Classification of Semiconductors


We are going to have a quick discussion of the two classifications of semiconductors, which will help us understand the basics of semiconductors better.

In order for a semiconductor crystal to be formed, atoms share electrons with neighboring atoms. For example, in a silicon crystal, every silicon atom inside has four neighboring atoms. Central silicon atoms share their four valence electrons with four neighboring silicon atoms to have eight electrons in its valence orbit, which produces a state of chemical stability. This sharing of valence electrons produces covalent bonds that hold the atoms together in a silicon crystal.

Silicon Crystal Structure
Carefully cooled silicon makes a nice, pretty crystalline structure

Now, if a crystal is purely made of silicon atoms, that silicon crystal is classified as an intrinsic semiconductor. This is because an intrinsic semiconductor is a pure semiconductor. At room temperature, the heat energy in the air allows the valence electrons in an intrinsic semiconductor to jump into the conduction band and become free electrons. In every electron that jumps into the conduction band, there’s also a vacancy, or more known as hole that is left in the valence band of the intrinsic semiconductor material. So in an intrinsic semiconductor, thermal energy just creates an equal number of free electrons and holes.

Intrinsic semiconductors act more like an insulator at room temperature. This is because the thermal energy at room temperature only produces a few free electrons and holes which are the current carriers in a semiconductor material. Increasing the operating temperature might produce more free electrons and holes but of course, operating at high temperature is not ideal and it will still just create an equal number of free electrons and holes.

Electrons moving from the valence band to the conduction band
When electrons jump from the valence band to the conduction band, they leave behind holes.

Yet we say that a semiconductor can be a very useful device in electronics by controlling its conductivity. However, this is not possible in an *intrinsic* semiconductor since they have poor conductivity. To increase its conductivity, intrinsic semiconductors must be doped with impurity atoms. A doped semiconductor is classified as an extrinsic semiconductor.

Doping can either increase the number of free electrons or holes in a semiconductor. Because of this, extrinsic semiconductors have two types, n-type and p-type. In the doping process, manufacturers melt a pure silicon crystal. This will break the covalent bonds and turns the solid silicon crystal to a liquid, meaning the atoms don’t have any set order or structure. Then impurity atoms are added. If we take atoms with five electrons in the valence orbit, known as pentavalent atoms, and add them to the liquified silicon, this increases the number of free electrons and creates an n-type semiconductor. Going the other way, we can add atoms with only three electrons in the valence orbit, or trivalent atoms, which will increase the number of holes, creating a p-type semiconductor.

Pentavalent and Trivalent Atoms
To get more free electrons, stick in an atom with five electrons in the valence band. For more free holes, put in an atom with three electrons in the valence band.

Manufacturers control the conductivity of a doped semiconductor through the amount of impurity atoms added. In this case, a semiconductor can be lightly or heavily doped. Lightly doped semiconductors have high resistance, while low resistance semiconductors are heavily doped. But, with careful manipulation of how semiconductor materials of different types interface, and the application of voltage in specific ways, that resistance can be further increased or decreased, depending on what you want to do.

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