What is PN Junction? โ€“ Formation | New Topic [2024]

In this note, we are going to learn about a topic called โ€œWhat is PN Junction?โ€œ, and about its formation of PN Junction as well. Welcome to Poly Notes Hub, a leading destination for engineering notes.

Author Name: Arun Paul.

What is PN Junction?

When a p-type semiconductor is properly linked to an n-type semiconductor, the resulting contact surface is known as a pn junction.

Below we show the pn junction and also the pn junction depletion region โ€“
what is pn junction - poly notes hub

Most semiconductor devices include one or more pn junctions. The pn junction is very important since it serves as the control element for semiconductor devices. A thorough understanding of the development and properties of pn junctions can help the reader understand semiconductor devices.

๐Ÿ“Œ Small Demonstration about P Type and N Type Semiconductor
Q1: What is P Type Semiconductor?

A1: When a small amount of trivalent impurity is added to a pure semiconductor, it is called a p-type semiconductor. The addition of trivalent impurity provides a large number of holes in the semiconductor. Typical examples of trivalent impurities are gallium (At. No. 31) and indium (At. No. 49). Such Impurities that produce p-type semiconductors are known as acceptor impurities because the holes created can accept the electrons. Learn More.

Q2: What is N Type Semiconductor?

A2: When a small amount of pentavalent is added to a pure semiconductor, it is known as an n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons in the semiconductor crystal. Typical pentavalent impurities are arsenic (At. No. 33) and antimony (At. No. 51). Such Impurities that produce n-type semiconductors are known as donor impurities because they donate or provide free electrons to the semiconductor crystal. Learn More.

Formation of PN Junction

Below we discuss about the PN Junction Formation โ€“

In fact, the characteristics of a pn junction will not be obvious if the p-type block is simply brought into contact with the n-type block. In reality, pn junctions are made using unique processes. Alloying is a typical approach for producing pn junctions. In this procedure, a tiny block of indium is put on an n-type germanium slab, as shown in Figure (i). The system is then heated to a temperature about 500 degrees Celsius.

what is pn junction - poly notes hub

The indium and some of the germanium melt together to form a small puddle of molten germanium-indium mixture, as shown in Figure (ii). The temperature is then decreased, and the puddle begins to harden. Under the right conditions, the indium impurity atoms in the germanium slab will align to form a single crystal. The addition of indium overcomes the excessive amount of electrons in n-type germanium, resulting in a p-type area.

As the process continues, the molten mixture gets progressively rich in indium. When all of the germanium has been redeposited, the leftover material forms an indium button that is frozen to the outside surface of the crystallized part, as illustrated in Figure (iii). This button serves as an appropriate basis for soldiers on leashes.

Properties of PN Junction

Below we listed some important PN junction properties โ€“
  • Forward Bias: Forward-biased junctions are those that have a positive voltage applied to the P-region and a negative voltage applied to the N-region. In this stage, the depletion layer width narrows, allowing current to easily pass through the junction.
  • Reverse Bias: When a negative voltage is applied to the P-region and a positive voltage to the N-region, the junction becomes reverse-biased. In this situation, the depletion layer expands and prevents considerable current flow.
  • Depletion Region: This is the region around the PN junction when mobile charge carriers (electrons and holes) have been depleted, leaving only stationary ions. This region acts as a barrier to the flow of current in the reverse-biased state.
  • Barrier Potential: When a PN junction is established, the dispersion of charge carriers creates a potential difference across it. This potential difference is known as the barrier potential. For silicon, forward bias is typically approximately 0.7 volts.
  • Rectification: A PN junction lets current to flow freely in one direction (forward bias) while blocking it in the other (reverse bias). This feature is critical for diodes, which are used in rectifiers to convert AC into DC.
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