Lecture 1OUTLINE• Basic Semiconductor Physics– Semiconductors–Intrinsic (undoped) siliconEE105 Fall 2007 Lecture 1, Slide 1–Intrinsic (undoped) silicon– Doping– Carrier concentrationsReading: Chapter 2.1What is a Semiconductor?• Low resistivity => “conductor”• High resistivity => “insulator”• Intermediate resistivity => “semiconductor”– conductivity lies between that of conductors and insulators–generally crystalline in structure for IC devicesEE105 Fall 2007 Lecture 1, Slide 2–generally crystalline in structure for IC devices• In recent years, however, non-crystalline semiconductors have become commercially very importantpolycrystalline amorphous crystallineSemiconductor MaterialsPhosphorusEE105 Fall 2007 Lecture 1, Slide 3Gallium(Ga)Phosphorus(P)Silicon• Atomic density: 5 x 1022atoms/cm3• Si has four valence electrons. Therefore, it can form covalent bonds with four of its nearest neighbors. • When temperature goes up, electrons can become free to move about the Si lattice. EE105 Fall 2007 Lecture 1, Slide 4free to move about the Si lattice.Electronic Properties of Si•••• Silicon is a semiconductor material.– Pure Si has a relatively high electrical resistivity at room temperature.•••• There are 2 types of mobile charge-carriers in Si:– Conduction electrons are negatively charged;– Holes are positively charged.EE105 Fall 2007 Lecture 1, Slide 5•••• The concentration (#/cm3) of conduction electrons & holes in a semiconductor can be modulated in several ways:1. by adding special impurity atoms ( dopants )2. by applying an electric field3. by changing the temperature4. by irradiationElectron-Hole Pair Generation• When a conduction electron is thermally generated, a “hole” is also generated.• A hole is associated with a positive charge, and is free to move about the Si lattice as well.EE105 Fall 2007 Lecture 1, Slide 6Carrier Concentrations in Intrinsic Si• The “band-gap energy” Egis the amount of energy needed to remove an electron from a covalent bond. • The concentration of conduction electrons in intrinsic silicon, ni, depends exponentially on Egand the absolute temperature (T):EE105 Fall 2007 Lecture 1, Slide 7absolute temperature (T):600Kat /101300Kat /101/2exp102.531531032/315cmelectronsncmelectronsncmelectronskTETniigi×≅×≅−×=Doping (N type)• Si can be “doped” with other elements to change its electrical properties.• For example, if Si is doped with phosphorus (P), each P atom can contribute a conduction electron, so that the Si lattice has more electrons than holes, i.e. it becomes “N type”:EE105 Fall 2007 Lecture 1, Slide 8becomes “N type”:Notation:n = conduction electron concentrationDoping (P type)• If Si is doped with Boron (B), each B atom can contribute a hole, so that the Si lattice has more holes than electrons, i.e. it becomes “P type”:Notation:p = hole concentrationEE105 Fall 2007 Lecture 1, Slide 9Summary of Charge CarriersEE105 Fall 2007 Lecture 1, Slide 10Electron and Hole Concentrations• Under thermal equilibrium conditions, the product of the conduction-electron density and the hole density is ALWAYS equal to the square of ni:2innp =P-type materialN-type materialEE105 Fall 2007 Lecture 1, Slide 11P-type materialAiANnnNp2≈≈DiDNnpNn2≈≈N-type materialTerminologydonor: impurity atom that increases nacceptor: impurity atom that increases pN-type material: contains more electrons than holesP-typematerial: contains more holes than electronsEE105 Fall 2007 Lecture 1, Slide 12P-typematerial: contains more holes than electronsmajority carrier: the most abundant carrier minority carrier: the least abundant carrier intrinsic semiconductor: n = p = niextrinsic semiconductor: doped semiconductorSummary• The band gap energy is the energy required to free an electron from a covalent bond.– Egfor Si at 300K = 1.12eV• In a pure Si crystal, conduction electrons and holes are formed in pairs.EE105 Fall 2007 Lecture 1, Slide 13formed in pairs.– Holes can be considered as positively charged mobile particles which exist inside a semiconductor.– Both holes and electrons can conduct current.• Substitutional dopants in Si:– Group-V elements (donors) contribute conduction electrons– Group-III elements (acceptors) contribute holes– Very low ionization energies (<50
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