3_1_3semicond_diode

3_1_3semicond_diode - Review of Semiconductor Physics, PN...

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Unformatted text preview: Review of Semiconductor Physics, PN Junction Diodes and Resistors and s s s s s s s Semiconductor fundamentals Doping Pn junction The Diode Equation Zener diode LED Resistors What Is a Semiconductor? What •Many materials, such as most metals, allow electrical current to Many flow through them flow •These are known as conductors •Materials that do not allow electrical current to flow through Materials them are called insulators them •Pure silicon, the base material of most transistors, is considered Pure a semiconductor because its conductivity can be modulated by the introduction of impurities the Semiconductors Semiconductors s s A material whose properties are such that it is not quite a material conductor, not quite an insulator conductor, Some common semiconductors – elemental » Si - Silicon (most common) » Ge - Germanium – compound » » » » » GaAs - Gallium arsenide GaP - Gallium phosphide AlAs - Aluminum arsenide AlP - Aluminum phosphide InP - Indium Phosphide Crystalline Solids Crystalline s s In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal repeated Silicon crystal has a diamond lattice Silicon diamond Crystalline Nature of Silicon Crystalline s s s Silicon as utilized in integrated circuits is crystalline in nature As with all crystalline material, silicon consists of a repeating As basic unit structure called a unit cell unit For silicon, the unit cell consists of an atom surrounded by four For equidistant nearest neighbors which lie at the corners of the neighbors tetrahedron tetrahedron What’s so special about Silicon? What’s Cheap and abundant Amazing mechanical, chemical and electronic properties The material is very well-known to mankind SiO2: sand, glass Si is column IV of the periodic table Similar to the carbon (C) and the germanium (Ge) Has 3s² and 3p² valence electrons Nature of Intrinsic Silicon Nature s s Silicon that is free of doping impurities is called Silicon intrinsic intrinsic Silicon has a valence of 4 and forms covalent Silicon bonds with four other neighboring silicon atoms bonds silicon Semiconductor Crystalline Structure Semiconductor s s s Semiconductors have a regular Semiconductors crystalline structure crystalline – for monocrystal, extends for through entire structure through – for polycrystal, structure is for interrupted at irregular boundaries boundaries Monocrystal has uniform 3dimensional structure Atoms occupy fixed positions Atoms relative to one another, but relative are in constant vibration about are equilibrium equilibrium Semiconductor Crystalline Structure Semiconductor s s Silicon atoms have 4 Silicon electrons in outer shell electrons – inner electrons are very inner closely bound to atom closely These electrons are shared These with neighbor atoms on both sides to “fill” the shell both – resulting structure is resulting very stable very – electrons are fairly electrons tightly bound tightly » no “loose” electrons – at room temperature, if at battery applied, very little electric current flows flows Conduction in Crystal Lattices Conduction s s s Semiconductors (Si and Ge) have 4 electrons in their outer shell – 2 in the s subshell – 2 in the p subshell As the distance between atoms decreases the discrete subshells As spread out into bands spread As the distance decreases further, the bands overlap and then As separate separate – the subshell model doesn’t hold anymore, and the electrons the can be thought of as being part of the crystal, not part of the atom atom – 4 possible electrons in the lower band (valence band) – 4 possible electrons in the upper band (conduction band) Energy Bands in Semiconductors Energy s The space The between the bands is the energy gap, or energy or forbidden band forbidden Insulators, Semiconductors, and Metals Insulators, s s s s This separation of the valence and conduction bands determines This the electrical properties of the material the Insulators have a large energy gap – electrons can’t jump from valence to conduction bands – no current flows Conductors (metals) have a very small (or nonexistent) energy gap – electrons easily jump to conduction bands due to thermal electrons excitation excitation – current flows easily Semiconductors have a moderate energy gap – only a few electrons can jump to the conduction band » leaving “holes” – only a little current can flow Insulators, Semiconductors, and Metals (continued) (continued) Conduction Band Valence Band Conductor Semiconductor Insulator Hole - Electron Pairs Hole s s Sometimes thermal energy is enough to cause an electron to Sometimes jump from the valence band to the conduction band jump – produces a hole - electron pair Electrons also “fall” back out of the conduction band into the Electrons valence band, combining with a hole valence pair elimination pair creation hole electron Improving Conduction by Doping Improving s To make semiconductors better conductors, add impurities To (dopants) to contribute extra electrons or extra holes (dopants) – elements with 5 outer electrons contribute an extra electron to elements the lattice (donor dopant) the – elements with 3 outer electrons accept an electron from the elements silicon (acceptor dopant) silicon s Improving Conduction by Doping (cont.) (cont.) Phosphorus and arsenic are Phosphorus donor dopants donor – if phosphorus is if introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current electric – produces n type silicon produces silicon » very loosely bound to atom and can easily jump to conduction band » sometimes use + symbol to indicate heavier doping, so n+ silicon – phosphorus becomes phosphorus positive ion after giving up electron electron Improving Conduction by Doping (cont.) (cont.) s Boron has 3 electrons in its outer Boron shell, so it contributes a hole if it displaces a silicon atom displaces – boron is an acceptor dopant boron acceptor – yields p type silicon yields silicon – boron becomes negative ion boron after accepting an electron after Epitaxial Growth of Silicon Silicon s Epitaxy grows silicon on top of existing silicon existing s Silicon is placed in chamber at Silicon high temperature high Appropriate gases are fed into Appropriate the chamber the Can grow n type, then switch to Can p type very quickly type – uses chemical vapor uses deposition deposition – new silicon has same new crystal structure as original original – 1200 o C (2150 o F) 1200 s – other gases add other impurities to the mix impurities s Diffusion of Dopants Diffusion s It is also possible to introduce It dopants into silicon by heating them so they diffuse into the diffuse silicon silicon Can be done with constant Can concentration in atmosphere concentration top – no new silicon is added – high heat causes diffusion – close to straight line close concentration gradient concentration – predeposition – bell-shaped gradient s s Or with constant number of atoms Or per unit area per Diffusion causes spreading of Diffusion doped areas doped side s Diffusion of Dopants (continued) Diffusion Concentration of dopant in surrounding atmosphere kept constant per unit volume Dopant deposited on surface ­ constant amount per unit area Ion Implantation of Dopants Ion s s s s One way to reduce the spreading found with diffusion is to use ion One implantation implantation – also gives better uniformity of dopant – yields faster devices – lower temperature process Ions are accelerated from 5 Kev to 10 Mev and directed at silicon – higher energy gives greater depth penetration – total dose is measured by flux » number of ions per cm2 » typically 1012 per cm2 - 1016 per cm2 Flux is over entire surface of silicon – use masks to cover areas where implantation is not wanted Heat afterward to work into crystal lattice Hole and Electron Concentrations Hole To produce reasonable levels of conduction doesn’t To require much doping require – silicon has about 5 x 1022 atoms/cm3 – typical dopant levels are about 1015 atoms/cm3 s In undoped (intrinsic) silicon, the number of holes and In number of free electrons is equal, and their product equals a constant equals – actually, ni increases with increasing temperature s np = n s This equation holds true for doped silicon as well, so This increasing the number of free electrons decreases the number of holes number INTRINSIC (PURE) SILICON INTRINSIC At 0 Kelvin Silicon density is 5* 10²³ particles/cm³ Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice Higher temperatures create f ree charge carriers. A “ hole” is created in the absence of an electron. At 23C there are 10¹º particles/cm³ of free carriers DOPING DOPING There are two types of doping N-type and P-type. The N in N-type stands for negative. A column V ion is inserted. The extra valence electron is free to move about the lattice The P in P-type stands for positive. A column III ion is inserted. Electrons from the surrounding Silicon move to fill the “ hole.” Energy-band Diagram Energy-band s s s A very important concept in the study of semiconductors is the very energy-band diagram energy-band It is used to represent the range of energy a valence electron can It have have For semiconductors the electrons can have any one value of a For continuous range of energy levels while they occupy the valence shell of the atom shell – That band of energy levels is called the valence band That valence s Within the same valence shell, but at a slightly higher energy Within level, is yet another band of continuously variable, allowed energy levels levels – This is the conduction band This conduction Band Gap Band s s s s Between the valence and the conduction band is a range of energy Between levels where there are no allowed states for an electron levels This is the band gap E G In silicon at room temperature [in electron volts]: E G = 1.1 eV Electron volt is an atomic measurement unit, 1 eV energy is Electron necessary to decrease of the potential of the electron with 1 V. necessary 1eV = 1.602 × 10 −19 joule Impurities Impurities s s s Silicon crystal in pure form is Silicon good insulator - all electrons are bonded to silicon atom bonded Replacement of Si atoms can alter Replacement electrical properties of semiconductor semiconductor Group number - indicates number Group of electrons in valence level (Si Group IV) Group Impurities Impurities s s Replace Si atom in crystal with Group V atom – substitution of 5 electrons for 4 electrons in outer shell – extra electron not needed for crystal bonding structure » can move to other areas of semiconductor » current flows more easily - resistivity decreases » many extra electrons ­­> “donor” or n-type material Replace Si atom with Group III atom – substitution of 3 electrons for 4 electrons substitution – extra electron now needed for crystal bonding structure » “hole” created (missing electron) » hole can move to other areas of semiconductor if electrons continually fill holes » again, current flows more easily - resistivity decreases » electrons needed ­­> “acceptor” or p-type material COUNTER DOPING COUNTER Insert more than one type of Ion The extra electron and the extra hole cancel out A LITTLE MATH n= number of free electrons p=number of holes ni=number of electrons in intrinsic silicon=10¹º/cm³ pi-number of holes in intrinsic silicon= 10¹º/cm³ Mobile negative charge = -1.6* 10-19 Coulombs Mobile positive charge = 1.6* 10-19 Coulombs At thermal equilibrium (no applied voltage) n* p=(ni)2 (room temperature approximation) The substrate is called n-type when it has more than 10¹º free electrons (similar for p-type) P-N Junction P-N s s s s Also known as a diode One of the basics of semiconductor technology Created by placing n-type and p-type material in close Created contact contact Diffusion - mobile charges (holes) in p-type combine with Diffusion mobile charges (electrons) in n-type mobile P-N Junction P-N s Region of charges left behind (dopants fixed in crystal Region lattice) lattice) – Group III in p-type (one less proton than Si- negative Group charge) charge) – Group IV in n-type (one more proton than Si - positive Group charge) charge) s Region is totally depleted of mobile charges - “depletion Region region” region” – Electric field forms due to fixed charges in the depletion Electric region region – Depletion region has high resistance due to lack of mobile Depletion charges charges THE P-N JUNCTION THE The Junction The The “ potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region Biasing the P-N Diode Biasing THINK OF THE DIODE AS A SWITCH Forward Bias Applies - voltage to the n region and + voltage to the p region CURRENT! Reverse Bias Applies + voltage to n region and – voltage to p region NO CURRENT P-N Junction – Reverse Bias P-N s s s s positive voltage placed on n-type material electrons in n-type move closer to positive terminal, holes electrons in p-type move closer to negative terminal in width of depletion region increases allowed current is essentially zero (small “drift” current) P-N Junction – Forward Bias P-N s s s s positive voltage placed on p-type material holes in p-type move away from positive terminal, electrons in ntype move further from negative terminal depletion region becomes smaller - resistance of device decreases voltage increased until critical voltage is reached, depletion region voltage disappears, current can flow freely disappears, P-N Junction - V-I characteristics P-N Voltage-Current relationship for a p-n junction (diode) Voltage-Current Current-Voltage Characteristics Current-Voltage THE IDEAL DIODE Positive voltage yields f inite current Negative voltage yields zero current REAL DIODE The Ideal Diode Equation The qV I = I 0 exp − 1 , kT where I 0 = diode current with reverse bias q = 1.602 × 10 −19 coulomb , the electronic ch arg e eV k = 8.62 × 10 −5 , Boltzmann' s cons tan t K Semiconductor diode - opened region Semiconductor s s The p-side is the cathode, the n-side is the anode The dropped voltage, VD is measured from the cathode to the anode to Opened: VD ≥ VF: VD = VF s ID = circuit limited, in our model the VD cannot exceed VF circuit Semiconductor diode - cut-off region Semiconductor s Cut-off: 0 < VD < VF: ID ≅ 0 mA mA Semiconductor diode - closed region Semiconductor s Closed: VF < VD ≤ 0: 0: – VD is determined by the circuit, ID = 0 mA mA s Typical values of VF: 0.5 ¸ 0.7 V Zener Effect Zener s Zener break down: VD <= VZ: VD = VZ, ID is determined by the circuit. In case of standard diode the typical values of the break In down voltage VZ of the Zener effect -20 ... -100 V down Zener diode – Utilization of the Zener effect – Typical break down values of VZ : -4.5 ... -15 V s s LED LED s Light emitting diode, made from GaAs – VF=1.6 V – IF >= 6 mA Resistor in an Integrated Circuit Resistor ...
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This note was uploaded on 04/04/2010 for the course EE 3 taught by Professor Staff during the Winter '08 term at UCLA.

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