P-N Junction

What is P-N Junction?

Definition: A p-n junction is an interface or a boundary between two semiconductor material types, namely the p-type and the n-type, inside a semiconductor.

The p-side or the positive side of the semiconductor has an excess of holes and the n-side or the negative side has an excess of electrons. In a semiconductor, the p-n junction is created by the method of doping. The process of doping is explained in further detail in the next section.

Formation of P N JunctionForward BiasReverse BiasP N Junction Formula

Formation of P-N Junction

As we know if we use different semiconductor materials to make a p-n junction, there will be a grain boundary that would inhibit the movement of electrons from one side to the other by scattering the electrons and holes and thus we use the process of doping. We will understand the process of doping with the help of this example. Let us consider a thin p-type silicon semiconductor sheet. If we add a small amount of pentavalent impurity to this, a part of the p-type Si will get converted to n-type silicon. This sheet will now contain both p-type region and n-type region and a junction between these two regions. The processes that follow after the formation of a p-n junction are of two types – diffusion and drift. As we know, there is a difference in the concentration of holes and electrons at the two sides of a junction, the holes from the p-side diffuse to the n-side and the electrons from the n-side diffuse to the p-side. These give rise to a diffusion current across the junction.

unbiased P N junction

Also, when an electron diffuses from the n-side to the p-side, an ionized donor is left behind on the n-side, which is immobile. As the process goes on, a layer of positive charge is developed on the n-side of the junction. Similarly, when a hole goes from the p-side to the n-side, and ionized acceptor is left behind in the p-side, resulting in the formation of a layer of negative charges in the p-side of the junction. This region of positive charge and negative charge on either side of the junction is termed as the depletion region. Due to this positive space charge region on either side of the junction, an electric field direction from a positive charge towards the negative charge is developed. Due to this electric field, an electron on the p-side of the junction moves to the n-side of the junction. This motion is termed as the drift. Here, we see that the direction of drift current is opposite to that of the diffusion current.

Biasing conditions for the p-n Junction Diode

There are two operating regions in the p-n junction diode:

  • P-type
  • N-type

There are three biasing conditions for p-n junction diode and this is based on the voltage applied:

  • Zero bias: There is no external voltage applied to the p-n junction diode.
  • Forward bias: The positive terminal of the voltage potential is connected to the p-type while the negative terminal is connected to the n-type.
  • Reverse bias: The negative terminal of the voltage potential is connected to the p-type and the positive is connected to the n-type.

Forward Bias

Forward Bias p-n junction

When the p-type is connected to the positive terminal of the battery and the n-type to the negative terminal then the p-n junction is said to be forward-biased. When the p-n junction is forward biased, the built-in electric field at the p-n junction and the applied electric field are in opposite directions. When both the electric fields add up the resultant electric field has a magnitude lesser than the built-in electric field. This results in a less resistive and thinner depletion region. The depletion region’s resistance becomes negligible when the applied voltage is large. In silicon, at the voltage of 0.6 V, the resistance of the depletion region becomes completely negligible and the current flows across it unimpeded.

Reverse Bias

Reverse Bias p-n junction

When the p-type is connected to the negative terminal of the battery and the n-type is connected to the positive side then the p-n junction is said to be reverse biased. In this case, the built-in electric field and the applied electric field are in the same direction. When the two fields are added, the resultant electric field is in the same direction as the built-in electric field creating a more resistive, thicker depletion region. The depletion region becomes more resistive and thicker if the applied voltage becomes larger.

P-N Junction Formula

The formula used in the p-n junction depends upon the built-in potential difference created by the electric field is given as:

\(E_{0}=V_{T}ln[\frac{N_{D}.N_{A}}{n_{i}^{2}}]\)

Where,

  • E0 is the zero bias junction voltage
  • VT is the thermal voltage of 26mV at room temperature
  • ND and NA are the impurity concentrations
  • ni is the intrinsic concentration.

How does current flow in PN junction diode?

The flow of electrons from the n-side towards the p-side of the junction takes place when there is an increase in the voltage. Similarly, the flow of holes from the p-side towards the n-side of the junction takes place along with the increase in the voltage. This results in the concentration gradient between both sides of the terminals. Because of the formation of the concentration gradient, there will be a flow of charge carriers from higher concentration regions to lower concentration regions. The movement of charge carriers inside the pn junction is the reason behind the current flow in the circuit.

V-I Characteristics of PN Junction Diode

V I Characteristics of P N junction Diode

VI characteristics of PN junction diode is a curve between the voltage and current through the circuit. Voltage is taken along the x-axis while the current is taken along the y-axis. The above graph is the VI characteristics curve of the PN junction diode. With the help of the curve we can understand that there are three regions in which the diode works, and they are:

  • Zero bias 
  • Forward bias
  • Reverse bias

When the PN junction diode is under zero bias condition, there is no external voltage applied and this means that the potential barrier at the junction does not allow the flow of current.

When the PN junction diode is under forward bias condition, the p-type is connected to the positive terminal while the n-type is connected to the negative terminal of the external voltage. When the diode is arranged in this manner, there is a reduction in the potential barrier. For silicone diodes, when the voltage is 0.7 V and for germanium diodes, when the voltage is 0.3 V, the potential barriers decreases and there is a flow of current. 

When the diode is in forward bias, the current increases slowly and the curve obtained is non-linear as the voltage applied to the diode is overcoming the potential barrier. Once the potential barrier is overcome by the diode, the diode behaves normal and the curve rises sharply as the external voltage increases and the curve so obtained is linear.

When the PN junction diode is under negative bias condition, the p-type is connected to the negative terminal while the n-type is connected to the positive terminal of the external voltage. This results in an increase in the potential barrier. Reverse saturation current flows in the beginning as minority carriers are present in the junction. 

When the applied voltage is increased, the minority charges will have increased kinetic energy which affects the majority charges. This is the stage when the diode breaks down. This may also destroy the diode. 

Applications of PN Junction Diode

  • p-n junction diode can be used as a photodiode as the diode is sensitive to the light when the configuration of the diode is reverse-biased.
  • It can be used as a solar cell.
  • When the diode is forward-biased, it can be used in LED lighting applications.
  • It is used as rectifiers in many electric circuits and as a voltage-controlled oscillator in varactors.

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Frequently Asked Questions on p-n Junction

What happens when the battery voltage is increased in a forward-biased p-n junction?

The current through the junction increases when the battery voltage is increased in a forward-biased p-n junction.

What happens when a p-n junction is reverse biased?

The holes and electrons tend to move away from the junction.

What are the two breakdown mechanisms of the p-n junction?

The two breakdown mechanisms are Zener breakdown and Avalanche breakdown

What is static resistance of a diode?

Static resistance of a diode is defined as the ratio of the DC voltage applied across the diode to the DC current flowing through the diode.

What is the dynamic resistance of a diode?

Dynamic resistance of a diode is defined as the ratio of change in voltage to the change in current.

What is reverse resistance?

Reverse resistance is defined as the resistance offered by the p-n junction diode when it is reverse biased.

4 Comments

  1. what happens when p-n junction formed

    1. For the formation of p-n junction diode, p-type and n-type semiconductor materials are combined together. There is a movement of electrons to the p-type material and movement of holes to the n-type material. Due to this movement, there is a formation of the depletion region. There is an electric field formed at this junction which gives the voltage of the junction.

  2. Define the limitations in the operation conditions of a p-n junction.

    1. The pn junction will perform satisfactorily if it is operated within the following limiting values or conditions:

      • Maximum Forward Current– It is the highest instantaneous current under forward condition.
      • Peak Inverse Voltage – It is the maximum voltage a diode can withstand in the reverse-biased condition before it breakdowns.
      • Maximum Power Rating – It is the maximum power that a diode can dissipate without damaging the device itself.

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