Semiconductor
Valence Band: The band of energy level occupied by the valence electron It may be occupied by: (a) Partially filled with electrons e.g. Na (3s1), Al (3s2,3p1) (b) Completely filled with electrons e.g. Be(1s2,2s2), Mg(1s2,2s2,2p6,3s2 Conduction Band: The next higher band above the valence band is called conduction band & the electron occupying this band is called conduction electron. Electrons in the conduction band are free electrons & once the electrons enter the conduction band, it moves freely. The conduction band may be empty or partially filled with electron Energy gap: The gap between conduction band & valence band. No electron exist in the energy gap Conductor Semiconductor Insulator
Empty conduction band Conduction band Overlap Valence band Band energy Band energy
Empty conduction band
Large energy gap
Small energy gap
Band energy
Filled valence band
Valence band
Conductors: Conductors allow electricity to flow easily. Metals are good conductors because the outermost electrons in the atoms are so loosely bound that they move in the space between the atoms of that material by the influence of room-temperature heat energy. E.g: Silver, copper, gold, aluminum, iron, steel brass, bronze, dirty water A solid is conductor if it has either partially filled valence band (Li, Na, Al) or a filled valence band (Be, Mg) overlapping with the conduction band. No energy gap Insulators: Insulators don't allow electricity to flow easily The atoms electrons are tightly bound so the electron cannot move around. E.g: Glass, rubber, oil, fiberglass, porcelain, ceramic, wood plastic, diamond, pure water Solids in which valence band electrons is tightly bound to their parent nuclei & requires very large electric field to remove electrons to conduction band. These solids material possess: a) full valence band c)completely empty conduction band b) large energy gap(15 eV)
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Semiconductors: are materials which are neither conductors or insulators, having conductivities intermediate to those of conductors like copper and insulators like wood or plastic. The semiconductors have a) Almost filled valence band b) Narrow energy gap The reason semiconductors are important is that with some engineering they can sometimes both conduct and insulate depending on their connections. Thus they serve as the basis for switching and amplification, the fundamental actions of computer elements A semiconductor conducts electricity poorly at room temperature, but has increasing conductivity at higher temperatures Group IV are elemental semiconductors like Si, Ge & compound semiconductors can be formed by combining Group III (Ga , In )& V(P, As, Sb) or Group II(Cd, Pb) & VI(S, Se, Te). E.g gallium arsenite (GaAs) is an III & V group combination Carbon, silicon and germanium are a semiconductor has a unique property in their electron structure -- each has four electrons in its outer orbital. This allows them to form crystals lattice. The four electrons form covalent bonds with four neighbouring atom creating a crystal lattice. Germanium electron
Ge e
c) Almost empty conduction band
G e Ge G e Ge G e
Ge ee
G e Ge G e Ge G e
G e Ge G e e Ge G e
Ge
Covalent bond Covalent
Structure of the semiconductor Doping: Adding a small amount of the impurity in the semiconductor crystal 1. Adding group III (trivalent impurity)element into Group IV element 2. Adding group V (pentavalent impurity) element into Group IV element By doping pure silicon with Group V elements such as phosphorus, extra valence electrons are added that become unbonded from individual atoms and allow the compound to be an electrically conductive n-type semiconductor. Group V element is said to behave as an electron donor Doping with Group III elements, which are missing the fourth valence electron, creates "broken bonds" (holes) in the silicon lattice that is free to move. The result is an electrically conductive p-type semiconductor. Group III element acts as an acceptor.
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Difference between N-type Semiconductor and P-type semiconductor N type semiconductor 1 2 3 4 Semiconductor is formed by doping Si or Ge with pentavalent impurity(P, As) Semiconductor has an excess of electrons Conduction in such a semiconductor is due to movement of free-electrons Semiconductor conduction is due to movement of negative electric charge P type semiconductor Semiconductor is formed by doping Si or Ge with a trivalent impurity (Ga, Al, B ) Semiconductor has an excess of electron-holes Conduction in such a semiconductor is due to movement of electron-holes Semiconductor conduction is due to movement of positive electric charge
N-type semiconductor
P-type semiconductor
Excess electron
Ge e
G e G e
Ge
Ge ee
G e G e G e
Ge
Pentavalent impurity
G e Ge e G e
Ge
Ge e
G e G e
Ge
Ge ee
G e G e G e
Ge
G e Ge e G e
Ge
Holes
Trivalent impurity
G e
Ge
G e
Ge
G e
Ge
G e
Ge
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MOLE, MOLARITY, AVOGADRO NUMBER Mole: A mole is simply a unit of measurement or a unit of the amount of substances. The atomic masses or the molecular masses when expressed in grams is called mole. Units: moles 1mole of H atom is = 1 gm 1 mole of H2molecule is = 2 gm 1 mole of N atom is = 14 gm 1 mole of N2 molecule is = 28 gm 1 mole of NaCl atom is = 22.99+35.45=58.44 gm 1 mole of CO2 molecule is = 12+16X 2=44 gm
No. of Moles = Example 1. 2. 3. 4. 5.
Total mass At. mass or molecular mass
How many moles are present in 36 grams of carbon? (At.mass of carbon= 12 amu) What will be the mass of 10 moles of carbon dioxide in grams? Calculate mass of 4 mole of sodium chloride? Calculate the number of moles in 10 grams of sodium hydroxide. What mass of carbon dioxide is produced by the complete combustion of 100gm of the hydrocarbon pentane C5H12?
Avogadro's number: The number of particles (atoms, molecule and ions) in 1mole of substance is called Avogadro's number; its value is 6.023 x 1023 1mole of H atom = 6.023 x 1023 atoms=1gm 1mole of O2 molecule is = 6.023x 1023 molecules = 32 gm 1mole of H2Omolecule = 6.023 x 1023molecules = 18gm Similarly we can say 1 mole of Na+ ions=6.023 x 1023ions 1 mole of electron=6.023 x 1023 electron Examples 1. How many atoms of hydrogen are present in 0.5moles of hydrogen atoms? 2. A silicon chip is used as an integrated circuit of micrometer has a mass of 5.68 mg. How many Si atoms are present in this chip? 3. How many moles of O2 are in 0.85 g of baking soda in NaHCO3
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Molarity is the number of moles of a solute dissolved in a liter of solution Molarity = No. of moles of solute Volume of solution (liter) 1liter = 1dm3 1 ml = 1cm3 1liter = 1000 ml 1dm3 = 1000cm3
Example 1. Calculate the Molarity of 25 g of KBr dissolved in 750 mL 2. 80 g of glucose (C6H12O6) is dissolved in enough water to make 1 L of solution. What is its Molarity? 3. How many grams of KMnO4 are needed to make 500 mL of a 0.2 M solution? 4. 10 g of acetic acid (CH3COOH) is dissolved in 500 mL of solution. What is Molarity? 5. How many mL of solution will result when 15 g of H2SO4 is dissolved to make a 0.2 M solution?
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