P-Type Semiconductors
P-type semiconductors are elements of a group Ⅳ such as Silicon (Si) or Germanium (Ge) that are doped with elements of the group Ⅲ which are known as trivalent impurities such as Boron (B) or Aluminum (Al). In other words, we can say that an impurity is added to pure semiconductors to make P-type semiconductors. They have an extra hole or positive charge on them because the outermost shell has an unpaired electron.
N-Type Semiconductors
N-type semiconductors are elements of a group Ⅳ such as Silicon (Si) or Germanium (Ge) that are doped with elements of the group Ⅴ also known as pentavalent impurities such as Phosphorus (P) or Antimony (Sb). This type of semiconductor has an extra electron or negative charge on them because the outermost orbit of Silicon or Germanium gets an extra electron when it is doped with group Ⅴ elements.
Construction of PN Junction Diode
When a P-type and N-type semiconductor material are combined, a junction is formed between them that is known as a PN junction. During the formation of a PN junction, electrons from N-type material try to drift towards holes in P-type material, and holes from P-type material move towards electrons in N-type material. This results in the formation of a depletion region between P-type and N-type material that acts as a potential barrier for the flow of current.
So, the PN junction becomes a two-terminal semiconductor device that either allows current to flow from it or blocks current. In P-type material, holes are the majority of charge carriers, and in N-type material, free electrons are the majority of charge carriers.
Forward Biased Condition of PN Junction Diode
In the forward-biased condition of a PN junction diode, it behaves as a conductor and allows current to flow through it. The connections are made such that the positive terminal of the battery is integrated with the P-type material and the N-type material is integrated with the negative terminal of the battery.
The applied voltage must be greater than the potential barrier of the diode. For example, the Si diode has a potential barrier of 0.7V and the Germanium diode has a potential barrier of 0.3V. When forward voltage is applied to the PN junction, the electric field of the depletion region is opposite to the electric field applied by the battery.
When both electric fields add up, the resulting electric field is less in magnitude than both. So, the least resistance is offered to the flow of current and the diode conducts current.
Reverse Biased Condition of PN Junction Diode
In the reverse biased condition of the PN junction diode, it behaves as an insulator and does not allow current to flow through it. The connections are made such that the positive terminal of the battery is integrated with N-type material and the P-type material is integrated with the negative terminal of the battery.
When a reverse voltage is applied to the PN junction, the electric field of the depletion region is in the same direction as the electric field applied by the battery.
When both electric fields add up, the resulting electric field is greater in magnitude than both, and a very thick depletion region is formed. So, a large resistance is offered to the flow of current and the diode does not conduct current. If the reverse voltage is very large in magnitude, it may cause the diode to break down in reverse condition and conduct indefinitely.
I-V Characteristics for Semiconductor Diodes
The I-V characteristics of semiconductor diodes explain the currents and voltages of PN junctions in forward and reverse-biased conditions. In the I-V characteristics graph, voltage is taken along the x-axis and current is taken along the y-axis.
As the graph below indicates, in a forward-biased condition when voltage is applied to the PN junction, the diode starts conducting slowly. After some time, when the diode fully overcomes the potential barrier, it starts conducting completely. At this time, the current through the diode increases rapidly at the same voltage applied.
However, in reverse-biased conditions, when a reverse voltage is applied to the PN junction, it does not conduct current. However, a small reverse leakage current flows due to minority charge carriers present in P-type and N-type material. This current flow because holes are attracted by the negative side and electrons are attracted by the positive side of the battery. These minority charge carriers sweep across the depletion region and cause reverse leakage current.
Reverse Breakdown Voltage
In the reverse condition, if the applied reverse voltage is very large such that it overcomes the thick depletion region, it causes the diode to break down. In this condition, current surges through the diode are observed, and it starts conducting. The voltage beyond which a diode breaks down is known as the breakdown voltage.
Applications of PN Junction Diode
Diodes are the basic building block of modern-day electronics. PN junction diodes are mostly used as Light Emitting Diodes (LEDs) and photodiodes. Furthermore, they are also used in solar cells, rectifiers, clippers, clampers, logic gates, control circuits, and voltage-controlled oscillators.
Conclusion
PN junction semiconductor diodes are simple devices made through the combination of P-type and N-type materials. These materials are created by doping of trivalent or pentavalent elements of group Ⅲ and group Ⅴ in pure semiconductors. Diodes can behave as conductors as well as insulators in forward and reverse-biased conditions, respectively. They also have a number of applications in the field of modern electronics.
