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Priyanka ChSMBS8754584534Priyanka.c@vit.ac.in

The fear of the LORDisthe beginning of knowledge:butfools despise wisdom and instruction. Proverbs 1:7UNIT I7/16/2014UNIT I11Syllabus

Review Semiconductor devices Two terminal devices BJT, JFET, MOSFET Four terminal devices SCR, DIAC, TRIAC Photo devices: Photo diode Photo transition LED, LCD.

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BJT

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Three terminal Devices7/16/2014UNIT I5Classification of materials:Conductors: have low resistance - which allows electrical current flowInsulators: have high resistance - which suppresses electrical current flowSemiconductors: can allow and/or suppress electrical current flow

7/16/2014UNIT I6Review of Semiconductor materialsHistory of Semiconductor materials:

Semiconductor materials are of two typesSingle crystal (Eg. Germanium-Ge, Silicon-Si have a repetitive crystal structure)Compound (Eg. Gallium Arsenide-GaAs, Cadmium Sulfide-CdS , Gallium Nitrade- GaN )The three semiconductor most frequently used for the electronic devices are Ge, Si, GaAs.

7/16/2014UNIT I7Germanium(Ge):Was discovered in the early years that diodes and transistor were constructed using germanium as base material.Was most exclusively usedWas relatively easy to find and was available in fairly large quantities.Is sensitive to the change in the temperature.Limited number of areas of application.First Ge diode was discovered in the year 1939.First Ge transistor was discovered in the year 1947.

7/16/2014UNIT I8Silicon (Si):Has improved temperature sensitivities.(Less temperature sensitive than Ge)Is one of the most abundant material on the earth.Cheaper to manufacture.Used for wide range of applications.The first Si transistor introduced in the year 1954.Gallium Arsenide (GaAs):Meets the need of higher speed. The speed of operation is five time greater than silicon.Is more expensive.Used as a base material for high speed ,very large scale integrated circuit designsThe development of the first GaAs transistor was in the early 1970s.7/16/2014UNIT I9Review of Semiconductor materialsSemiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator.Atom is composed of three basic particles a) The Electrons b) The Protons c) The NeutronAtom is the smallest unit of the element that contains all its chemical properties.

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7/16/2014UNIT I12The bonding of atoms, strengthened by the sharing of electrons is called covalent bond.The external natural causes like light energy in the form of photons and the thermal energy (heat) from the surrounding medium causes to break the covalent bond leading to a free electron.The term intrinsic is applied to any semiconductor material that has been carefully refined to reduce the number of impurities to a very low level.Free electrons in a material due to only external causes are referred to as intrinsic carriers.Ge has the highest number of intrinsic carriers per cubic centimeter, GaAs the lowest.Relative mobility is also a factor to be concerned while choosing the semiconductor material.7/16/2014UNIT I13The free carriers in GaAs have more than five times the mobility of free carriers in Si.The addition of one part of impurity (of proper type)per million in a wafer of silicon material can change that material from a relatively poor conductor to a good conductor of electricity.Conductors have a positive temperature coefficient- resistance increases with increase in heat.Semiconductors materials have negative temperature coffecient-shows increased levels of conductivity with application of heat.7/16/2014UNIT I14Electrons that are in orbit farther from the nucleus have higher energy and are less tightly bound to the atom than those close to the nucleus.The valence electrons contribute to chemical reactions and bonding within the structure of a material.The process of losing a valance electron is known as ionization.Atoms that have four valence electrons are called tetravalent, and those with five are called pentavalent.The term valence is used to indicate that the potential required to remove any one of these electrons from the atomic structure and is significantly lower than that required for any other electron in the structure.

7/16/2014UNIT I15ConductorsGood conductors have low resistance so electrons flow through them with ease.Best element conductors include:Copper, silver, gold, aluminum, & nickel Alloys are also good conductors:Brass & steel Good conductors can also be liquid:Salt water

7/16/2014UNIT I16The atomic structure of good conductors usually includes only one electron in their outer shell. It is called a valence electron. It is easily striped from the atom, producing current flow.

Copper Atom7/16/2014UNIT I17InsulatorsInsulators have a high resistance so current does not flow in them.Good insulators include:Glass, ceramic, plastics, & woodMost insulators are compounds of several elements. The atoms are tightly bound to one another so electrons are difficult to strip away for current flow7/16/2014UNIT I18SemiconductorSemiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between.Common elements such as carbon, silicon, and germanium are semiconductors.Silicon is the best and most widely used semiconductor7/16/2014UNIT I19The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

7/16/2014UNIT I20The 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 outer electrons. These links are called covalent bonds

7/16/2014UNIT I21If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow.Good insulating semiconductor material is referred to as intrinsic.Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow.Silicon in this form is a great insulator.Semiconductor material is often used as an insulator.

7/16/2014UNIT I22DopingTo make the semiconductor conduct electricity, other atoms called impurities must be added.Impurities are different elements. This process is called doping.Doping increases the number of current carriers (electrons /holes).To increase the number of conduction band electrons in intrinsic silicon, pentavalent impurity atoms are added.A semiconductor material that has been subjected to the doping process is called an extrinsic material.

7/16/2014UNIT I23Semiconductors as ConductorsAn impurity, or element like arsenic (As),phosphorus (P), bismuth(Bi) has 5 valence electrons.Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms. The one electron left over for each arsenic atom becomes available to conduct current flow.

7/16/2014UNIT I24If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely.If pentavalent impurities are added N-type semiconductor

7/16/2014UNIT I25You can also dope a semiconductor material with an atom such as boron(B), indium(In), gallium(Ga) that has only 3 valence electrons.The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond.This place where a fourth electron should be is referred to as a hole. The hole assumes a positive charge so it can attract electrons from some other source.Holes become a type of current carrier like the electron to support current flowIn n-type material the electron is called the majority carrier and the hole the minority carrier.In p-type material the hole is called the majority carrier and the electron the minority carrier.7/16/2014UNIT I26If trivalent impurity atoms are added then the semiconductor is called P-type semiconductor

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DiodeIf a block of silicon is doped with a trivalent impurity and the other part with a pentavalent impurity, a boundary called pn- junction is formed between the resulting p-type and n-type portion and a basic diode is formed.The boundary is also called as depletion region, potential barrier.A diode is a device that conducts current in only one direction.Current under no bias condition is zero.The bias refers to the application of an external voltage across the two terminal s of the device to extract a response.7/16/2014UNIT I28

7/16/2014UNIT I29Free electrons on the n-side and free holes on the p-side can initially diffuse across the junction. Uncovered charges are left in the neighborhood of the junction. This region is depleted of mobile carriers and is called the depletion region (thickness 0.5 1.0 m).

7/16/2014UNIT I30The diffusion of electrons and holes stop due to the barrier potential (potential difference across the junction) reaching some critical value. The barrier potential (or the contact potential) depends on the type of semiconductor, temperature and doping densities. At room temperature, typical values of barrier potential difference are: Ge ~ 0.2 0.4 V Si ~ 0.6 0.8 V

7/16/2014UNIT I31FORWARD BIASED PN JUNCTION When an external voltage is applied to the P-N junction making the P side positive with respect to the N side the diode is said to be forward biased (F.B). The barrier potential difference is decreased by the external applied voltage. The depletion band narrows which urges majority carriers to flow across the junction.A F.B. diode has a very low resistance.7/16/2014UNIT I32

7/16/2014UNIT I33The depletion region shrinks slightly in width. With this shrinking the energy required for charge carriers to cross the depletion region decreases exponentially. Therefore, as the applied voltage increases, current starts to flow across the junction. The barrier potential of the diode is the voltage at which appreciable current starts to flow through the diode. The barrier potential varies for different materials.7/16/2014UNIT I34REVERSE BIAS PN JUNCTIONWhen an external voltage is applied to the PN junction making the P side negative with respect to the N side the diode is said to be Reverse Biased (R.B.).The barrier Potential difference increases. The depletion band widens preventing the movement of majority carriers across the junction.A R.B. diode has a very high resistance.

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7/16/2014UNIT I36Only thermally generated minority carriers are urged across the p-n junction. Therefore the magnitude of the reverse saturation current (or reverse leakage current) depends on the temperature of the semiconductor. When the PN junction is reversed biased the width of the depletion layer increases, however if the reverse voltage gets too large a phenomenon known as diode breakdown occurs.

7/16/2014UNIT I37The reverse current is so small that it can be neglected. however , if the external reverse bias voltage is increased to a value called the breakdown voltage , the reverse current will drastically increase.The high reverse bias voltage imparts energy to the free minority electrons so that as they speed through the p region, they collide with atoms with enough energy to knock valence electrons out of orbit into the conduction band.7/16/2014UNIT I38The multiplication of conduction electrons is called avalanche and results in a very high reverse current that can damage the diode because of excessive heat dissipation.7/16/2014UNIT I39VI CHARACTERISTICS

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