the p-type semiconductor, and the anode is connected to the n-type semiconductor, the system is said to be “forward biased.” In this scenario, electrons flow from the anode toward the cathode pole and charge flows across the junction. If the connectivity is reversed, with the battery anode connected to the p-type semiconductor and the cathode connected to the n-type semiconductor, the system is said to be “reverse biased” and negligible charge flows across the junction. Combining n-type and p-type semiconductors creates a system which has useful applications in modern electronics.

If the cathode of a battery is connected to the p-type semiconductor while the anode is connected to the n-type semiconductor, the system is said to be forward biased and current flows through the junction.

If the battery anode is connected to the p-type semiconductor and the cathode connected to the n-type semiconductor, the system is said to be reverse biased and negligible current passes.

Electronic devices and instruments, such as digital alarm clocks, mp3 players, computer processors, and the electronics in cell phones, all take advantage of semiconductor technology. Doping provides a way to modulate the properties of semiconductors that have broad applications in daily life.