Electrical Properties
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Introduction to Electrical Properties
Apart from general physical characteristics like density and rigidity, solids depict another important property called Conductivity. Conductivity is defined as the measure of a solid to ease the flow of electrons and conduct electricity. The values of conductivity are diverged and vary from 10-20 to 107. The unit of conductivity is ohm-1 m-1.
Types of Solids
On the basis of conductivity solids are classified into three types, namely:
Conductors
The solids which allow current to flow through them very easily are called Conductors. The conductivity of solids lies between 104 to 107 ohm-1m-1. Metals are generally good conductors of electricity as their conductivity is generally near to 107 ohm-1 m-1.
Insulators
The solids which don’t allow electricity to flow through them are called Insulators. The conductivity of insulators is majorly low with the range of 10-20 to 10-10 ohm-1 m-1. Some common examples of insulators are plastics, rubber, wood etc.
Image 2: Common examples of conductors and insulators
Semiconductors
The solids in which the value of conductivity lies between to that of conductors and insulators are known as Semiconductors. They neither allow complete electricity to pass through them and neither resists the flow of electrons entirely. The value of conductivity lies between 10-6 to 104 ohm-1 m-1. Some good examples of semiconductors are as follows:
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Silicon
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Germanium
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Selenium
Conduction
The process through which transfer of heat or electricity takes place inside a material due to the potential difference between the end points is called Conduction. In solids conduction can be seen in two ways:
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Conduction in Metals
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Conduction in Semiconductors
Conduction in Metals
A conductor can conduct current in two ways either by the flow of electrons or by the flow of ions. In metallic conductors, the flow of electric current is possible due to the flow of electrons and in electrolytes, the same is possible due to the flow of ions.
Metals have the potential to conduct electricity in solid and molten phase. The value of conductivity relies upon a number of valence electrons available per atom. The atomic orbitals of metal atoms are responsible for forming molecular orbitals. The energy level of molecular orbitals is nearly same to form bands. When the band is half-filled or partially filled the electrons can flow easily from one band to another, thereby making the electron to flow under an applied electric field.
But in case the gap between valence band and conduction band (also called unoccupied band) is large the flow of electrons is just not possible. Summarizing the points, the drawn conclusions are:
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When the gap is high, electrons don’t jump from one band to another thereby the substance is said to have low conductivity
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When the gap is low, electrons can easily move from one band to another thereby the substance is said to have high conductivity
Image 3: Energy Band Gap
Conduction in Semiconductors
Taking the case of semiconductors, the gap between valence band and conduction band is small. Due to this small gap, some electrons can easily jump from valence band to conduction band, thereby showing some conductivity. Electrical conductivity is directly proportional to temperature as the electrons get more energy to move from one band to another.
Semiconductors which are made from only one or pure semiconductor are called Intrinsic Semiconductor. Some common examples of intrinsic semiconductors are silicon and germanium. Also, the conductivity of an intrinsic semiconductor is very low and of no practical use. To make intrinsic semiconductor applicable we increase its conductivity with the help of doping.
Doping
Image 4: Electron Distribution
The process of adding an adequate amount of impurity in a semiconductor to increase its conductivity is called doping. Doping is done in two ways, namely:
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By adding electron rich impurities
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By adding electron deficient impurities
By adding electron rich impurities
Silicon and germanium are present in group 14 of the periodic table with four valence electrons each. They can form four covalent bonds with other neighbor atoms. When we dope silicon and germanium with electron rich impurities like Phosphorus P the five electrons present in P form bonds with Si and Ge. One extra electron left becomes delocalized and is responsible for increasing conductivity of silicon and germanium. The semiconductors from this type of doping are also called n-type semiconductors as the conductivity is enhanced due to the addition of negatively charged species.
By adding electron deficient impurities
When we dope silicon or germanium with electron deficient species like Boron (B) or Aluminium (Al) we obtain p-type semiconductors. Out of four electrons present in silicon, three are used to form covalent bonds with Boron and 1 electron is left unshared. The place where the missing fourth electron is located is called Electron Vacancy or Electron Hole. An electron from nearby atoms comes and fulfills the vacancy.
Image 5: Types of semiconductors
Applications of p-type and n-type semiconductors
Semiconductors are used worldwide and have many practical applications. Some are as follows:
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Combinations of p-type and n-type semiconductors are used in manufacturing of electronic components
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Diode which is a general combination is utilized as a rectifier
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Transistors which are used in detecting and amplifying sound signals are made by placing one layer of semiconductor in between two similar semiconductor layers
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Photo diode is widely utilized in converting light energy into electrical energy
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