How semiconductor material differ from conductor and insulator?

In order to understand semiconductor material, we have to understand about resistivity, conductor and insulator.

Resistivity

According to Ohm’s Law, resistance is the proportion of the voltage difference to the flow of current across any electrical or electronic circuit. The issue with involving opposition as an estimation is that it relies especially upon the actual size of the material being estimated as well as the material out of which it is made. For instance, if we somehow managed to expand the length of the material (making it longer) its opposition would likewise increment relatively.

Moreover, assuming we expanded its width or size (making it thicker) its obstruction worth would diminish. So we need to have the option to characterize the material so as to demonstrate its capacity to one or the other lead or go against the progression of electrical flow through it regardless of anything else its size or shape is.

The quantity that is used to indicate this specific resistance is called Resistivity and is given the Greek symbol of ρ, (Rho). Resistivity is measured in Ohm-meters, (Ω.m). Resistivity is the inverse to conductivity.

If the resistivity of various materials is compared, they can be classified into three main groups, Conductors, Insulators and Semi-conductors as shown below chart of resistivity: 

 

Here, we observed that a very little margin between the resistivity of the conductors like gold and silver in comparison to a much larger margin for the resistivity of the insulators between quartz and glass. Because of the variation of ambient temperature, there is a difference in resistivity. Metals are much better conductors of heat than insulators.

Conductors

The resistivity of conductors is very little which is typically in the micro-ohms per meter. There are lots of free electrons in the basic atomic structure of a conductor which assists to flow electrical current conveniently. When electrical voltage is applied, these electrons will only flow through a conductor.

If a positive voltage potential is applied to the material, these “free electrons” leave their parent atom and travel together through the material forming an electron drift, more commonly known as a current. How “freely” these electrons can move through a conductor depends on how easily they can break free from their basic atoms when a voltage is applied. Then the amount of electrons that flow depends on the amount of resistivity the conductor has.

Metals like Copper, Aluminum, Silver or non-metals like Carbon are usually good conductor as they contain very few electrons in their outer “Valence Shell” or ring. It causes them being easily knocked out of the atom’s orbit. This helps them to flow freely through the material until they intersect with other atoms, producing a “Domino Effect” through the material thereby creating an electrical current. Copper and Aluminum is the main conductor used in electrical cables as shown.

As metals contain very little values of resistance, they are good conductors of electricity. Typically, it is micro-ohms per meter, (μΩ.m). Copper and aluminum are examples of very good conductors of electricity. They still have some resistance to the flow of electrons and subsequently do not conduct seamlessly. The resistivity of conductors increases with ambient temperature. Therefore, during carrying electrical current, resistors become hot.

Insulators

Insulators are built with non-metals materials which don’t have any free electrons in their basic atomic structure. In this case, the electrons in the outer valence shell are intensely attracted by the positively charged inner nucleus. As there are no free electrons, no current will flow even though a potential voltage is applied to the material. It provides these materials their insulating properties.

Insulators are not affected by normal temperature changes because of having their very high resistivity. Typically, it is millions of ohms per meter. Although at very high temperatures wood becomes charcoal and changes from an insulator to a conductor. Some examples of good insulators are rubber, marble, fused quartz, PVC plastics etc.

Without insulators, electrical circuits would short together and not work. Therefore, insulators play a vital role in electrical and electronic circuits. For insulating and supporting overhead transmission cables, insulators made of glass or porcelain are used. Some other examples: epoxy-glass resin materials are used to make printed circuit boards, PCB’s etc. while PVC is used to insulate electrical cables.

Semiconductor materials

Semiconductors materials like silicon (Si), germanium (Ge) and gallium arsenide (GaAs), contains electrical properties in between a “conductor” and an “insulator”. They are neither good conductors nor good insulators. Therefore, they are called “semi”-conductors. Their atoms are closely assembled together in a crystalline form called a “crystal lattice”. Therefore, they have very little “free electrons”. Under special conditions, these electrons can flow.

By adding or replacing, certain donor or acceptor atoms to this crystalline structure, the ability of semiconductors to conduct electricity can be significantly developed. In this way, thereby, more free electrons can be produced than holes or vice versa. That is by adding a little percentage of another element to the base material, either silicon or germanium.

Germanium and Silicon contain semi-conductive material property and chemically pure. They are classified as intrinsic semiconductors. We can control their conductivity by controlling the amount of impurities added to this intrinsic semiconductor material. Free electrons or holes can be produced correspondingly by adding numerous impurities called donors or acceptors to this intrinsic material. This process of adding donor or acceptor atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is known as Doping. Because of not having purity, these donor and acceptor atoms of doped silicon are together known as “impurities”. After doping these silicon material with a adequate number of impurities, it can be converted into an N-type or P-type semi-conductor material.

Most widely used basic semiconductor material is the silicon. It contains four valence electrons in its outermost shell. It shares this electron with its neighboring silicon atoms to form full orbital’s of eight electrons. The bond structure between the two silicon atoms is such that each atom shares one electron with its neighbor making the bond very firm.

Very little free electrons are available to move around the silicon crystal. As a result, crystals of pure silicon (or germanium) are good insulators, or at the very least very high value resistors.

Silicon atoms are arranged in a definite regular shape making them a crystalline solid structure. A crystal of pure silica (silicon dioxide or glass) is usually called an intrinsic crystal (it has no impurities) and hence has no free electrons.

Just connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we require to build a “positive” and a “negative” pole within the silicon permitting electrons and therefore electric current to flow out of the silicon. These poles are built by doping the silicon with certain impurities.