WHAT ARE THE TYPES OF SEMI-CONDUCTORS
Types of Semiconductors
Semiconductors can be classified as:
1. Intrinsic Semiconductor
2. Extrinsic Semiconductor
An intrinsic type of semiconductor material is made to be very pure chemically. It is made up of only a single type of element.
Conduction Mechanism in Intrinsic Semiconductors
Conduction Mechanism in Case of Intrinsic Semiconductors (a) In absence of electric field (b) In presence of electric Field
Germanium (Ge) and Silicon (Si) are the most common type of intrinsic semiconductor elements. They have four valence electrons (tetravalent). They are bound to the atom by covalent bond at absolute zero temperature.
When the temperature rises, due to collisions, few electrons are unbounded and become free to move through the lattice, thus creating an absence in its original position (hole). These free electrons and holes contribute to the conduction of electricity in the semiconductor. The negative and positive charge carriers are equal in number.
The thermal energy is capable of ionizing a few atoms in the lattice, and hence their conductivity is less.
Lattice of Pure Silicon Semiconductor at Different Temperatures
At absolute zero kelvin temperature: At this temperature, the covalent bonds are very strong and there are no free electrons and the semiconductor behaves as a perfect insulator.
Above absolute temperature: With the increase in temperature few valence electrons jump into the conduction band and hence it behaves like a poor conductor.
Energy Band Diagram of Intrinsic Semiconductor
The energy band diagram of an intrinsic semiconductor is shown below:
(a) Intrinsic Semiconductor at T = 0 Kelvin, behaves like an insulator (b) At t>0, four thermally generated electron pairs
In intrinsic semiconductors, current flows due to the motion of free electrons as well as holes. The total current is the sum of the electron current Ie due to thermally generated electrons and the hole current (Ih)
Total Current (I) = Ie + Ih
For an intrinsic semiconductor, at finite temperature, the probability of electrons to exist in conduction band decreases exponentially with increasing bandgap (Eg)
n = n0e-Eg/2.Kb.T
Eg = Energy bandgap
Kb = Boltzmann’s constants
The conductivity of semiconductors can be greatly improved by introducing a small number of suitable replacement atoms called IMPURITIES. The process of adding impurity atoms to the pure semiconductor is called DOPING. Usually, only 1 atom in 107 is replaced by a dopant atom in the doped semiconductor. An extrinsic semiconductor can be further classified into:
1. N-type Semiconductor
2. P-type Semiconductor
Classification of Extrinsic Semiconductor:
Mainly due to electrons
I = Ih and nh >> ne
Majority – Electrons and Minority – Holes
When a pure semiconductor (Silicon or Germanium) is doped by pentavalent impurity (P, As, Sb, Bi) then, four electrons out of five valence electrons bonds with the four electrons of Ge or Si.
The fifth electron of the dopant is set free. Thus the impurity atom donates a free electron for conduction in the lattice and is called “Donar“.
Since the number of free electron increases by the addition of an impurity, the negative charge carriers increase. Hence it is called n-type semiconductor.
Crystal as a whole is neutral, but the donor atom becomes an immobile positive ion. As conduction is due to a large number of free electrons, the electrons in the n-type semiconductor are the MAJORITY CARRIERS and holes are the MINORITY CARRIERS.
Mainly due to holes
I = Ih and nh >> ne
Majority – Holes and Minority – Electrons
When a pure semiconductor is doped with a trivalent impurity (B, Al, In, Ga ) then, the three valence electrons of the impurity bonds with three of the four valence electrons of the semiconductor.
This leaves an absence of electron (hole) in the impurity. These impurity atoms which are ready to accept bonded electrons are called “Acceptors“.
With the increase in the number of impurities, holes (the positive charge carriers) are increased. Hence, it is called p-type semiconductor.
Crystal as a whole is neutral, but the acceptors become an immobile negative ion. As conduction is due to a large number of holes, the holes in the p-type semiconductor are MAJORITY CARRIERS and electrons are MINORITY CARRIERS.
Difference between Intrinsic and Extrinsic Semiconductors
- Pure semiconductors
- Density of electrons is equal to density of holes
- Electrical conductivity is low
- Dependence on temperature only
- No impurities
- Impure semiconductors
- Density of electrons is not equal to density of holes
- Electrical conductivity is high
- It is doped with trivalent, pentavalent impurity atoms
Applications of Semiconductors
Let us now understand the uses of semiconductors in daily life. Semiconductors are used in almost all electronic devices. Without them, our life would be much different.
Their reliability, compactness, low cost and controlled conduction of electricity make them ideal to be used for various purposes in a wide range of components and devices. transistors, diodes, photosensors, microcontrollers, integrated chips and much more are made up of semiconductors.
Uses of Semiconductors in Everyday life
Temperature sensors are made with semiconductor devices.
They are used in 3D printing machines
Used in microchips and self-driving cars
Used in calculators, solar plates, computers and other electronic devices.
Transistor and MOSFET used as a switch in Electrical Circuits are manufactured using the semiconductors.
Industrial Uses of Semiconductors
The physical and chemical properties of semiconductors make them capable of designing technological wonders like microchips, transistors, LEDs, solar cells, etc.
The microprocessor used for controlling the operation of space vehicles, trains, robots, etc is made up of transistors and other controlling devices which are manufactured by semiconductor materials.
Importance of Semiconductors
Here we have discussed some advantages of semiconductors which makes them highly useful everywhere.
They are highly portable due to the smaller size
They require less input power
Semiconductor devices are shockproof
They have a longer lifespan
They are noise-free while operating.