Characteristics of a PN Junction Diode

A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in one direction only. However, unlike a resistor, a diode is non ohmic; does not behave linearly with respect to the applied voltage as the diode has an exponential I-V relationship and therefore we cannot described its operation by simply using an equation such as Ohm's law.

If a suitable positive voltage (forward bias- positive terminal connected to the p-type side) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased. 

By applying a negative voltage (reverse bias- positive terminal connected to the n-type side) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction.

Junction Diode Symbol and Static I-V Characteristics

Before using a PN junction as a practical device or as a rectifying device we need to firstly bias the junction, i.e. connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases the potential barrier. As the potential barrier is increased, current is then blocked from flowing, only very little current (leakage current), at very large external voltage, passes through.

An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction. As the potential barrier, current can easily flow through the diode (the diode is more conductive). Current increases as voltage is increased however, not linearly (non-ohmic)

In summary;

The PN junction region of a Junction Diode has the following important characteristics:

1. A semiconductor can be doped with donor impurities (Pentavalent atoms) such as Antimony (N-type doping), so that it contains mobile and free electrons- the charge carries. 
2. A semiconductor may be doped with acceptor impurities (trivalent atoms)such as Boron (P-type doping), so that it contains holes- the charge carriers. 
3. Semiconductors contain two types of mobile charge carriers, Holes and Electrons. The holes are positively charged while the electrons negatively charged.
4. The p-n junction region has no charge carriers and is known as the depletion layer.
5. The junction (depletion) layer has a physical thickness that varies with the applied voltage. A reverse bias voltage widens it while a forward bias voltage decreases it.
6. When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer.
7. When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow. 
8. When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).