Class 12 Physics Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits
Electronic devices are characterised by the ability to control the flow of electron. Such devices are made of semiconductors. To understand what are semiconductors, one must know about conductors and insulators. Material having high conductivity and low resistivity are conductors. However, material with low conductivity and high resistivity are insulators. There is another material whose resistivity or conductivity intermediate to conductors and insulators. Such material are called semiconductors.
All metals are conductors. Material like rubber, plastic and putty are insulators. Materials such as Silicon, Germanium, CdS, GaAs are semiconductors.
History of Semiconductors
Before 1948 most of electronic devices are made up of vacuum tubes or valves. For example, a diode valve have two electrodes- anode and cathode, triode valve have three electrodes- plate, cathode and grid. Similarly, tetrode and pentode valve have 4 and 5 electrodes.
In vacuum tube, heated cathode supply electrons. The flow of electrons can be controlled by varying voltage between its different electrodes. If air is present then electrons may collide with air molecules and so they may lose their energy. Therefore vacuum is required in the inter electrode space. Since electrons flow in only one direction, therefore such devices are referred as valves. The major drawback to use vacuum tubes is that these are very bulky, operates at high voltages, consumes high power with limited life and low reliability. Semiconductor devices such as diodes and transistors provide an efficient solution for all these drawback. Semiconductors are of small size, operate at low voltages, consume small power, they have long life and are durable.
Classification of Semiconductors
Semiconductors are classified into 2 types Elemental semiconductor or compound semiconductor. Elemental semiconductors like silicon and germanium are most extensively used. However, compound semiconductors exist in compound form that can be Inorganic, Organic or in the form of organic polymers. Inorganic semiconductors are CdS, GaAs, CdSe, InP, etc. organic semiconductor includes anthracene, doped phthalocyanines, etc. Some organic polymers behaving as semiconductors are polypyrrole, polyaniline, polythiophene, etc.
Band theory of solids
Bands in solids are separated by energy gap Eg. There are many bands in solids but out of them valence band, conduction band and forbidden energy gap are most important.
- Valence band: Under normal condition, all the valence electrons are engaged in covalent bond with other atom and therefore these electrons are not free.
The energy band formed due to merging of energy levels associated with energy levels of valence electron is called valence band.
In valence band, electrons cannot move freely. Therefore electrons in valence band do not contribute in electric current.
- Conduction band:
Energy band formed due to merging of energy levels associated with free electrons is called conduction band.
In conduction band, electrons are free to move. Conduction band is always at the top of valence band.
- Forbidden gap:
Forbidden gap is the energy gap that separates the conduction band from valence band. It is also known as band gap or forbidden band gap.
It is denoted by Eg. To bring the electron from valence band to conduction band, energy given to the electron must be greater than the energy associated with forbidden gap.
On the basis of energy band concept, materials can be classified as
- Conductors: Some solids have partial overlap of conduction band over the valence band, hence, there is no band gap between both the bands. Electrons in the valence band can easily move to conduction band. Thus such solids show electrical conductivity and are called conductors. All metals are good conductors.
- Insulators: In insulators band gap is very large, thus, electron can’t reach the conduction band from valence band. Therefore, such solids could not show electrical conductivity. For example, rubber and putty are insulators.
- Semiconductors: In some solids, band gap is very narrow. At low temperatures, conduction band is empty thus , no conduction is possible. For such condition these solids behave like insulators. At room temperature some electrons gain energy to cross the band gap and jump into conduction band. So electric conduction is possible at room temperature. Such solids are called semiconductors. Germanium, silicon are the example of semiconductors.
Sample of semiconductor in its purest form is called intrinsic semiconductor.
The impurity content or atoms of other elements are very very small i.e. one part in 100 million part of semiconductor.
Crystal structure of intrinsic semiconductor
Take an example of silicon. The silicon atom has four electron in the outermost shell and can accomodate more four electrons in order to achieve noble gas configuration. Thus silicon atom forms covalent bond with other four silicon atom, thereby forming three dimensional uniform pattern called crystal. The crystal structure of silicon have regular pattern arranged in the form of tetrahedron with each atom at the vertex.
Electrons and holes in intrinsic semiconductor
At low temperatures, all the shared electron remain intact and no electron is free. Thus semiconductor behaves like an insulator. As the temperature increases, some electrons absorb thermal energy, thereby, breaking covalent bond, thus becoming free. While leaving the place, electron of negative charge creates vacancy of positive charge. This vacancy of positive charge is called hole. In intrinsic semiconductor a free electron and hole are always generated in pair. Such a generation of electron hole pair due to thermal energy is called thermal generation. Thus number of electron is equal to number of holes, ne = nh = ni. Here, ni is called as intrinsic carrier concentration.
Conduction in intrinsic semiconductor
With increase in temperature covalent bond breaks to form electron hole pair. However, the free electron move towards the positive terminal of battery thus, allowing the current to flow in the circuit. Neighbouring electron of the hole tries to occupy the hole, thereby leaving a hole in that place. Process of shifting of hole continues towards the negative terminal of battery. Thus the total current in the intrinsic semiconductor is due to electron and hole I = Ie + Ih. Intrinsic semiconductors are not suitable for electronic circuits, this is because of poor conductivity at room temperature and insufficient conductivity at applied voltage.
To improve the conductivity there is need to change properties of semiconductor. One way to do this, is adding other material to crystal structure of intrinsic semiconductors. This process is called doping. The material added to pure intrinsic semiconductor is called as dopant.
Doped semiconductor material is called extrinsic semiconductor.
Conditions for proper doping
- The dopant must not distort the crystal structure
- Dopant should take position of semiconductor atom in the crystal structure
- The size of dopant atom must be comparable to the crystal atom
- The concentration of impurity should be small.
Extrinsic semiconductor are of p type or n type.
N type semiconductor are those in which small amount of pentavalent impurity is added. Example of such impurities are Arsenic, Phosphorus, Antimony, etc. When small amount of trivalent impurity is added to semiconductor a p type semiconductor is formed. Example of such impurities are Boron, Aluminium, Gallium, etc.
Keywords: p type semiconductor, n type semiconductor, Intrinsic semiconductor, Extrinsic semiconductor