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 details in the next section.

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. This gives 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, an 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 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.

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.

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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 p-n junction?

The two breakdown mechanism 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.

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