Silicon in Semiconductor devices Document Transcript
Electronic Material andTheir ApplicationsTerm PaperTopic : Silicon In Semiconductor DevicesAshish SinghB10007
2Electronic Material and Their ApplicationsSilicon as elementSilicon is the most abundant electropositive element in The Earth’s crust. It’s ametalloid with a marked metallic lustre and very brittle .Silicon is group IV element.Silicon is the eighth most common element in the universe by mass, but very rarelyoccurs as the pure free element in nature. It is most widely distributed in dusts,sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates.Over 90% of the Earths crust is composed of silicate minerals, making silicon thesecond most abundant element in the Earths crust (about 28% by mass) afteroxygen.Silicon is denoted by the symbol Si. Its atomic number is 14 and atomic weight is28.08. It is a tetravalent element and contains four electrons in its outermost orbit.Itis less reactive than its chemical analog carbon, the non-metal directly above it in theperiodic table, but more reactive than germanium.Physical propertiesSilicon is a solid at room temperature, with relatively high melting and boiling pointsof 1414 and 3265 degrees Celsius respectively. It has a greater density in a liquidstate than a solid stateIn its crystalline form, pure silicon has a gray color and ametallic lustre.Chemical PropertiesSilicon is a relatively inactive element at room temperature. It does not combine with oxygenor most other elements. Water, steam, and most acids have very little affect on the element.At higher temperatures, however, silicon becomes much more reactive. In the molten(melted) state, for example, it combines with oxygen, nitrogen, sulphur, phosphorus, andother elements. It also forms a number of alloys very easily in the molten state.IsotopesThere are three naturally occurring isotopes of silicon: silicon-28, silicon-29, andsilicon-30. Isotopes are two or more forms of an element. Isotopes differ from eachother according to their mass number. The number written to the right of the
3Electronic Material and Their Applicationselements name is the mass number. The mass number represents the number ofprotons plus neutrons in the nucleus of an atom of the element. The number ofprotons determines the element, but the number of neutrons in the atom of any oneelement can vary. Each variation is an isotope.Five radioactive isotopes of silicon are known also. A radioactive isotope is one thatbreaks apart and gives off some form of radiation. Radioactive isotopes areproduced when very small particles are fired at atoms. These particles stick in theatoms and make them radioactive.Figure 1 Pure SiliconSilicon –a giant covalent StructureSilicon is a non-metal, and has a giant covalent structure exactly the same as carbonin diamond - hence the high melting point. You have to break strong covalent bondsin order to melt it. The element silicon would be expected to form 4 covalent bond(s)in order to obey the octet rule. Si is a nonmetal in group 4A, and therefore has 4valence electrons. In order to obey the octet rule, it needs to gain 4 electrons. It cando this by forming 4 single covalent bonds.
4Electronic Material and Their ApplicationsFigure 2 arrangement of Silicon atoms in unit CellSilicon crystallizes in the same pattern as diamond, in a structure which Ashcroft andMermin call "two interpenetrating face-centered cubic" primitive lattices. The linesbetween silicon atoms in the lattice illustration indicate nearest-neighbor bonds. Thecube side for silicon is 0.543 nm. Germanium has the same diamond structure with acell dimension of .566 nmSemiconductorA semiconductor is a material which has electrical conductivity between that of aconductor such as copper and an insulator such as glass. The conductivity of asemiconductor increases with increasing temperature, behavior opposite to that of ametal. Current conduction in a semiconductor occurs via free electrons and "holes",collectively known as charge carriers.Conduction in Semiconductors
5Electronic Material and Their ApplicationsThe most important parameters for conduction in semiconductor material the band gap the number of free carriers (electrons or holes) available for conduction; and the "generation" and recombination of free carriers (electrons or holes) inresponse to light shining on the material.Figure 3 semiconductor energy bandCurrent is the rate of flow of charge , we shall be able calculate currents flowing inreal devices since we know the number of charge carriers. There are two currentmechanisms which cause charges to move in semiconductors. The two mechanismsare drift and diffusion.Drift CurrentFigure 4 crystal lattice where the free electrons move randomly in BrownianmotionFree electrons collide with the stationary atoms and get deflected in a differentdirection. The average distance travelled between two collisions is called the meanfree path. In absence of any electric field, no net movement in any direction takesplace as shown by solid arrows.
6Electronic Material and Their ApplicationsWith application of the electric field, electrons get accelerated in opposite direction ofthe electric field and hence, although the motion is still random, the net movementtakes place as indicated by arrow from left to right. This movement is called drift.Figure 5 (a) Drift of electrons (b) Drift of holesDiffusion CurrentFigure 6 Diffusion of holes in Brownian motionConcentration of carriers (holes) is much high on the left side of the surface YY′.Since the holes are in Brownian motion, they cross over the surface YY′ randomlyfrom left to right and right to left. But as the concentration of holes on the left side ismuch higher than that on the right side, the average number holes going from left toright is more than number of holes going from right to left of the surface YY′. Thisconstitutes a net flow of holes (carriers) from left to right, i.e. a net current flow due toflow of holes from left to right. This is called diffusion current because it resultsfrom diffusion of carriers. Thus, diffusion current is due to flow of charge carriersfrom higher concentration to lower concentration region.The diffusion current is proportional to the concentration gradient
7Electronic Material and Their ApplicationsP –type and N-type SemiconductorsThe addition of a small percentage of foreign atoms in the regular crystal lattice ofsilicon or germanium produces dramatic changes in their electrical properties,producing n-type and p-type semiconductors.Figure 7 P -and N - Type SemiconductorP-TypeThe addition of trivalent impurities such as boron, aluminum or gallium to an intrinsicsemiconductor creates deficiencies of valence electrons,called "holes".N-TypeThe addition of pentavalent impurities such as antimony, arsenic or phosphorouscontributes free electrons, greatly increasing the conductivity of the semiconductor.
8Electronic Material and Their ApplicationsSemiconductor DevicesDiodesThe diode is a device made from a single p–n junction. At the junction of a p-typeand an n-type semiconductor there forms a region called the depletion zone orregion which blocks current conduction from the n-type region to the p-type region,but allows current to conduct from the p-type region to the n-type region. Thus, whenthe device is forward biased, with the p-side at higher electric potential, the diodeconducts current easily; but the current is very small when the diode is reversebiased.Exposing a semiconductor to light can generate electron–hole pairs, which increasesthe number of free carriers and its conductivity. Diodes optimized to take advantageof this phenomenon are known as photodiodes. Compound semiconductor diodescan also be used to generate light, as in light-emitting diodes and laser diodes.Silicon DiodesSilicon rectifier diodes, are used in many applications from high voltage, high currentpower supplies, where they rectify the incoming mains (line) voltage and must passall of the current required by whatever circuit they are supplying; this may be severaltens of Amperes or more.
9Electronic Material and Their ApplicationsCarrying such currents requires a large junction area so that the forward resistanceof the diode is kept as low as possible. Even so the diode is likely to get quite warm.The black resin case helps dissipate the heat.The resistance to current in the reverse direction (when the diode is "off") must behigh, and the insulation offered by the depletion layer between the P and N layersextremely good to avoid the possibility of "reverse breakdown", where the insulationof the diode fails due to the high reverse voltage across the junction.Silicon diodes are made in many different forms with widely differing parameters.They vary in current carrying ability from milli-amps to tens of amps, some will havereverse breakdown voltages of thousands of volts; others use their junctioncapacitance to act as tuning devices in radio and TV tuners.Silicon Zener DiodeFigure 8 Zener DiodeZener diodes are special silicon diodes which have a relatively low, definedbreakdown voltage, called the Zener voltage.At low reverse voltages a Zener diodebehaves in a simi-lar manner to an ordinary silicon diode, that is, it passes only avery small leakage current. If, however, the reverse bias is increased until it reachesthe breakdown region, then a small reverse voltage increase causes a considerableincrease in leakage current; the reverse current is then called the Zener current.The characteris-tics of a Zener diode operating under reverse breakdown conditionsare similar to those of a struck glow dis-charge tube. Because of this, Zener diodescan be used in a similar way, i. e. as stabilizers, limiters, ripple reduc-tion elements,reference voltage sources, and also as DC coupling elements with a constantvoltage drop.
10Electronic Material and Their ApplicationsTunnel DiodeA tunnel diode or Esaki diode is a type of semiconductor diode that is capable ofvery fast operation, well into the microwave frequency region, by using the quantummechanical effect called tunneling.These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. Theheavy doping results in a broken bandgap, where conduction band electron stateson the n-side are more or less aligned with valence band hole states on the p-side.Tunnel diodes are usually made from germanium, but can also be made in galliumarsenide and silicon materials. They are used in frequency converters anddetectors.They have negative differential resistance in part of their operatingrange, and therefore are also used as oscillators, amplifiers, and in switching circuitsusing hysteresisSilicon Solar cellsThe "photovoltaic effect" is the basic physical process through which a PV cellconverts sunlight into electricity. Sunlight is composed of photons, or particles ofsolar energy. These photons contain various amounts of energy corresponding tothe different wavelengths of the solar spectrum. When photons strike a PV cell, theymay be reflected or absorbed, or they may pass right through. Only the absorbedphotons generate electricity. When this happens, the energy of the photon istransferred to an electron in an atom of the cell (which is actually a semiconductor).With its newfound energy, the electron is able to escape from its normal positionassociated with that atom to become part of the current in an electrical circuit. Byleaving this position, the electron causes a "hole" to form. Special electricalproperties of the PV cell—a built-in electric field—provide the voltage needed to drivethe current through an external load (such as a light bulb).
11Electronic Material and Their ApplicationsTo induce the electric field within a PV cell, two separate semiconductors aresandwiched together. The "p" and "n" types of semiconductors correspond to"positive" and "negative" because of their abundance of holes or electrons (the extraelectrons make an "n" type because an electron actually has a negative charge).Although both materials are electrically neutral, n-type silicon has excess electronsand p-type silicon has excess holes. Sandwiching these together creates a p/njunction at their interface, thereby creating an electric field.When the p-type and n-type semiconductors are sandwiched together, the excesselectrons in the n-type material flow to the p-type, and the holes thereby vacatedduring this process flow to the n-type. (The concept of a hole moving is somewhatlike looking at a bubble in a liquid. Although its the liquid that is actually moving, itseasier to describe the motion of the bubble as it moves in the opposite direction.)Through this electron and hole flow, the two semiconductors act as a battery,creating an electric field at the surface where they meet (known as the "junction"). Itsthis field that causes the electrons to jump from the semiconductor out toward thesurface and make them available for the electrical circuit. At this same time, theholes move in the opposite direction, toward the positive surface, where they awaitincoming electrons.
12Electronic Material and Their ApplicationsSilicon controlled rectifier(Thyristor)A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name "silicon controlled rectifier" orSCR is General Electrics trade name for a type of thyristor.Figure 9 SCR schematic diagramSCRs are unidirectional devices (i.e. can conduct current only in one direction) asopposed to TRIACs which are bidirectional (i.e. current can flow through them ineither direction). SCRs can be triggered normally only by currents going into the gateas opposed to TRIACs which can be triggered normally by either a positive or anegative current applied to its gate electrode.This device is generally used in switching applications. In the normal "off" state, thedevice restricts current to the leakage current. When the gate-to-cathode voltageexceeds a certain threshold, the device turns "on" and conducts current. The devicewill remain in the "on" state even after gate current is removed so long as currentthrough the device remains above the holding current. Once current falls below theholding current for an appropriate period of time, the device will switch "off". If thegate is pulsed and the current through the device is below the latching current, thedevice will remain in the "off" state.TransistorA transistor is a semiconductor device used to amplify and switch electronic signalsand electrical power. It is composed of semiconductor material with at least three
13Electronic Material and Their Applicationsterminals for connection to an external circuit. A voltage or current applied to onepair of the transistors terminals changes the current through another pair ofterminals. The transistor is the fundamental building block of modern electronicdevices, and is ubiquitous in modern electronic systems.Bipolar Junction TransistorFigure 10 BJT siliconFigure 11 BJT N-P-N TransistorA bipolar junction transistor (BJT or bipolar transistor) is a type of transistor thatrelies on the contact of two types of semiconductor for its operation. BJTs can beused as amplifiers, switches, or in oscillators. BJTs can be found either as individualdiscrete components, or in large numbers as parts of integrated circuits.Bipolar transistors are so named because their operation involves both electrons andholes. These two kinds of charge carriers are characteristic of the two kinds of doped
14Electronic Material and Their Applicationssemiconductor material. In contrast, unipolar transistors such as the field-effecttransistors have only one kind of charge carrier.A Bipolar Junction Transistor is made up of a piece of silicon that has been treated tocreate a channel in the silicon. First, an undoped (untreated) piece of silicon isselected. (Undoped silicon acts as an insulator, preventing the flow of current.) Next,a process is used to create an N-type channel in the silicon. This is silicon that hasan overabundance of negative charge carriers (electrons). A second channel iscreated in the first, this time with P-type silicon. P-type silicon carries currentbecause of the overabundance of positive charge carriers (holes, theyre called).The process has created three regions in the silicon, an N-type region, a P-typeregion, and another N-type region. The base terminal is connected to the P-typeregion, while the collector and emitter are connected to the opposing N-type regions.Darlington TransistorFigure 12 Darlington Pair diagramThe Darlington transistor (often called a Darlington pair) is a compound structureconsisting of two bipolar transistors (either integrated or separated devices)connected in such a way that the current amplified by the first transistor is amplifiedfurther by the second one.This configuration gives a much higher common/emittercurrent gain than each transistor taken separately and, in the case of integrateddevices, can take less space than two individual transistors because they can use ashared collector. Integrated Darlington pairs come packaged singly in transistor-likepackages or as an array of devices (usually eight) in an integrated circuit.
15Electronic Material and Their ApplicationsSilicon Quantum Dot LEDA quantum dot display is a type of display technology used in flat panel displays asan electronic visual display. Quantum dots (QD) or semiconductor nanocrystals are aform of light emitting technology and consist of nano-scale crystals that can providean alternative for applications such as display technology.Quantum-dot-based LEDs are characterized by pure and saturated emission colorswith narrow bandwidth, and their emission wavelength is easily tuned by changingthe size of the quantum dots. Moreover, QD-LED combine the color purity anddurability of QDs with efficiency, flexibility, and low processing cost of organic light-emitting devices. QD-LED structure can be tuned over the entire visible wavelengthrange from 460 nm (blue) to 650 nm (red).The structure of QD-LED is similar to basic design of OLED. The major difference isthat the light emitting centers are cadmium selenide (CdSe) nanocrystals, orquantum dots. A layer of cadmium-selenium quantum dots is sandwiched betweenlayers of electron-transporting and hole-transporting organic materials. An appliedelectric field causes electrons and holes to move into the quantum dot layer, wherethey are captured in the quantum dot and recombine, emitting photons. Thespectrum of photon emission is narrow, characterized by its full width at half themaximum value.Bringing electrons and holes together in small regions for efficient recombination toemit photons without escaping or dissipating was one of the major challenges. Toaddress this problem, a thin emissive layer sandwiched between a hole-transporterlayer (HTL) and an electron-transport layer (ETL). By making an emissive layer insingle layer of quantum dots, electrons and holes may be transferred directly fromthe surfaces of the ETL and HTL., and resulting high recombination efficiency.Both ETL and HTL consist of organic materials. It is well known that most organicelectroluminescent materials are in favor of injection and transport of holes ratherthan electrons. Thus, the electron-hole recombination generally occurs near thecathode, which could lead to the quenching of the exciton produced. In order toprevent the produced excitons or holes from approaching cathode, a hole-blocking
16Electronic Material and Their Applicationslayer plays dual roles in blocking holes moving towards the cathode and transportingthe electrons to the emitting layer, QD layer. Tris-Aluminium (Alq3), bathocuproine(BCP), and TAZ are most commonly used hole-blocking materials. These materialscan be used as both electron-transporting layer and hole blocking layer.The array of quantum dots is manufactured by self-assembly in process known asspin-casting; a solution of quantum dots in an organic material is poured into asubstrate, which is then set spinning to spread the solution evenly.Quantum dot LEDs often rely on toxic heavy metals such as cadmium to emit light.Now, researchers report new colors of LEDs that use silicon quantum dots.Silicon could be a less toxic alternative, but scientists have struggled to fabricatesilicon-based devices that glow in colors across the visible spectrum. Current siliconLEDs emit only red and near-infrared light.