Lecture 1What is a Semiconductor?Semiconductor MaterialsSiliconElectronic Properties of SiElectron-Hole Pair GenerationCarrier Concentrations in Intrinsic SiDoping (N type)Doping (P type)Summary of Charge CarriersElectron and Hole ConcentrationsTerminologySummaryEE105 Fall 2007 Lecture 1, Slide 1Lecture 1OUTLINE•Basic Semiconductor Physics–Semiconductors–Intrinsic (undoped) silicon–Doping–Carrier concentrationsReading: Chapter 2.1EE105 Fall 2007 Lecture 1, Slide 2What 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 devices•In recent years, however, non-crystalline semiconductors have become commercially very importantpolycrystalline amorphous crystallineEE105 Fall 2007 Lecture 1, Slide 3Semiconductor MaterialsGallium(Ga)Phosphorus(P)EE105 Fall 2007 Lecture 1, Slide 4Silicon•Atomic density: 5 x 1022 atoms/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 5Electronic 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.- 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 irradiationEE105 Fall 2007 Lecture 1, Slide 6Electron-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 7Carrier Concentrations in Intrinsic Si•The “band-gap energy” Eg is 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 Eg and the absolute temperature (T):600Kat /101300Kat /101/2exp102.531531032/315cmelectronsncmelectronsncmelectronskTETniigiEE105 Fall 2007 Lecture 1, Slide 8Doping (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”:Notation:n = conduction electron concentrationEE105 Fall 2007 Lecture 1, Slide 9Doping (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 10Summary of Charge CarriersEE105 Fall 2007 Lecture 1, Slide 11Electron 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 materialAiANnnNp2DiDNnpNn2N-type materialEE105 Fall 2007 Lecture 1, Slide 12Terminologydonor: impurity atom that increases nacceptor: impurity atom that increases pN-type material: contains more electrons than holesP-type material: contains more holes than electronsmajority carrier: the most abundant carrier minority carrier: the least abundant carrier intrinsic semiconductor: n = p = niextrinsic semiconductor: doped semiconductorEE105 Fall 2007 Lecture 1, Slide 13Summary•The band gap energy is the energy required to free an electron from a covalent bond.–Eg for Si at 300K = 1.12eV•In a pure Si crystal, conduction electrons and holes are formed 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
View Full Document
Unlocking...