Lecture 2 OUTLINE Electrons and holes Energyband model

Lecture #2 OUTLINE • Electrons and holes • Energy-band model Read: Chapter 2 (Section 2. 2)

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 (#/cm 3) of conduction electrons & holes in a semiconductor can be modulated in several ways: 1. 2. 3. 4. by adding special impurity atoms ( dopants ) by applying an electric field by changing the temperature by irradiation 2 Spring 2007 EE 130 Lecture 2, Slide 2

Bond Model of Electrons and Holes 2 -D representation: When an electron breaks loose and becomes a conduction electron, a hole is also created. Si Si Si 3 Spring 2007 EE 130 Lecture 2, Slide 3

What is a Hole? • Mobile positive charge associated with a half-filled covalent bond – Treat as positively charged mobile particle in the semiconductor • Fluid analogy: 4 Spring 2007 EE 130 Lecture 2, Slide 4

The Hole as a Positive Mobile Charge 5 Spring 2007 EE 130 Lecture 2, Slide 5

Pure Si conduction ni 1010 cm-3 at room temperature 6 Spring 2007 EE 130 Lecture 2, Slide 6

Definition of Terms n = number of electrons/cm 3 p = number of holes/cm 3 ni = intrinsic carrier concentration In a pure semiconductor, n = p = ni 7 Spring 2007 EE 130 Lecture 2, Slide 7

Si: From Atom to Crystal Energy 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 band 8 Spring 2007 EE 130 Lecture 2, Slide 8

Energy Band Diagram electron energy Ec Ev distance Simplified version of energy band model, indicating • bottom edge of the conduction band (Ec) • top edge of the valence band (Ev) Ø Ec and Ev are separated by the band gap energy EG 9 Spring 2007 EE 130 Lecture 2, Slide 9

Summary • 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 band 10 Spring 2007 EE 130 Lecture 2, Slide 10

– 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 EG 11 Spring 2007 EE 130 Lecture 2, Slide 11