Lecture #2Electronic Properties of SiBond Model of Electrons and HolesWhat is a Hole?The Hole as a Positive Mobile ChargePure SiDefinition of TermsSi: From Atom to CrystalEnergy Band DiagramSummarySlide 11Lecture #2OUTLINE• Electrons and holes• Energy-band modelRead: Chapter 2 (Section 2.2)EE130 Lecture 2, Slide 2Spring 2007Electronic 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 irradiationEE130 Lecture 2, Slide 3Spring 2007Bond Model of Electrons and HolesS i S i S iS i S i S iS i S i S iSi Si SiSi Si SiSi Si SiWhen an electron breaks loose and becomes a conduction electron, a hole is also created.2-D representation:EE130 Lecture 2, Slide 4Spring 2007What is a Hole?•Mobile positive charge associated with a half-filled covalent bond–Treat as positively charged mobile particle in the semiconductor•Fluid analogy:EE130 Lecture 2, Slide 5Spring 2007The Hole as a Positive Mobile ChargeEE130 Lecture 2, Slide 6Spring 2007ni 1010 cm-3 at room temperaturePure SiconductionEE130 Lecture 2, Slide 7Spring 2007Definition of Termsn = number of electrons/cm3p = number of holes/cm3ni = intrinsic carrier concentrationIn a pure semiconductor,n = p = niEE130 Lecture 2, Slide 8Spring 2007Si: From Atom to CrystalEnergy states in Si atom energy bands in Si crystal•The highest nearly-filled band is the valence band•The lowest nearly-empty band is the conduction bandEE130 Lecture 2, Slide 9Spring 2007Energy Band DiagramEcEvSimplified version of energy band model, indicating•bottom edge of the conduction band (Ec)•top edge of the valence band (Ev)Ec and Ev are separated by the band gap energy EGelectron energydistanceEE130 Lecture 2, Slide 10Spring 2007Summary•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.•Splitting of allowed atomic energy levels occurs in a crystal–Separation between energy levels is small, so we can consider them as bands of continuous energy levels•Highest nearly-filled band is the valence band•Lowest nearly-empty band is the conduction bandEE130 Lecture 2, Slide 11Spring 2007–Energy-band diagram:•Shows only bottom edge of conduction band Ec and top edge of valence band Ev•Ec and Ev are separated by the band-gap energy
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