What is Forward Biasing?

Biasing means applying a potential difference to a diode. A diode is a semiconductor device with two terminals and it conducts current primarily in one direction. In forward biased p-n junction diodes, the p-side is connected to the positive terminal and the n-side is connected to the negative terminal. It is the process by which a p-n junction diode allows electric current to pass through the diode in the presence of applied voltage. If the diode is forward biased and a voltage of 0.7 volts is applied for silicon diode and a voltage of 0.3 volts is applied for germanium diodes it starts conducting current.

How does current flow in forward-biased diodes?

When the battery connected to the forward-biased diode has a voltage greater than the barrier voltage the majority carriers will have enough energy to pass through the depletion region. The complete elimination of the barrier constitutes the low resistance path for the flow of current. Thus, the current starts flowing through the junction. The forward bias set up the electric field across the potential which reduces the strength of the potential barrier In forward biasing the device operates as a conductor 

What is Reverse Biasing?

In reverse bias the negative region is connected to the positive terminal of the battery and the positive region is connected to the negative terminal. The depletion region widens and the barrier potential increases. As the majority carriers cannot cross the junction, the pn-junction does not conduct. However, a small current flows through the circuit. This reverse current caused by the thermally produced minority carriers is called saturation current. The saturation current is independent of the applied voltage. The reverse potential increases the strength of the potential barrier. The flow of charge carriers is blocked by the potential barrier and a high resistive path is created. Therefore, there will not be any current flow through the circuit. In reverse bias, the device acts as an insulator.

Capacitance of reverse-biased pn-junction diode

The capacitors are devices that store electric charge in the form of an electric field. The ability of a material to store electric charge is called capacitance ability of a material to store electric charge is called capacitance. For a capacitor, the capacitance depends on the size of the electrodes or plates and inversely proportional to the distance between these plates. 

Just like the capacitors, a reverse-biased pn-junction diode will also store electric charge in the depletion region. These electric charges (positive ions and negative ions)  stored in the depletion region are immobile. These electric charges exert an electric field or electric force.  Therefore, the charge is stored in the depletion region in the form of an electric field. Therefore there is a capacitance in the depletion region.

The p-n junction diode with narrow depletion region stores more charge and dipole with wider depletion region stores less number of charges. Therefore, the capacitance of the reverse bias p-n junction diode decreases when voltage increases