Class 12 th semiconductor part 1
- 1. Class 12 th Physics Priyanka Jakhar Physics Lecturer GGIC Vijay Nagar Ghaziabad U.P. Semiconductor Part 1
- 2. Electronics Electronics is the combination of two words electrons and dynamics. Electronics is a science of flow and control of electrons in vacuum and solids. Modern electronics is a part of solid state Physics in which flow of electrons is channelised in solid before the introduction of modern semiconductor electronics. Electronics was based on vacuum tubes like that diode ,triode and tetrode valves in vacuum tubes . The flow of electron from heating cathode was directed by various electrodes like grid in vacuum the device based on these tubes but bulk consume high power less reliable and have limited life also they require vacuum for electrons flow. First type of semiconductor diode was introduced by Sir Jagdish Chandra Bose and Indian brain the solid state electronics was first come into light after the invention of first practical transistor by William Shockley and his team at Bell Labs in 1947. Semiconductor electronics devices were started referred as they are cheap small in size operate at low power produce almost no heat and reliable as compared to the vacuum tube .
- 4. Energy Band In Solid :-- According to Quantum Mechanical Laws, the energies of electrons in a free atom can not have arbitrary values but only some definite (quantized) values. However, if an atom belongs to a crystal, then the energy levels are modified. This modification is not appreciable in the case of energy levels of electrons in the inner shells (completely filled). But in the outermost shells, modification is appreciable because the electrons are shared by many neighbouring atoms. Due to influence of high electric field between the core of the atoms and the shared electrons, energy levels are split-up or spread out forming energy bands. Consider a single crystal of silicon having N atoms. Each atom can be associated with a lattice site. Electronic configuration of Si is 1s2, 2s2, 2p6,3s2, 3p2.(Atomic No. is 14) When all the atoms of the Crystal are well separated ( region A ) :-- There is no interaction between the atoms so each of an atom has same energy levels .This is represented by straight line in region A. In this region out of four an outermost electrons 2N electrons are in 2N impossible is 3s states all have same energy (and remaining 2N are in 6N possible 3p states ,all have same energy .Only 2N levels of 3p states are filled so some 3p e states are empty. There is a gap between 3s and 3p states.
- 5. O • • • • • • • • • • • • • • 2p6 2s2 1s2 3p2 3s2 Energy a b c d Inter atomic spacing (r) Conduction Band Forbidden Energy Gap Valence Band Ion core state Formation of Energy Bands in Solids: Region A RegionB RegionD RegionC
- 6. When interatomic distance reduce (region B ):-- The interaction between the atom becomes significant the 3s and 3p States which earlier have identical energies spread out and form energy band .The gap between the bands decrease .The different energy levels of e with continuous energy variation are called energy bands. When interatomic distance reduce further ( region C ) :--The bands merge with each other .No energy gap is visible since, the upper and lower energy states are mixed together . When interatomic distance reduce further (region D ):-- The energy band again split apart . As r becomes equal to 𝒓 𝟎 the actual interatomic distance of the Crystal the energy band of 8 N states split is apart into two 4N field and 4N empty states separated by a gap called energy gap or energy band gap 𝑬 𝒈 . The lower band which is completely filled by 4N valence electrons at absolute zero temperature is called Valence band . The upper band which is completely empty at absolute zero temperature is called conduction band . The collection of very closely spaced energy levels is called an energy band. Note: 1. The exact energy band picture depends on the relative orientation of atoms in a crystal. 2. If the bands in a solid are completely filled, the electrons are not permitted to move about, because there are no vacant energy levels available.
- 8. Valence band :-- This is the lower end of a material having valence electron . It may be completely filled or partially empty. The electron lying in the valence band are not usually taking part in electrical conduction as they cannot be agitated by external electric field. Conduction band :-- This is the upper band of the material with no electron at absolute zero. It may be completely empty or partially filled . The electrons lying in conduction band can be agitated by external electric field . They impart electrical conduction . The band gap or energy gap :-- The separation between the valence band and conduction is known as the energy gap of the two bands. This is the minimum energy required to move an electron from the valence band to conduction band. It has no electron . It is also called the forbidden gap . Conduction Band Forbidden Energy Gap Valence Band • • • • • • ≈6 eV
- 9. Conduction Band and Valence Band in Semiconductors:-- Valence Band:-- The energy band involving the energy levels of valence electrons is known as the valence band. It is the highest occupied energy band. When compared with insulators, the bandgap in semiconductors is smaller. It allows the electrons in the valence band to jump into the conduction band on receiving any external energy. Conduction Band:-- It is the lowest unoccupied band that includes the energy levels of positive (holes) or negative (free electrons) charge carriers. It has conducting electrons resulting in the flow of current. The conduction band possess high energy level and are generally empty. The conduction band in semiconductors accepts the electrons from the valence band.
- 10. The first possible energy band diagram shows that the conduction band is only partially filled with electrons. With a little extra energy the electrons can easily reach the empty energy levels above the filled ones and the conduction is possible. The second possible energy band diagram shows that the conduction band is overlapping with the valence band. This is because the lowest levels in the conduction band needs less energy than the highest levels in the valence band. The electrons in valence band overflow into conduction band and are free to move about in the crystal for conduction. Conductor / Metal Conduction Band • • • • • • Valence Band Partially filled Conduction Band • • • • • • Conduction Band Valence Band Forbidden Energy Gap • • • •• •
- 11. Semiconductor At absolute zero temperature, no electron has energy to jump from valence band to conduction band and hence the crystal is an insulator. At room temperature, some valence electrons gain energy more than the energy gap and move to conduction band to conduct even under the influence of a weak electric field. As an electron leaves the valence band, it leaves some energy level in band as unfilled. Such unfilled regions are termed as ‘holes’ in the valence band. They are mathematically taken as positive charge carriers. Any movement of this region is referred to a positive hole moving from one position to another. Conduction Band Valence Band Forbidden Energy Gap ≈1 eV Eg-Si = 1.1 eV EgGe= 0.74 eV • • • •• • Since 𝑬 𝒈 is small, therefore, the fraction is sizeable for semiconductors. Insulators Electrons, however heated, can not practically jump to conduction band from valence band due to a large energy gap. Therefore, conduction is not possible in insulators. Eg-Diamond = 7 eV Conduction Band Forbidden Energy Gap Valence Band • • • • • • ≈6 eV
- 12. Fermi Level :-- The highest energy level which can be occupied by electrons in a crystal, at absolute 0 temperature , is called Fermi Level. If the electrons get enough energy to go beyond this level, then conduction takes place. Fermi energy :-- The highest possessed of free electron in a material at absolute zero known as Fermi energy .The energy corresponding to fermi energy level is called Fermi energy. The electron cannot interact unless possessed the Fermi level of energy. This was named after physicist Enrico fermi, the creator of world first nuclear reactor . At absolute zero the Fermi energy and Fermi level are equal but different at other temperatures . The Fermi level lies between valence and conduction bands of the material .
- 13. Difference between Fermi energy and Fermi level:-- The Fermi energy is only defined at absolute zero, while the Fermi level is defined for any temperature. The Fermi energy is an energy difference (usually corresponding to a kinetic energy), whereas the Fermi level is a total energy level including kinetic energy and potential energy. Fermi Level in Semiconductors:-- Fermi level (denoted by EF) is present between the valence and conduction bands. It is the highest occupied molecular orbital at absolute zero. The charge carriers in this state have their own quantum states and generally do not interact with each other. When the temperature rises above absolute zero, these charge carriers will begin to occupy states above Fermi level. In a p-type semiconductor, there is an increase in the density of unfilled states. Thus, accommodating more electrons at the lower energy levels.
- 14. BASIS OF COMPARISON CONDUCTOR SEMICONDUCTOR INSULATOR Description Conductor is a material which permits the electric current or heat to pass through it. Semi-conductor is a material or substance that may act as a conductor as well as insulators under different conditions. Insulators are materials or substances which do not allow heat or electricity to pass through it. Conductivity Have very high conductivity, (107 Ʊ/m) thus they can conduct electrical current easily. Have intermediate conductivity (between 10-7 Ʊ/m to 10-13 Ʊ/m), thus they can act as insulator and conductor at different conditions. They have very low conductivity (1013 Ʊ/m), thus they do not allow current flow. Difference Between Conductor, Insulator And Semi-Conductor
- 15. Reason For Conductivity Or Otherwise The conduction in conductors is due to the free electrons in metal bonding. The conduction in semiconductors is due to the movement of electrons and holes. Non conduction is due to absence free electrons or holes. Resistance Vs Temperature The resistance of a conductor increases with an increase in temperature. The resistance of a semiconductor decreases with increase in temperature. Insulators have very high resistance but it decreases with temperature. Charge Carrier In conductors, electrons are charge carriers. In semiconductors, intrinsic charge carriers are holes and electrons. Insulators do not have any charge carriers. Valence Electrons They have only one valence electron in the outermost shell. They have four valence electrons in the outermost shell. They have eight valence electrons in the outermost shell.
- 16. Band Gap In conductors, there is low energy gap between the conduction and valence band of a conductor. It does not need extra energy for the conduction state. The band gap of semiconductor is greater than that of conductor but smaller than that of an insulator. The band gap in insulators is huge, which needs an enormous amount of energy like lightning to push electrons into the conduction band. Coefficient Of Resistivity Conductors have a positive coefficient of resistivity. Its resistance increases with increase in temperature. Semiconductors have a negative coefficient of resistivity. The coefficient of resistivity of an insulator is also negative but it has very huge resistance. Absolute Zero Some special conductors turn into superconductors when supercooled down while other have finite resistance. The semiconductors turn into insulators at absolute zero. The insulator resistance increases when cooled down to absolute zero.
- 17. Valence Band & Conduction Band Valence band and conduction band of conductors is completely filled. Valence band is partially empty and conduction band is partially filled. Valence band of insulators is completely filled and conduction band is completely empty. Temperature Increase Vs Number Of Carriers On increasing temperature, the number of carriers decreases. On increasing temperature, the number of carriers increases. On increasing temperature, number of carries increase. Overlapping Of Bands The valence and conduction bands are overlapped. Valence band and conduction band are separated energy gap of 1.1eV. Both the bands get divided by an energy gap of 6eV -10eV. Examples Gold, Copper, Silver, Aluminum etc. Silicon, Germanium, Selenium, Antimony, Boron etc. Rubber, Glass, Wood, Mica, Plastic, Paper etc.
- 18. Application The metals like iron, copper, aluminum etc that conduct electricity are made into wires and capable for carrying electric current. Semiconductors are used every day electronic devices such as cellphone, computer, solar panel etc as switches, energy converters, amplifiers etc. The insulators are used for protection against high voltages and prevention of electrical short between cables in circuits. Type Of Ionic Bonding Conductors have metallic type of bonding in their structure. Semiconductors have ionic bond and covalent type of bonding in their structure. Insulators have covalent type of bonding in their structure. Effect Of Electric Field On Current Flow In conductors, current flow under the influence of electric field takes place easily. In semiconductors, current flow does not take place under the influence of electric field. In insulators, current flows slowly under the influence of electric field.
- 20. Holes and Electrons in Semiconductors:-- 1. Holes and electrons are the types of charge carriers accountable for the flow of current in semiconductors. 2. Holes (valence electrons) are the positively charged electric charge carrier whereas electrons are the negatively charged particles. 3. Both electrons and holes are equal in magnitude but opposite in polarity. Mobility of Electrons and Holes:-- 1. In a semiconductor, the mobility of electrons is higher than that of the holes. 2. It is mainly because of their different band structures and scattering mechanisms. 3. Electrons travel in the conduction band whereas holes travel in the valence band. 4. When an electric field is applied, holes cannot move as freely as electrons due to their restricted movment. 5. The elevation of electrons from their inner shells to higher shells results in the creation of holes in semiconductors.