#Technical studies- study 1
We know that conductors are those materials who allow current to flow through them freely and insulators are those materials who does not allow current to flow but semiconductors are the material which property lies between conductor and insulator.
The best conductors have one valence electron and the best insulators have eight valence electrons.
Whereas the best semiconductors have four valence electrons. Germanium and silicon are examples for semiconductors.
Many years ago germanium was the only material for semiconductor but it had a fatal flow that engineers could not overcome.
Silicon is the most abundant element on the earth, but there were some refining problems that prevented the use of silicon.
Once problems were solved it became the popular semiconductor for use. Without it modern electronics, communications and computers would be impossible.
Semiconductors are classified into two following categories:-
Intrinsic semiconductors are the purest semiconductors. At room temperature it behaves like an insulator. Its free electron and hole number are same.
Voltage applied to an intrinsic semiconductor will force the free electrons to left side and the holes to the right side of the semiconductor body.
In this way the free electrons reach to the left end of the crystal and flow to the positive battery terminal. The free electrons at the negative battery terminal flow to the right end of the crystal and combine with holes.
By adding some impurity to intrinsic semiconductors the conductivity of the semiconductor can be increased. This method is called doping and the doped semiconductor is called extrinsic semiconductor.
A doped semiconductor can have extra holes or extra free electrons.
Thus extrinsic semiconductor is classified into two types.
N-type semiconductor, P-type semiconductor.
When we add a pentavalent atom to an intrinsic semiconductor the product is called n- type semiconductors.
N stands for negative.
Some pentavalent atoms are arsenic,antimony,phosphorus.
When a phosphorus atom added to a silicon atom there is a extra free electron in the product. Because silicon has four valence electron and phosphorous has 5 valence electron.
One one electron from each atom forms four covalent bonds and one electron of phosphorus atom remains unused. Thus it is a free electron. So pentavalent atom is called donor atom.
Let’s consider the conduction of a n-type semiconductor.
When a voltage source applied to a n-type semiconductor the free electrons of the crystal flows left side of the crystal towards the positive terminal and the holes flows right side of the crystal towards the negative terminal.
When a hole reaches the end of the crystal a free electron from external circuit goes there and recombines with the hole. Conduction happens because of free electrons in n-type semiconductors.
Free electrons are present in more numbers than holes so free electrons are the majority carriers and holes are the minority carriers.
When a trivalent atom is added to silicon crystal it is called a p-type semiconductor.
P stands for positive.
Some trivalent atoms are boron, gallium, aluminium.
When a boron atom added to a silicon atom it creates a hole, because silicon has four valance electron and boron has three valance electron. One one electron from both atoms created three covalent bonds for fourth covalent bond silicon has one electron but boron has no electron so it is called a hole.
To complete the bond boron accepts a free electron, so trivalent atoms are called acceptors.
Let’s consider the conduction of a p-type semiconductor.
When voltage source is applied to the semiconductor the free electrons of the semiconductor flows towards left side of the crystal and holes to the right side of the crystal.
The holes reaching the right end of the crystal recombines with a free electron from external circuit. Conduction happens due to flow of holes.
In p-type semiconductor the number of holes are more than the number of free electrons. So holes are majority carriers and free electrons are minority carriers.
We studied that extrinsic semiconductors are two types, p-type and n-type. Both types are equal to a carbon resistor.
When both semiconductors are added we get a new device called pn junction. By understanding this one can understand all kinds of semiconductors devices.
As discussed earlier each trivalent atom added to a semiconductor is a hole, so in figure p-type semiconductor the circled negative sign is the trivalent atom and the outside positive signs are the holes.
Similarly in figure n-type semiconductor the circled positive signs are pentavalent atoms and the outside negative signs are the free electrons.
In figure the free electrons on the n side spread in all directions. Some of them spread across the junction and enters to the p region. It recombines with the hole then the free electron becomes a valence electron.
This process continuous, each time an electron diffuses across the junction and creates a pair of ions. When an free electron leaves the n side it leaves a pentavalent atom behind with shortage of a electron. This pentavalent atom becomes a positive ion.
Similarly in p-side after the hole combined with free electron the trivalent atom becomes a negative ion. These ions remain fix across the junction and can’t move around like free electrons because of covalent bond.
Each pair of positive and negative ion is called dipole.
When the dipole increases the region near the junction the number of carriers decreases. This region is called depletion layer.
PN junction is called diode. Diode theory is explained in the next study.
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