Lecture 2Dr. Abdul Majid SandhuDr. Abdul Majid Sandhu, Department of Physics, University of Gujrat.

N- and P-type Semiconductors

Atomic StructureElements are made of atoms110 Elements; each has an atomic structure Bohr ModelAtoms have planetary structure Atoms are made of nucleus (Protons (+) & Neutrons) and electrons (-)

110 th element is called Darmstadtium (Ds)

Atomic StructureElectrons go around the nucleus in their orbits at discrete distances Each orbit has some energy level The closer the orbit to the nucleus the less energy it has Electrons on the same shell have similar energy levelValence shell is the outmost shell. Its electrons are ready to be freed Number of electrons (Ne) on each shell (n)

First shell has 2 electronsSecond shell has 8 electrons (not shown here)

Ne = 2n2

Valence ShellAtoms are made of valence shell and coreCore includes nucleus and other inner shellsFor a Carbon atom the atomic number is 6Core charge = 6 P + 2 e = (+6)+(-2)=(+4) Remember the first shell has 2 electrons

ElementsBasic categoriesConductorsExamples: Copper, silverOne valence electron , the e can easily be freedInsulatorsValence electrons are tightly bounded to the atomSemiconductorsSilicon, germanium (single element) Gallium arsenide, indium phosphide (compounds)They can act as conductors or insulators

Conduction band is where the electron leaves the valence shell and becomes freeValence band is where the outmost shell isAlways free electronsFree electrons

What is a Semiconductor?Low resistivity => conductorHigh resistivity => insulatorIntermediate resistivity => semiconductorconductivity lies between that of conductors and insulatorsgenerally crystalline in structure for IC devices

Semiconductor MaterialsGallium(Ga)Phosphorus(P)

Semiconductor typesA material whose properties are such that it is not quite a conductor, not quite an insulatorSome common semiconductorselementalSi - Silicon (most common)Ge - GermaniumcompoundGaAs - Gallium arsenideGaP - Gallium phosphideAlAs - Aluminum arsenideAlP - Aluminum phosphideInP - Indium Phosphide

Semiconductor Crystalline StructureSemiconductors have a regular crystalline structurefor monocrystal, extends through entire structurefor polycrystal, structure is interrupted at irregular boundariesMonocrystal has uniform 3-dimensional structureAtoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium

Semiconductors Remember the further away from the nucleus the less energy is required to free the electrons Germanium is less stableLess energy is required to make the electron to jump to the conduction band

When atoms combine to form a solid, they arrange themselves in a symmetrical patterns Semiconductor atoms (silicon) form crystalsIntrinsic crystals have no impurities

Conduction Electrons and HolesElectrons exist only within prescribed energy bands These bands are separated by energy gaps When an electron jumps to the conduction band it causes a hole When electron falls back to its initial valence recombination occurs Consequently there are two different types of currentsHole current (electrons are the minority carriers)Electron current (holes are the minority carriers)Remember: We are interested in electrical current!

Improving Conduction by DopingTo make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holeselements with 5 outer electrons contribute an extra electron to the lattice (donor dopant)elements with 3 outer electrons accept an electron from the silicon (acceptor dopant)

DopingBy adding impurities to the intrinsic semiconductor we can change the conductivity of the material this is called doping N-type doping P-type dopingN-type: pentavalent (atom with 5 valence electrons) impurity atoms are added [Sb(Antimony) + Si] Negative charges (electrons) are generated N-type has lots of free electronsP-type: trivalent (atom with 3 valence electrons) impurity atoms are added [B(Boron) + Si] Positive charges (holes) are generatedP-type has lots of holes

Electron and Hole ConcentrationsUnder thermal equilibrium conditions, the product of the conduction-electron density and the hole density is ALWAYS equal to the square of ni:P-type materialN-type material

DiodesN region has lots of free electrons P region has lots of holesAt equilibrium: total number positive and negative charges is the same (@ room temp)At the pn junction the electrons and holes with different charges form an electric fieldIn order to move electrons through the electric field (generate current) we need some force (voltage)This potential difference is called barrier voltageWhen enough voltage is applied such that electrons are moved then we are biasing the diodeTwo layers of positive and negative charges for depletion region the region near the pn-junction is depleted of charge carriers)

Biasing Types of a DiodeForward biasBias voltage VBias > barrier voltage VBar Reduction in + and ions smaller depletion region VBar Depends on material, doping, temp, etc. (e.g., for silicon it is 0.7 V)Reverse biasEssentially a condition that prevents electrons to pass through the diode Very small reverse break down currentLarger depletion region is generated

Connected to the negative side of the batteryConnected to the positive side of the batteryAK

Biasing Types of a Diode (Forward)AKMoving electronsSmall dynamic resistanceVBiasnpConventional Current FlowConventional Current FlowI (Forward)

Very SmallMoving Electrons:Reverse Current)Biasing Types of a Diode (Reverse)AKLarge resistanceVBiasnpConventional Current FlowHoles are left behind; large depletion region Instant pull of electrons

I-V Characteristic of a DiodeForward bias: current passes through The knee is where VBias=VBarAt point B VBias < VBar Very little currentNote that at the knee the current increases rapidly but V(forward) stays almost the same

Reveres bias: No current passes through When VBias < VBar Very little current (mu or nano Amp)At the knee, the reverse current increases rapidly but the reverse voltage remains almost the same Large reverse current can result in overheating and possibly damaging the diode (V=50V or higher typically)

Overheating results from high-speed electrons in the p-region knocking out electrons of atoms in n-region from their orbit to the conduction bandHence, we use limiting resistors

Electrons moving from n to p region

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