Difference Between Intrinsic and Extrinsic Semiconductors

A semiconductor is a material with electrical properties between a conductor (like metal) and an insulator (like rubber or plastic). Semiconductors have a unique ability to conduct electricity under certain conditions and act as insulators under different conditions. This property makes them crucial in everything from microchips and transistors to solar cells and LEDs. There are two main types of semiconductors – Intrinsic and Extrinsic. The main difference between intrinsic and extrinsic semiconductors is that intrinsic semiconductors are pure semiconducting materials without any intentional doping, while extrinsic semiconductors are intentionally doped to modify their electrical properties. This blog will cover the difference between intrinsic and extrinsic semiconductors.

Before we discuss the differences between the two types of conductors, let's first understand the basics of intrinsic and extrinsic semiconductors.

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What are Intrinsic Semiconductors?

An intrinsic semiconductor is a pure semiconductor material without intentional doping with other elements. A single type of atom forms it, usually silicon (Se) and Germanium (Ge).  

In intrinsic semiconductors, semiconductor atoms have 4 electrons in their outer orbit, which they share with adjacent atoms and form 4 covalent bonds. Thus, each atom has 8 electrons in its outermost shell. This forms a very strong network between atoms and their electrons.

Fig 1 – The above image shows the structure of an intrinsic semiconductor with all bonds intact at low temperatures. 

Under normal temperature and pressure conditions, an intrinsic semiconductor has an equal number of free electrons and holes (Holes are the empty spaces left by electrons excited to a higher energy band). Free electrons are not attached to any atom and can move freely through the material. 

Let us understand this with an analogy –

Think of an intrinsic semiconductor like a dough your mother kneaded to make chapatis for your dinner. It’s neutral and hasn’t been altered. Similarly, intrinsic semiconductors are pure semiconductor materials without intentional doping.

What are Extrinsic Semiconductors?

An extrinsic semiconductor is the result of introducing atoms of other elements so that the primitive semiconductor loses its purity and gains conductivity. This impurification process is known as “doping.” Depending on the impurity type added, there are two types of extrinsic semiconductors.

Coming back to the dough analogy –

Now, imagine adding salt, spices and oil to that dough. This transformed dough can be used for making dishes other than plain chapatis and represents an extrinsic semiconductor. The added salt, spices and oil change its properties, just as intentionally introduced impurities modify extrinsic semiconductors.

P-Type (Positive Type) Semiconductor

In the P-type semiconductor, trivalent elements with 3 valence electrons are used as dopants. The most common P-type semiconductors are Boron (B), Indium (In), and Gallium (Ga). The four covalent bonds we saw in the intrinsic semiconductor cannot be formed by only contributing three electrons. 

So, these trivalent dopants introduce “holes” into the crystal lattice of the semiconductor material. Because these dopants have one fewer valence electron than the host semiconductor material, they create regions or holes where there is effectively a positive charge due to the absence of an electron.

Fig 2 – The figure represents a P-type Extrinsic semiconductor with a trivalent impurity atom of Boron (B) and a void space formed in the covalent bond with a neighboring silicon atom. This hole attracts electrons and participates in conduction.

The network these atoms now make presents a series of holes that allow electrons to move more easily, leading to electrical conduction.

Remember – In a p-type extrinsic semiconductor, Number of holes >> Number of free electrons

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N-Type (Negative Type) Semiconductor

An N-type semiconductor uses pentavalent elements with five valence electrons as dopants. The most commonly used elements are phosphorus (P), Arsenic (As), and Antimony. These dopants introduce extra electrons into the semiconductor material's crystal lattice.

Fig 3 – The above image shows the structure of an n-type extrinsic semiconductor. When a pentavalent atom takes the place of a Si atom, all its four electrons bond with four neighbouring Si atoms. However, the fifth electron remains loosely bound to the parent atom.

These extra valence electrons act as majority charge carriers, which means they are the dominant charge carriers responsible for electric current in the semiconductor material. Because they have a negative charge, their movement constitutes an electric current.

Remember – In an n-type extrinsic semiconductor Number of free electrons >> Number of holes 

Difference Between Intrinsic and Extrinsic Semiconductors

Semiconductors play a crucial role in modern electronics, enabling the development of devices like transistors, integrated circuits, and solar cells. The key distinction between intrinsic and extrinsic semiconductors lies in their purity and the way they conduct electricity. We hope this blog was helpful.