PN Junction Diode Theory and VI Characteristics of PN Junction Diode

The P-N junction diode is appeared in the year 1950. It is the most essential and the basic building block of the electronic device. The PN junction diode is a two terminal device, which is formed when one side of the PN junction diode is made with p-type and doped with the N-type material. The PN-junction is the root for semiconductor diodes. The various electronic components like BJTs, JFETs, MOSFETs (metal–oxide–semiconductor FET), LEDs and analog or digital ICs all supports semiconductor technology. The main function of the semiconductor diode is, it facilitates the electrons to flow totally in one direction across it. Finally, it acts as a rectifier. This article gives a brief information about the PN junction diode, PN junction diode in forward bias and reverse bias and the VI characteristics of PN junction diode

What is a PN Junction Diode?

There are three possible  biasing  conditions and two operating regions for the typical PN-Junction Diode, they are: zero bias, forward bias and reverse bias.

When no voltage is applied across the PN junction diode then the electrons will diffuse to P-side and holes will diffuse to N-side through the junction and they combine with each other. Therefore, the acceptor atom close to the P-type and donor atom near to the N-side are left unutilized. An electronic field is generated by these charge carriers. This opposes further diffusion of charge carriers. Thus, no movement of the region is known as depletion region or space charge.

PN Junction Diode
PN Junction Diode

If we apply forward bias to the PN-junction diode, that means negative terminal is connected to the P-type material and the positive terminal is connected to the N-type material across the diode which has the effect of decreasing the width of the PN junction diode.

If we apply reverse bias to the PN-junction diode, that means positive terminal is connected to the P-type material and the negative terminal is connected to the N-type material across the diode which has the effect of increasing the width of the PN junction diode and no charge can flow across the junction

VI Characteristics of PN Junction Diode
VI Characteristics of PN Junction Diode

Zero Biased PN Junction Diode

In the zero bias junction, potential provides higher potential energy to the holes on the P and N side terminals. When the terminals of the junction diode are shorted, few majority charge carriers in the P-side with plenty energy to overcome the potential barrier to travel across the depletion region. Therefore, with the help of majority charge carriers, the current starts to flow in the diode and it is denoted to as forward current. In the same way, minority charge carriers in the N-side move across the depletion region in reverse direction and it is referred to as reverse current.

Zero Biased PN Junction Diode
Zero Biased PN Junction Diode

Potential barrier opposes the movement of electrons & holes across the junction and permits the minority charge carriers to drift across the PN junction. However, the potential barrier helps minority charge carriers in P-type and N-type to drift across the PN-junction, then an equilibrium will be established when the majority charge carriers are equal and both moving in reverse directions, so that the net result is zero current flowing in the circuit. This junction is said to be in a state of dynamic equilibrium.

When the temperature of the semiconductor is increased, minority charge carriers have been endlessly generated and thus leakage current starts to rise. But, electric current cannot flow since no external source has been connected to the PN-junction.

PN Junction Diode in Forward Bias

When a PN-junction diode is connected in a forward bias by giving a positive voltage to the N-type and a negative voltage to the P-type material. If the external voltage becomes more than the value of the potential barrier (estimate 0.7 V for Si and 0.3V for Ge, the opposition of the potential barriers will be overcome and the flow of current will start.Because the negative voltage repels electrons near to the junction by giving them the energy to combine and cross over with the holes being pushed in the opposite direction to the junction by the positive voltage.

PN Junction Diode in Forward Bias
PN Junction Diode in Forward Bias

The result of this in a characteristic curve of zero current flowing up to built in potential is  called as “knee current” on the static curves & then a high current flow through the diode with a slight increase in the external voltage as shown below.

 VI Characteristics of PN Junction Diode in Forward Bias

The VI characteristics of PN junction diode in forward bias are non linear, that is, not a straight line. This nonlinear characteristic illustrates that during the operation of the N junction, the resistance is not constant. The slope of the PN junction diode in forward bias shows the resistance is very low. When forward bias is applied to the diode then it causes a low impedance path and permits to conduct a large amount of current which is known as infinite current. This current starts to flow above the knee point with a small amount of external potential.

PN Junction Diode VI Characteristics in Forward Bias
PN Junction Diode VI Characteristics in Forward Bias

The potential difference across the PN junction is maintained constant by the depletion layer action. The max amount of current to be conducted is kept incomplete by the load resistor, because when the PN junction diode conducts more current than the normal specifications of the diode, the extra current results in the heat dissipation and also leads to serve damage to the device.

PN Junction Diode in Reverse Bias

When a PN junction diode is connected in a Reverse Bias condition, a positive (+ Ve) voltage is connected to the N type material & a negative (-Ve) voltage is connected  to the P-type material.

When the +Ve  voltage is applied to the N-type material, then it attracts the electrons near the positive electrode and goes away from the junction, whereas the holes in the P-type end are also attracted away from the junction near the negative electrode.

PN Junction Diode in Reverse Bias
PN Junction Diode in Reverse Bias

In this type of biasing, current flow through the PN junction diode is zero. Though, the current leakage due to minority charge carriers flows in the diode that can be measured in a uA (micro amperes). As the potential of the reverse bias to the PN junction diode ultimately increases and leads to PN junction reverse voltage breakdown and the current of the PN junction diode is controlled by an external circuit. Reverse breakdown depends on the doping levels of the P & N regions. Further, with the increase in reverse bias the diode will become short circuited due to overheat in the circuit and max circuit current flows in the PN junction diode.

VI Characteristics of PN Junction Diode in Reverse Bias

In this type of biasing, the characteristic curve of diode is shown in the fourth quadrant of the below figure. The current in this biasing is low till breakdown is reached and hence the diode looks like as open circuit. When the input voltage of the reverse bias has reached the breakdown voltage, reverse current increases enormously.

PN Junction Diode VI Characteristics in Reverse Bias
PN Junction Diode VI Characteristics in Reverse Bias

Therefore, this is all about PN junction diode in zero bias, forward bias and reverse bias conditions and  VI characteristics of PN junction diode.We hope that you have got a better understanding of this concept.Furthermore, any doubts regarding this article, or electronics projects please give your feedback by commenting in the comment section below.Here is a question for you, which diode is used in the photo transistor ?

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4 Comments

  1. shaik sameena says:

    Why we represent VI characteristics only in 1 st and 3 Rd quaderents only?…….plz tell me sir……..

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