PN junction Diode Characteristics and Biasing

Pn junction is one of the most important structures in today’s semiconductor technology used in transistors, FET and many types of integrated circuits. In our previous articles, we have seen  p-n junction formation from a p-type and n-type semiconductor and Semiconductor electronics. We have also learned about diffusion current, drift current and depletion region. Here we are going to explain how p-n junction diode characteristics.

PN junction diode image

[Image source]

P-n junction diode:

To understand the behavior of p-n junction we make it conducting by applying an external voltage over a range of 0v, 5v, 10v and determine how the current passed through the p-n junction varies with increasing voltage levels. We usually connect two metallic contacts at the ends of the p-n junction to apply external voltage. A p-n junction with two metallic contacts is known as p-n junction diode or semiconductor diode.

As we know diode is a specialized electronic component with two electrodes called the anode and cathode conducts current in one direction. As shown in the figure above, the direction of the arrow indicates the direction of conventional current.  The process of applying an external voltage to the p-n junction is known as biasing. We generally bias p-n junction in following ways:

  • Forward bias: In forward bias negative terminal is connected to n-type material and the positive terminal is connected to p-type material across the diode shows the decrease in the built-in potential barrier.
  • Reverse bias: In reverse bias negative terminal is connected to p-type material and the positive terminal is connected to n-type material across the diode shows the increase in the built-in potential barrier.
  • Zero bias: In zero bias no external voltage is applied.

Pn junction diode biasing:

In the modern electronics p-n junction possesses special properties that are useful in many applications. We can bias p-n junction in following ways by applying an external voltage or not. Three possible biasing conditions and two operating regions for the typical p-n junction diode are explained below.

Forward and reverse bias image

[Image source]

(i) Zero bias diode:

In zero bias or thermal equilibrium state, junction potential provides higher potential energy to the holes on the p-side than the n-side. When we short the terminals of the junction diode, few majority carriers in the p-side with plenty energy starts travel across the depletion region. With the help of majority charge carriers, the current start flowing in the diode known as forward current. Similarly, minority charge carriers in the n-side move across the depletion region in reverse direction give rise to a reverse current. Due to this, the potential barrier opposes the movement of electrons and holes across the junction and permits the minority carriers to drift across the p-n junction.  Therefore, the potential barrier helps minority charge carriers in p-type and n-type to drift across the p-n junction. After this when the majority charge carriers are equal and both are moving in a reverse direction, the equilibrium will be established indicates the zero current flowing in the circuit. That’s why this junction is said to be in a state of dynamic equilibrium.

(ii) Forward biased diode:

Forward biasing a p-n junction diode is very simple, into this, we connect its positive terminal to the p-side and negative terminal to the n-side of the of the p-n junction diode. When an external voltage is applied, the majority charge carriers in N and P regions are attracted towards the PN junction and the width of the depletion layer decreases with the diffusion of the majority charge carriers. So an electric field induced in the direction converse to that of the incorporated field. When the forward bias is greater than the built-in potential, the depletion region becomes very much thinner so that a large number of majority charge carriers can cross the PN junction and conducts an electric current. The current flowing up to built-in potential is known as KNEE current.

(iii) Reverse bias diode:

In reverse bias condition, we connect positive terminal to the n-side and negative terminal to the p-side of the of the p-n junction diode. When an external voltage is applied, 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 which results in the increase in the width of the potential barrier. With the increase in the potential barrier width, the electric field at the junction also starts increasing and the p-n junction act as a resistor. Minority charge carriers generated at the depletion region cause the small leakage current in the junction diode. This indicates that the increase in the width of the depletion layer presents a high impedance path which acts as an insulator. When the reverse bias potential across the p-n junction diode increases, the reverse breakdown voltage occurs causing the diode current to be controlled by an external circuit. If we increase reverse bias further, p-n junction diode becomes short circuited due to overheat in the circuit and maximum circuit current flows in the PN junction diode.

 Pn junction diode characteristics:

P-n junction diode shows zero resistance in the forward direction and infinite resistance in the reverse direction. i.e., it is not a perfect diode. Before using this diode, it is necessary to know a little about its characteristics and properties with forward bias and reverse bias.

To know about its characteristics we plot a graph between voltage and current along the x-axis and y-axis which shows the behavior of p-n junction diode in forward biasing and in reverse biasing.

pn junction diode characteristics image

[Image source]

(i) Forward characteristic for a junction diode:

As shown in the figure above, the VI characteristics of junction diode are not linear i.e., not a straight line. This nonlinear characteristic indicates that the resistance is not constant during the operation of N junction. When forward biased is applied to the diode then due to low impedance path, a large amount of current starts flowing known as infinite current. This current starts to flow above the knee point with a small amount of external potential. If we increase further current then it may damage the diode, to overcome this we use load resistor which controls the flow of current and safe the device from damaging.

(ii) Reverse characteristic for a junction diode:

In this type of biasing the current is low till breakdown is reached and hence the diode likes an open circuit. The characteristic curve of this diode is shown in the fourth quadrant of the given figure above. When the input voltage of the reverse bias has reached the breakdown voltage, reverse current increases enormously. In reverse direction, a perfect diode would not allow any current to flow.

Pn junction diode Equation:

The pn junction diode equation for an ideal diode is given below:

I = IS[exp(eV/KBT) – 1]

Here,

IS = reverse saturation current

e = charge of electron

KB = Boltzmann constant

T = temperature

For a normal p-n junction diode, the equation becomes

I = IS[exp(eV/ɳKBT) – 1]

Here, ɳ = emission co-efficient, which is a number between 1 and 2, which typically increases as the current  increases.

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