What is a P-N Junction?
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.
Unbiased P-N Junction
- 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 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.
P-N Junction Formation
- 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 the 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.
- 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 the depletion region.
- Due to this positive space charge region on either side of the junction, an electric field direction from the 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 the drift. Here, we see that the direction of the drift current is opposite to that of the diffusion current.
Try yourself:The p-region has a greater concentration of __________ as compared to the n-region in a P-N junction.
Holes are the majority charge carriers in p-type material.
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.
1. Forward Bias
Forward Bias of the P-N Junction
- In the figure, 1 represents battery induced electric field, 2 represents Build-in electric field, 3 represents conventional current, and 4 represents Electron current.
- 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.
2. Reverse Bias
Reverse Bias of the P-N Junction
- In the figure, 1 represents a battery-induced electric field, and 2 represents Build-in electric field.
- 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:
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 the P-N 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 concentration gradient, there will be a flow of charge carriers from higher concentration region to lower concentration region.
- The movement of charge carriers inside the p-n junction is the reason behind the current flow in the circuit.
V-I Characteristics of P-N Junction Diode
Applications of P-N 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 a rectifier in many electric circuits and as a voltage-controlled oscillator in varactors.
Frequently Asked Questions on P-N Junction
Q.1. What happens when the battery voltage is increased in a forward-biased p-n junction?
Ans. The current through the junction increases when the battery voltage is increased in a forward-biased p-n junction.
Q.2. What happens when a P-N junction is reverse biased?
Ans. The holes and electrons tend to move away from the junction.
Q.3. What are the two breakdown mechanisms of the P-N junction?
Ans. The two breakdown mechanisms are Zener breakdown and Avalanche breakdown.
Q.4. What is the static resistance of a diode?
Ans. 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.
Q.5. What is the dynamic resistance of a diode?
Ans. Dynamic resistance of a diode is defined as the ratio of change in voltage to the change in current.
Q.6. What is reverse resistance?
Ans. Reverse resistance is defined as the resistance offered by the p-n junction diode when it is reverse biased.