Biasing means application of voltage.To make a p-n junction to conduct, you have to make electrons move from the n-type region to the p-type region and holes moving in the reverse direction.
To do so, you have to overcome the potential barrier across the junction by connecting a battery to the two ends of the p-n junction diode. The battery can be connected to the p-n junction in two ways:
When a junction is forward biased and the bias exceeds barrier potential, holes are compelled to move towards the junction and cross it from the p-region to the n-region. Similarly, electrons cross the junction in the reverse direction. This sets in forward current in the diode.
The current increases with voltage and is of the order of a few milliampere. Under the forward bias condition, the junction offers low resistance to flow of current. The value of junction resistance, called forward resistance, is in the range 10Ω to 30Ω.
When the p-n junction is reverse biased, holes in the p-region and electrons in the n-region move away from the junction. A small current flow even now because of the fewer number of electron hole pairs generated due to thermal excitations. This small current caused by minority carriers is called reverse saturation current or leakage current.
In most of the commercially available diodes, the reverse current is almost constant and independent of the applied reverse bias. Its magnitude is of the order of a few microamperes for Ge diodes and nanoamperes in Si diodes.
A p-n junction offers low resistance when forward biased, and high resistance when reverse biased. This property of p-n junction is used for ac rectification.
When the reverse bias voltage is of the order of a few hundred volt, the current through the p-n junction increases rapidly and damages it due to excessive power dissipation. The voltage at which a diode breaks down is termed as breakdown voltage.
When a reverse bias is applied, a large electric field is established across the junction. This field
(i) accelerates the available minority carriers, which in turn, collide with the atoms of the semiconductor material and eject more electrons through energy transfer (avalanche effect)
(ii) breaks covalent bonds by exerting large force on electrons bound by the bonds. This results in creation of additional electron-hole pairs in the junction region (Zener effect).
Both these processes give rise to large reverse current even for a small increment in reverse bias voltage. This process is termed as Zener breakdown.