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device chemistry.pptx
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- 2. Introduction Semiconductors arematerials whose electronic properties are intermediate between those of Metals and Insulators. They have conductivities in the range of 10^-4 to 10^+4 ohm^−1 m^−1. Their conductivity and other properties can be altered with the introduction of impurities, called Doping. It has negative temperature co-efficient of resistance which means the resistance of a semiconductor decreases with the increase in temperature and vice-versa. When a suitable metallic impurity is added to a semiconductor, its current conducting property change appreciably.
- 3. Crystal Lattice Structure The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice. The atoms link together with one another sharing their valence electrons. These links are called covalent bonds.
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- 5. Majority and MinorityCarriers N-type material has a large number of free electrons and a small number of holes in which case free electrons are considered majority carriers and holes are the minority carriers . P-type material has a large number of holes and a small number of electrons in which case holes are considered majority carriers and free electrons are the minority carriers.
- 6. Majority and MinorityCarriers
- 7. What Are Valence& Conduction Bands? Semiconductors are also classified by their bands: a fully occupied (with electrons) valence band, and an unoccupied conduction band. A valence band contains an electron orbital where electrons can jump off and move to the conduction band when they become excited. A conduction band is defined as that energy band that consists of free electrons that are responsible for conduction.
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- 9. FERMI ENERGY &FERMI LEVEL Fermi Level is the maximum energy level that an electron could reach at zero temperature. The Fermi level lies between the valence band and conduction band because at absolute zero temperature, the electrons are all in the lowest energy state. Fermi Energy is the maximum kinetic energy an electron can attain at 0K.
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- 11. What is anIntrinsic Semiconductor? A semiconductor material in its pure form is known as an intrinsic semiconductor. The intrinsic semiconductors are chemically pure, i.e. they are free from impurities. The number of free electrons is equal to the number of holes in the intrinsic semiconductor. The common examples of the intrinsic semiconductors are germanium (Ge) and silicon (Si).
- 12. What is anExtrinsic Semiconductor? A semiconductor to which an impurity at a controlled rate is added to make it conductive is known as an extrinsic semiconductor. The process by which an impurity is added to a semiconductor is known as Doping. The purpose of adding impurity in the semiconductor crystal is to increase the number of free electrons or holes to make it conductive. If a Pentavalent impurity, having five valence electrons is added to a pure semiconductor a large number of free electrons will exist. If a trivalent impurity having three valence electrons is added, a large number of holes will exist in the semiconductor. Depending upon the type of impurity added the extrinsic semiconductor may be classified as n type semiconductor and p type semiconductor.
- 13. N-type semiconductor AnN-type semiconductor is formed when a small amount of pentavalent impurity is added to a pure germanium or silicon crystal. The pentavalent atom has 5 valance electrons, but only 4 form covalent bonds with the neighbouring atoms. The 5th electron finds no place in the covalent bonding so becomes free. The majority carrier in N-type semiconductor are free electrons. Holes are also present in the N-type semiconductor as Minority Carriers. The pentavalent impurity atom is called donor because each donates a free electron.
- 14. P-type semiconductor Inp-type semiconductors, holes are the majority carriers, and electrons are the minority carriers. A p-type semiconductor is formed by adding a III group element(such as B, Al) as a doping element. The impurity atom is surrounded by four silicon atoms. It provides the atoms to make only three covalent bonds as it has only three valence electrons. The vacancy that exists in the fourth bond constitutes the hole. Trivalent impurities are called the acceptor impurities Because each hole it contributes can accept a free electron during recombination.
- 15. Law of MassAction The law of Mass action asserts that at a constant temperature, the product of the number of electrons in the conduction band and the number of holes in the valence band remains constant. It is expressed Mathematically as ni 2 =nxp Where ni is the intrinsic carrier concentration n is number of electrons in the conduction band p is number of holes in the valence band.
- 16. DRIFT CURRENT Driftcurrent arises from the movement of carriers in response to an applied electric field. Positive carriers (holes) move in the same direction as the electric field while negative carriers (electrons) move in the opposite direction. The drift velocity increases with increasing electric field and contributes to the mobility μ of the carriers.
- 17. DIFFUSION CURRENT Themovement of charge carriers from higher concentration to lower concentration generates diffusion current. This occurs when a semiconductor is doped non-uniformly then there is a non- uniform distribution of carriers or a concentration gradient. The current moves to the direction where there is initially a higher concentration of electrons or a lower concentration of holes.
- 18. Effect of temperatureon semiconductor parameters Intrinsic concentration (ni) : The number of holes or electrons present in an intrinsic semiconductor at any temperature is called intrinsic carrier concentration (ni). In N type semiconductor, the number of free electrons (n) does not change appreciably with the increase in temperature, but number of holes (p) increases. In P type semiconductor, the number of free electrons (n) increases with the increase in temperature, but number of holes remains constant.
- 19. Mobility (µ) : The mobility means the movement of charge carriers. The mobility of intrinsic semiconductor decreases with increase in temperature. Due to the higher collision rate,their effective mobility of the charge carriers decreases. Conductivity (σ) : The conductivity of an intrinsic semiconductor depends upon the number of hole electron pairs and mobility. The conductivity of an intrinsic semiconductor increases with increase in temperature. Forbidden energy gap (EG) : The energy required to break a covalent bond in a semiconductor is known as energy gap. The forbidden energy gap decreases with the increase in temperature.
- 20. Semiconductors Pn Junction : The P-N junction is formed between the p-type and the n-type semiconductors. The p-side or the positive side of the semiconductor has an excess of holes, and the n-side or the negative side has an excess of electrons. 20
- 21. Semiconductors Depletion Layer: Thefree electrons of n region begin to diffuse across the junction into the p region where they combine with the holes near the junction. The p region also loses holes as the electrons and holes combine. The result is that there are layers of positive charge (pentavalent ions) and negative charge (trivalent ions). These two layers form depletion layer or depletion region This layer is free of charge carriers 21
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- 23. BJT(BIPOLAR JUNCTION TRANSISTOR) BJT stands for Bipolar Junction Transistor which is a type of transistor that uses both electrons and holes as charge carriers. It’s a 3 terminal device. Used in amplification of weak signals and switching operations. Plays a vital role in both Analog and digital Electronics.
- 24. History of itsInvention. W. Shockley, J. Barden, and W. Brattain invented the first transistor at Bell Labs in 1948. The Inventors received the nobel Prize in physics in 1956.
- 25. Physical structure ofBJT. Two types of BJT 1.NPN and 2.PNP.
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- 27. Working of theTransistor The emitter-base junction of a transistor is forward biased whereas collector base junction is reverse biased. At BE junction, the potential barrier decreases with forward bias. So, electron start flowing from emitter terminal to base terminal. As the base is lightly doped terminal, so very little number of electrons from emitter terminal combine with holes in base terminal. Due to combination of electrons and holes, current from base terminal will start flowing known as Base current (ib). While the remaining electrons will flow from the reverse bias collector junction known as Collector current (ic). The total emitter current will be the combination of base current & collector current given by: ie = ib+ic
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- 33. Applications of BJT Used as an Amplifiers. Converters. Temperatures sensors. Electronic switches.
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