Lecture 1 OUTLINE Basic Semiconductor Physics Semiconductors Intrinsic undoped silicon Doping Carrier concentrations Reading Chapter 2 1 EE105 Fall 2007 Lecture 1 Slide 1 What 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 important polycrystalline amorphous crystalline EE105 Fall 2007 Lecture 1 Slide 2 Semiconductor Materials Phosphorus P Gallium Ga EE105 Fall 2007 Lecture 1 Slide 3 Silicon 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 4 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 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 field 3 by changing the temperature 4 by irradiation EE105 Fall 2007 Lecture 1 Slide 5 Electron 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 6 Carrier 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 15 ni 5 2 10 T 3 2 exp Eg 2kT electrons cm3 ni 1 1010 electrons cm3 at 300K ni 1 1015 electrons cm3 at 600K EE105 Fall 2007 Lecture 1 Slide 7 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 Notation n conduction electron concentration EE105 Fall 2007 Lecture 1 Slide 8 Doping 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 concentration EE105 Fall 2007 Lecture 1 Slide 9 Summary of Charge Carriers EE105 Fall 2007 Lecture 1 Slide 10 Electron 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 2 np ni N type material P type material n N D p N A 2 n n i NA 2 n p i ND EE105 Fall 2007 Lecture 1 Slide 11 Terminology donor impurity atom that increases n acceptor impurity atom that increases p N type material contains more electrons than holes P type material contains more holes than electrons majority carrier the most abundant carrier minority carrier the least abundant carrier intrinsic semiconductor n p ni extrinsic semiconductor doped semiconductor EE105 Fall 2007 Lecture 1 Slide 12 Summary 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 meV EE105 Fall 2007 Lecture 1 Slide 13
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