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Semiconductors

Semiconductors

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Semi conductor Prepared By: Mohammed Qashlan Energy Band  In any material, there are 2 energy band: 1. Valence band : band  : the  the outermost shell that determines the conductivity 2. Conduction band : the band outside the valence shell. The 2 bands are separated by one energy gap called – forbidden gap. gap .  The valence band contains with electrons.   The electrons can move to the conduction conduction band if it have enough enough energy ( eg: eg: light or heat).   hen the electron absorbs enough energy to to !ump from valence band to the conduction band, the electron is said to be in e"cited state. The concept of energy bands is particularly important in classifying materials as conductors, semiconductors, and insulators.  • smaller forbidden forbidden band and re#uires less energy to Semiconductor : has a smaller  move an electron from the valence band to the conduction band. $ Therefore, for a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator.   semiconducting elements: elements : – low electrical conductivity conductivity at room temperature – %lectrical conductivity increases with with temp.   &ap between valence and conduction band is intermediate in si'e.  emiconducting elements form the basis of solid state electronic devices.   etalloids (such as silicon or germanium) are semiconducting elements whose electrical conductivity increases as temperature increases.   * stri+ing property of these elements is that their conductivities increase mar+edly when they are doped with small #uantities of other elements.   ade from materials that have four valence electrons in their outer orbitals.  &ermanium and silicon are the most common. comm on.  ilicon is preferred due to its ability to withstand heat.   * pure semiconductor semiconductor material such such as silicon silicon or germanium has no special properties and will ma+e a poor conductive material.   hen silicon is doped with phosphorus, it becomes an n-type semiconductor , in which electrical current is carried by negatively charged electrons.   hen silicon is doped with boron, it becomes a p-type semiconductor , in which an electrical current is carried by positively charged holes.  oining a p-type semiconductor to an n-type semiconductor produces a p-n p-n junction  junction,, which can function as a rectifier.   * rectifier is a device that allows current to flow in one direction, but not the other.  Types of Semiconductor :  emiconductors are mainly classified into two categories: i. Intrinsic ii. Extrinsic i.  Intrinsic Intrinsic : chemically very pure and possesses poor conductivity. conductiv ity. -  It It has e#ual numbers of negative carriers (electrons) and positive carriers (holes). -  Impurities Impurities do not affect its electrical behavior. Intrinsic Semiconductor ilicon has  outer shell valence electrons /orms into a lattice structure to share electrons The pure semiconductor semiconductor material ithout impurities atoms. example: !ilicon and "ermanium Extrinsic semiconductor :  improved intrinsic semiconductor with a small amount of impurities added by a process, +nown as doping process process,, which alters the electrical properties of the semiconductor and improves its conductivity.  Introducing impurities into the semiconductor materials (doping process) can control their  conductivity.. conductivity  *dding impurities impurities atom into intrinsic intrinsic semiconductor # e"trinsic semiconductor.   The process of adding specific types of atoms to a semiconductor to favorably alter electric characteristics – $oping  2 types of e"trinsic (impure) semiconductor0  %-type  &-type  hen an impurity increases the number of free electrons, the doped semiconductor is negati'e or n-type .  *n impurity that that reduces the number of free electrons, causing more holes, creates a positi'e or positi'e  or p-type semiconductor.  Doping  $oping : *dding impurities to the silicon crystal lattice to increase the number of carriers.  *dd a small number of atoms to increase increase either the number of electrons or holes.  Donors n-Type Material $onors -(dd atoms ith ) 'alence-band electrons -ex. &hosphorous *&+ -,$onates an extra e - that can freely tra'el around -ea'es behind a positi'ely charged nucleus *cannot mo'e+ -/'erall0 the crystal is still electrically neutral -Called ,n-type material *added negati'e carriers+ 1 % type material - iffused iffused impurities im purities with ) valence electrons are called donor  donor atoms atoms.. (ntimony *!b+ impurity in n-type material Acceptors Make p-Type Material (cceptors $ (dd atoms ith only 2 'alenceband electrons $ ex. 3oron *3+ h1  – $ ,(ccepts e  and pro'ides extra h4 to freely tra'el around $ ea'es behind a negati'ely charged nucleus *cannot mo'e+ $ /'erall0 the crystal is still electrically neutral $ Called ,p-type silicon *added positi'e carriers+ &-type material he diffused impurities ith 3 valence electrons re called acceptor  atoms 3oron *3+ impurity in p-type material PN Junction Formation   * 45 !unction !unction is fabricated fabricated from a single slice of semiconductor.  6ne side doped with acceptor impurity im purity atoms  atoms   p region  6ne side doped with donor impurity atoms – atoms  – n region  The interface separating the n and p regions is referred as the metallurgical !unction. The &% junction  * p-n !unction as .a rectifier  78 Semiconductor Properties /or T 9 ; %lectron sha+en free and can cause current to flow h1 e  -"eneration – . because they are charged particles – when they move, they carry current. -Therefore,, semiconductors can conduct -Therefore electricity for T 9 ; ? but not much current (at room temperature (3;), pure silicon has only 7 free electron per 3 trillion atoms).