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PN Junction is formed at the interface or boundary between a p-type and the n-type semiconductor. The positive side of the semiconductor (p side) has an excess of holes. The negative side of the semiconductor (n side) has an excess of electrons. The PN junction is created by the process of doping. Doping is done to provide a seamless movement of electrons between the p side and n side of the semiconductor.
| Table of Content |
Key Terms: Semiconductors, PN Junction, Biasing, Forward Bias, Reverse Bias, Diode, Junction, Battery, Electron, Equilibrium
What is PN Junction Diode?
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- Fusing a p-type semiconductor with a n-type semiconductor creates a potential barrier voltage across the diode junction.
- This interface creates a two terminal device better known as the PN Junction.
- The p and the n region are separated by the charge rich depletion region.
- The junction stays in equilibrium without the application of any external voltage.
- The potential barrier can be overcomed by establishing a connection between the p and n type and connecting it to a battery.
- The electrons thus cross the depletion region.
PN Junction Diode
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| Important Concepts Related to PN Junction | ||
|---|---|---|
| Zener Diode | Application of Junction Diode | Light Emitting Diode |
| Integrated Circuits | Light Emitting Diode | Varactor Diode |
| Schottky Diode | Pin Diode | Laser Diode |
Formation of PN Junction
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- The p-n junction in a semiconductor is formed by the doping method.
- The method of doping is used because if the semiconductor is formed by combining different semiconductor materials, it will result in the creation of a grain boundary.
- Grain boundary will scatter the holes and electrons and will stop the movement of electrons from one side to another side.
Drift Current and Diffusion
Doping in Semiconductors
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Let us look at the step-by-step process of doping:
- Consider there is a thin Silicon semiconductor of p-type.
- Now, as a small amount of pentavalent impurity is added to the semiconductor wafer, it turns to be n-type silicon.
- The wafer now has both p and n-type regions which are separated by an interface or a junction.
- After that, the processes of diffusion and drift take place.
- Due to the difference of electrons at one side of the junction and the concentration of holes at another, the electrons from the n side diffuse to the p side.
- Similarly, the holes that are in the p-side diffuse to the n-side.
- Therefore, across the junction, there is a diffusion current.
- As the holes from the p side diffuse to the n side, there is an ionized acceptor that is left behind on the p side.
- Hence, a layer of negative charge develops on the p side of the junction.
Doping in Semiconductors
- Similarly, as the electrons from the n side diffuse to the p side, there is an immobile ionized donor which is left behind on the n side.
- Due to the continuation of the process, on the n side of the junction, a positive charge layer is developed.
- The region with both negative and positive that is on both sides of the junction is called the depletion region.
- Therefore, it leads to the development of an electric field due to which an electron that is on the p side of the junction moves to the n side.
- This leads to a drift current and is employed in making PN junction diodes.
- The direction of diffusion and drift current is opposite to each other.
Biasing Conditions of PN Junction
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Based on the voltage applied, there are 3 biasing conditions of a pn junction
Forward Bias
- When the positive terminal of the battery is connected to the p side of the junction and the negative terminal is connected to the n side of the junction, the pn junction is forward biased.
- Here, the applied electric field and the built-in electric field, both are in opposite directions.
- The resulting field is thus less than the built- in electric field.
- The depletion region thus becomes thin and less resistive.
Reverse Bias
- When the positive terminal of the battery is connected to the n side of the junction and the negative terminal is connected to the p side of the junction, the pn junction is reverse biased.
- Here, the applied electric field and the built-in electric field, both are in the same direction.
- The resulting field is thus in the same direction as the built-in electric field.
- The depletion region thus becomes thicker and more resistive.
Forward and Reverse Bias
Zero Bias
When no external voltage is applied to the pn junction, it is zero biased.
Zero Bias
Formula of PN Junction
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The formula of PN junction is,
E0 = \(V_T \ ln \frac{N_D.N_A}{n_i^2}\)
Here,
E0 is the junction voltage at zero bias
VT is the thermal voltage at room temperature of 26mV
ND and NA are the concentrations of impurity
ni is the intrinsic concentration
Applications of PN Junction
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- PN junction is used in many semiconductor electronics.
- PN junction can be used as a solar cell.
- When forward biased, PN junctions is can be used in LED lights.
- In LED, the PN junction can be used when it's forward biased.
- When reverse biased, the pn junction is used as a photodiode.
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Things to Remember
- An interface or a boundary that is between p-type and n-type, the two types of semiconductor material, is known as a p-n junction.
- The positive side of the semiconductor, which is the p side, has an excess of holes while the negative side of the semiconductor, which is the n side, has an excess of electrons.
- The p-n junction is formed by doping.
- Doping combines different semiconductor materials together.
- Based on the voltage applied, there are 3 biasing conditions of a p-n junction Forward Bias, Reverse Bias, Zero Bias.
- The P-N junction can be used as a solar cell.
- In LED, the PN junction can be used when it's forward biased.
Sample Questions
Ques: What is a p-n junction? (1 mark)
Ans. A p-n junction is an interface or a boundary that is between the two types of semiconductor material p-type and n-type.
Ques: What are the two processes that take place in the formation of a p-n junction? (1 mark)
Ans. In the formation of a p-n junction, the processes of diffusion and drift take place.
Ques: When is a p-n junction said to be reverse biased? (1 mark)
Ans. When the positive terminal of the battery is connected to the n side of the junction and the negative terminal is connected to the p side of the junction, the p-n junction is said to be reverse biased.
Ques. Give reason to explain why n and p regions of a Zener diode are heavily doped. Find the current through the Zener diode in the circuit given below. (3 marks)
(Zener breakdown voltage is 15 V) (2019 outside Delhi)
Ans. When both the sides of the junction, p and n are heavily doped the depletion region that forms is very thin i.e., <10? m. Therefore the electric field throughout the junction is very high 
even when there is a little reverse bias voltage. This could be the cause of breakdown during the reverse bias.
Ques. (a) A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labeled circuit diagram she would use and explain how it works.
(b) Give the truth table and circuit symbol for the NAND gate. (2018)
Ans.
In the positive half cycle of the AC, end A changes to (+) and A1 changes to forward biased and D2 is reverse-biased, hence D1 conducts and D2 doesn't. So traditionally current flows through the D1, RL, and the top half of the secondary winding. Likewise, the (-) half cycle of the AC diode D2 changes to forward biased and D1 turns to reversed biased, current flows through D2, RL, and the bottom half of secondary winding. Thus the current flows in a similar direction in the two half cycles of input AC voltage.
Ques. Name the junction diode whose I-V characteristics are drawn below. (2017)
Ans. This is the attribute of the solar cell.
Ques. Write the two processes that take place in the formation of a p-n junction. Explain with the help of a diagram, the formation of depletion region and barrier potential in a p-n junction. (2017)
Ans. Diffusion and drift are the two processes that take place in the formation of a p-n junction. Diffusion of an electron:
Development of the barrier potential and depletion region:
At this particular point of the p-n junction development the independent electrons closer to the junction diffuse throughout the junction into the p region and merge with holes. Therefore on merging with the hole, it forms a negative ion and leaves a positive ion on the n-side. These two particular strata of the immobile (+) and (-) charges form the depletion region.
Furthermore, the negative charge helps in repelling any more diffusion of electrons when they reach a point through the continuous diffusion of electrons across the junction. This depletion region now forms the barrier and the energy from the exterior is provided to get the electrons to shift across the barrier of the electric field. This possible difference needed to shift the electrons through the electric field is known as the barrier potential.
Ques. (a) In the following diagram, is the junction diode forward biased or reverse biased?
(b) Draw the circuit diagram of a full-wave rectifier and state how it works. (2017 outside Delhi)
Ans.
(a) The junction diode is reverse biased.
(b) When the junction can be stated as forward based only then diode D1 conducts. Therefore in the first half cycle of the input AC, we can see that D1 conducts and D2 doesn’t and the current will flow to B from A. D2 can be stated as reverse biased and in the second half cycle of input AC, diode D2 conducts while D1 doesn’t. The current in this case also flows from A to B. Therefore we can say that the full cycle will be unidirectional.
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