When a p-type semiconductor is brought in close contact with an n-type semiconductor by stable means, the arrangement of both the semiconductors is known as the PN junction.
The interface of p-type and n-type semiconductors is called Junction. Two independent p-type and n-type semiconductors pressed against each other do not form a PN junction.
PN junction Diode and Characteristics of PN junction Diode
What is pn junction diode
When a single piece of semiconductor material (either Si or Ge) whose one portion is doped with an n-type impurity and the other portion is doped with p-type impurity behave as a PN junction. A PN junction having metallic contact at its end is known as the PN junction diode.
How pn junction diode works
The most basic property of junction diodes is that it conducts an electric current in one direction and does not allow it to flow in the other direction.
Junction diode is very useful in a wide variety of applications including the rectification of AC signals ( converting AC to DC), the detection of radio signals, conversion of solar power to electricity, and in the generation and detection of light.
It is also used in a variety of electronic circuits as a switch, as a voltage Reference, or even as a tunable capacitor.
The PN junction is the building block of other electronic devices like the junction transistor. For this reason, a study of the properties and behavior of PN junction diode theory is important.
Formation of PN Junction
A small sphere of trivalent impurity says indium is pressed on a thin wafer of n-type germanium or silicon slab. The system is heated so that the Indium to the surface of Germanium and produces P-type Germanium just below the source of contact.
This p – types along with the n type Germanium wafer from a P-N junction. Both the upper and lower portion of the system have metallic contacts.
Similarly, a PN junction can be made by diffusion of a pentavalent impurity like Phosphorus into a p-type semiconductor. In this process, a p-type semiconductor is heated in Phosphorus gas to result in diffused n-type layer on the semiconductor.
Formation of depletion layer in a PN junction
There is a high concentration of holes in the p-region and a high concentration of electrons in the n-region of the PN junction. The holes from the p-region and electrons from the n-region diffuse through the junction. The electron which diffuses through the junction to p-region recombine with holes.
In fact, on the p-side of a p-n Junction, there are negative ions fixed in their position in the crystal lattice surrounded by holes. When a hole diffuses through the Junction to the n-region of the semiconductor, a negative ion is left Behind near the junction.
The Negative Ion is fixed or immobile. Similarly, on the n side of the p-n junction, there are positive ions fixed in their respective position in the crystal lattice surrounded by free electrons.
When an electron diffuses through the Junction to the p – region of the semiconductor, a positive Ion is left behind near the junction.
This positive ion is fixed or immobile. These positive and negative ions on both sides of the junction form a deflection layer or depletion region or space charge region or transition region.
This layer is known as a depletion layer because it is depleted of free and mobile charge carriers. The thickness of the depletion layer is about 10−3 mm or 10-6 m.
Junction barrier or barrier potential
The depletion layer contains positive and negative immobile ions. These positive and negative Ions are separated by a distance equal to the thickness of the depletion layer.
Thus, a potential difference is set up across the junction which opposes the further diffusion of electrons and holes through the junction. The potential difference is called a potential barrier or junction barrier.
Difference between Forward and Reverse Biasing of pn junction diode
Forward biased pn junction diode
When a battery of an e.m.f. greater than the barrier potential (Vb) is connected to a PN junction diode in such a way that the positive terminal of the battery is connected to the p-region and the negative terminal of the battery is connected to the n-region of the junction diode, then the PN junction diode is said to be forward-biased.
Thus, the flow of electrons to the left side and hole to the right side of the junction begin. The movement of holes and electrons constitute hole current (Ih) and electron current (Ie) respectively.
In the region of the p-n junction, electrons and holes recombine. For every electron-hole combination near the junction, a covalent bond of a p-type semiconductor connected to the positive terminal of the battery breaks.
The electron liberated enters the positive terminal of the battery and the hole moves to the right side of the junction. Similarly, more electrons from the negative terminal of the battery enter the n-region to compensate for the electron lost by the combination with the hole at the junction.
These electrons diffuse through the junction and enter the p-region. Here, they again combine with the holes. Thus a continuous current flows through the junction diode.
Reversed biased pn junction diode
A PN junction is said to be reversed biased when the positive terminal of the battery is connected to the n-region and the negative terminal of the battery is connected to the p-region of the PN junction diode.
Hence the junction resistance increases. The holes and electrons (majority carriers) in the p-region and n-region respectively are attracted by the negative and positive terminal of the battery.
Thus, both holes and electrons are displaced away from the junction. As a result of this, holes in the p – region and electrons in the n – region cannot cross through the junction. Therefore, the flow of current in the diode is almost stopped.
There is a small current due to the minority carrier in the p-region and n-region, which exists under reverse-bias conditions. The current is called saturation current and denoted by Is. This current is not affected by the applied voltage but increases with the increase in temperature.
Forward and Reverse bias characteristics of PN junction diode
The variation of diode current with the applied voltage to the junction diode is known as the characteristics of PN junction diodes.
Forward bias characteristics
When the battery voltage is zero, diode does not conduct and the diode current is zero. As the battery voltage (V) is increased, the barrier potential starts decreasing and a small current begins to flow.
The forward current increases slowly at first but as soon as the battery voltage becomes greater than the barrier potential (Vb), the forward current increases rapidly. The battery voltage at which the forward current starts increasing rapidly is known as knee voltage (Vk).
After knee voltage, the junction diode behaves almost like a conductor. And the variation of current with voltage applied across the junction diode is almost linear.
However, there is a limit of current that can pass through the diode without damaging it. This maximum current is known as maximum forward current. A high current through the diode produces a large amount of heat which may burn the diode.
Reverse bias characteristics
When the p-n junction is Reverse biased, the majority carriers in the p and n region are repelled away from the junction. There is a small current due to the minority carriers.
This current attains its maximum or saturation value immediately and is independent of the applied reverse voltage. it depends on the temperature of the junction diode.
As the reverse voltage is increased to a certain value, called breakdown voltage, large amounts of covalent bonds are broken. As a result of this, the last electron-hole pair is produced which diffuses through the junction, and hence there is a sudden rise in the reverse current.
Once breakdown voltage is reached, the high reverse current may damage the junction diode.The maximum reverse voltage that can applied to PN junction without damaging the junction is called peak inverse voltage (PIV).
The forward and reverse bias characteristics of junction diode is also known as voltage ampere characteristics.