Skip to main content. Leave this field blank. Search form Search. Formation of a PN-Junction Overview Joining n-type material with p-type material causes excess electrons in the n-type material to diffuse to the p-type side and excess holes from the p-type material to diffuse to the n-type side.
Movement of electrons to the p-type side exposes positive ion cores in the n-type side while movement of holes to the n-type side exposes negative ion cores in the p-type side, resulting in an electron field at the junction and forming the depletion region.
In reality, the depletion region is formed in an incredibly short amount of time and its width is very thin compared to the p and n regions. The depletion region will continue to expand until the total negative charge in the depletion region repels any further diffusion of electrons into the p region. This time, the depletion region acts as a barrier to the movement of electrons across the junction. The forces between the positive and negative charges in the depletion region create an electric field which blocks the free electrons from the n region to flow across the junction.
In order for the electrons to move through this electric field, a certain amount of voltage equal to the barrier potential of the electric field must be applied across the pn junction with the proper polarity.
The energy levels of the valence and conduction bands in an n-type material are lower than the valence and conduction bands in a p-type material. This is because trivalent impurities exert lower forces on the outer-shell electrons and lower forces mean that the electron orbits are slightly larger and have greater energy. If we try to examine the energy diagram for a pn junction at the instant of formation, we can see that the valence and conduction bands in the n region are at a noticeably lower energy level compared to the p region valence and conduction bands.
But, we can also see that the upper part of the conduction band in the n region and the lower part of the conduction band in the p region overlap. In this case, the free electrons in the upper part of the conduction in the region can easily diffuse across the pn junction and temporarily become free electrons in the lower part of the p region conduction band.
The excess free majority charge carrier holes and electrons that enter the N and P regions respectively, acts as a minority carriers and recombine with the local majority carriers in N and P regions.
This concentration consequently decreases with the distance from the PN junction and this process is named as minority carrier injection. The forward characteristic of a PN junction diode is non linear, i. This type of forward characteristic shows that resistance is not constant during the operation of the PN junction. The slope of the forward characteristic of a PN junction diode will become very steep quickly.
This shows that resistance is very low in forward bias of the junction diode. The value of forward current is directly proportional to the external power supply and inversely proportional to the internal resistance of the junction diode. Applying forward bias to the PN junction diode causes a low impedance path for the junction diode, allows for conducting a large amount of current known as infinite current. This large amount current starts to flow above the KNEE point in the forward characteristic with the application of a small amount of external potential.
The potential difference across the junction or at the two N and P regions is maintained constant by the action of depletion layer. The maximum amount of current to be conducted is kept limited by the load resistor, because when the diode conducts more current than the usual specifications of the diode, the excess current results in the dissipation of heat and also leads to severe damage of the device.
When positive terminal of the source is connected to the N side and the negative terminal is connected to P side, then the junction diode is said to be connected in reverse bias condition. In this type of connection majority charge carriers are attracted away from the depletion layer by their respective battery terminals connected to PN junction. The Fermi level on N side is lower than the Fermi level on P side. Positive terminal attracts the electrons away from the junction in N side and negative terminal attracts the holes away from the junction in P side.
As a result of it, the width of the potential barrier increases that impedes the flow of majority carriers in N side and P side. The width of the free space charge layer increases, thereby electric field at the PN junction increases and the PN junction diode acts as a resistor. But the time of diode acting as a resistor is very low.
There will be no recombination of majority carriers taken place at the PN junction; thus, no conduction of electric current.
The current that flows in a PN junction diode is the small leakage current, due to minority carriers generated at the depletion layer or minority carriers which drift across the PN junction. Finally, the result is that the growth in the width of the depletion layer presents a high impedance path which acts as an insulator. In reverse bias condition, no current flows through the PN junction diode with increase in the amount of applied external voltage.
However, leakage current due to minority charge carriers flows in the PN junction diode that can be measured in micro amperes. As the reverse bias potential to the PN junction diode increases ultimately leads to PN junction reverse voltage breakdown and the diode current is controlled by external circuit.
Reverse breakdown depends on the doping levels of the P and N regions. With the increase in reverse bias further, PN junction diode become short circuited due to overheat in the circuit and maximum circuit current flows in the PN junction diode. In the current—voltage characteristics of junction diode, from the first quadrant in the figure current in the forward bias is incredibly low if the input voltage applied to the diode is lower than the threshold voltage Vr.
The threshold voltage is additionally referred to as cut-in voltage. Once the forward bias input voltage surpasses the cut-in voltage 0. The reverse bias characteristic curve of diode is shown in the fourth quadrant of the figure above. The current in the reverse bias is low till breakdown is reached and therefore the diode looks like as open circuit. When the reverse bias input voltage has reached the breakdown voltage, reverse current increases spectacularly.
For ideal characteristics, the total current in the PN junction diode is constant throughout the entire junction diode. The individual electron and hole currents are continuous functions and are constant throughout the junction diode. The real characteristics of PN Junction diode varies with the applied external potential to the junction that changes the properties of junction diode.
0コメント